DE102024126349A1 - Heat-curable underfill material, method for bonding a substrate to at least one joining partner and use as underfill - Google Patents

Abstract

A heat-curable backer material comprising (A) an epoxy; (B) a radical-curable component comprising a (meth)acrylate (B1); (C) a cyclic carbodiimide, wherein the cyclic carbodiimide has a ring structure comprising a carbodiimide group whose nitrogen atoms are linked together via a linker; (D) a nitrogen-based hardener comprising a latent nitrogen-based hardener (D1); (E) a radical initiator component comprising a radical initiator for heat curing (E1); and (F) a filler.
Furthermore, a method for bonding a substrate to at least one joining partner and its use as an underfill are described.

Description

AREA OF INVENTION

The invention relates to a heat-curable underfill material. Furthermore, the invention relates to a method for bonding a substrate to at least one joining partner using the underfill material and the use of the underfill material as an underfill.

TECHNICAL BACKGROUND

Underfills are frequently used to reinforce the solder joints of BGA (Ball Grid Array), CSP (Chip Scale Package), or flip-chip components at the substrate or wafer level. Underfilling involves applying a mostly epoxy-based adhesive between the electronic component and the substrate to protect the components from mechanical stresses such as shock and vibration. Another important property of the adhesive is its ability to compensate for differences in thermal expansion between the electronic component and the substrate, for example, between a chip and a printed circuit board, in order to prevent stress and increase the reliability and lifespan of the resulting bond.

For underfilling, the liquid underfill material is usually dispensed along the side of a chip or housing and can flow into the gap between the electronic component and the substrate, which is sometimes only a few hundred micrometers high, via capillary action. To increase flowability, the circuit board can be heated during the dispensing process. The underfill material is then hardened.

To meet the requirements for the mechanically compensating properties of the cured underfill, high filler concentrations are sometimes necessary. This makes the formulation of underfills with simultaneously good flow properties particularly challenging. For example, solvents for reducing viscosity are unsuitable, as the required adhesion values and glass transition temperatures cannot be achieved. Additionally, solvents can lead to bubble formation during the heat curing process.

Numerous epoxy-based underfill materials are described in the prior art.

US Patent 6,893,736 B2 describes epoxy resin-based backing compounds for reinforcing the solder joint of a semiconductor component. These compounds are characterized by their high resistance to temperature and humidity, thus compensating for the disadvantages of a pure solder joint. Additionally, the compounds possess high flowability and a long working time.

The advantages of cyclic carbodiimides are revealed in the prior art. WO 2010/071211 A1 This describes the use of cyclic carbodiimides for stabilizing hydrolysis-sensitive polymers, such as polyesters. Furthermore, cyclic carbodiimides offer the advantage that no volatile isocyanates are released as a byproduct.

EP 3 127 933 B1 describes curable epoxy resins comprising a cyclic carbodiimide. The cured compounds exhibit particularly good mechanical properties and high resistance to damp heat. Use as a backer rod is not intended.

Epoxy resins for underfills comprising a cyclic carbodiimide are also known from WO 2019/138919 A1. The cured adhesives have a high glass transition temperature and exhibit particularly low mass loss at elevated temperatures.

To prevent contamination of the substrate due to uncontrolled flow of underfill materials, dual-curing formulations can be used. EP 2 948 506 B1 This describes an epoxy-based underfill material comprising a light-curing resin component and a photoinitiator. To ensure complete curing of the light-curing component, preferably consisting of (meth)acrylates, in shaded areas, radical initiators based on peroxide or azo can optionally be used.

There is therefore a continued need for curable underfill materials with particularly good flow properties combined with excellent mechanical properties in the cured state, which meet the ever-increasing demands of the industry. Furthermore, it is required that the underfill materials exhibit good stability against segregation even at very low viscosity and simultaneously high filler levels.

SUMMARY OF THE INVENTION

The object of the present invention is to provide curable underfill materials that enable the flow and bonding of electronic components and reliably fix them to a substrate even under load over their entire service life.

The invention relates in particular to underfill materials characterized by high flowability and, in the cured state, exhibiting a low coefficient of thermal expansion (CTE) combined with a high glass transition temperature ( _Tg ). Furthermore, the underfill materials should cure at low temperatures to avoid damaging temperature-sensitive components.

Another objective of the invention is to ensure a homogeneous distribution of fillers in the cured adhesive.

This problem is solved according to the invention by a hardenable underfilling compound according to claim 1.

Further embodiments of the invention are specified in the dependent claims, which can optionally be combined with one another.

• (A) ein Epoxid;
• (B) eine radikalisch härtbare Komponente, die ein (Meth)acrylat (B1) umfasst;
• (C) ein cyclisches Carbodiimid, wobei das cyclische Carbodiimid eine Ringstruktur aufweist, die eine Carbodiimidgruppe umfasst, deren Stickstoffatome über einen Linker miteinander verbunden sind;
• (D) einen Härter auf Stickstoffbasis, der einen latenten Härter auf Stickstoffbasis (D1) umfasst;
• (E) eine radikalische Initiatorkomponente, die einen radikalischen Initiator für die Warmhärtung (E1) umfasst; und
• (F) einen Füllstoff.

The heat-curable underfill material according to the invention comprises

• (A) an epoxy;
• (B) a radically curable component comprising a (meth)acrylate (B1);
• (C) a cyclic carbodiimide, wherein the cyclic carbodiimide has a ring structure comprising a carbodiimide group whose nitrogen atoms are linked together via a linker;
• (D) a nitrogen-based hardener comprising a latent nitrogen-based hardener (D1);
• (E) a radical initiator component comprising a radical initiator for heat curing (E1); and
• (F) a filler.

By combining the components, particularly flowable underfill materials can be formulated, which are especially suitable for underfilling electronic components.

The cured compounds are characterized by a high glass transition temperature ( _Tg ) and a low coefficient of thermal expansion (CTE). This ensures that components joined using the backer rod are reliably and permanently fixed, even at high temperatures.

The curable underfills are formulated based on epoxy (A) and can be reliably cured at particularly low temperatures, which is especially advantageous when bonding temperature-sensitive components. At the same time, low energy input enables particularly resource-efficient curing processes. Reliable curing at low temperatures is made possible by the latent nitrogen-based hardener (D1).

The use of (meth)acrylate (B1) in component (B) results in particularly low-viscosity curable underfills, enabling high filler levels while maintaining high flowability. After flow, the (meth)acrylates (B1) can be reliably cured by adding the radical initiator component (E), which includes a radical initiator for heat curing (E1). This is particularly advantageous for reliable and complete curing in shadowed areas, which are especially prevalent when underfilling components such as electronic components. Optionally, a radical photoinitiator (E2) can be added for rapid light curing of the bond.

In this case, the radical initiator component (E) comprises the radical initiator for heat curing (E1) and a radical photoinitiator (E2).

The strength of the bond is further ensured by a particularly high glass transition temperature of the cured material. Preferably, the cured underfill has a glass transition temperature of 120 °C or higher, which is advantageous for use on electronic components. This glass transition temperature is achieved not only through the use of the filler (F) but also through the addition of cyclic carbodiimide (C), which further increases the bond density of the adhesive network. The increased network density and glass transition temperature also result in higher resistance to moisture, giving the cured underfill excellent damp-heat resistance.

The combination of cyclic carbodiimide (C) and filler (F) gives the underfill a low coefficient of thermal expansion, in particular a coefficient of thermal expansion of less than 40 ppm/K. This further increases the stability of the bond over a wide temperature range, enabling a particularly reliable and durable connection between the joining partners.

Furthermore, the cured underfill material, through the use of cyclic carbodiimide (C), can be free of undesirable byproducts that arise when using non-cyclic carbodiimides. In particular, the cured underfill material is free of volatile isocyanates, which can lead to an undesirable decrease in the adhesion of the bonded joint. Accordingly, the curable underfill material is specifically free of non-cyclic carbodiimides.

It is understood that at least one of the components (A) to (D) is at least difunctional in order to enable crosslinking of the components contained in the curable underfill material.

1. a) Bereitstellen des Substrats und Platzieren des wenigstens einen Fügepartners auf dem Substrat, wobei ein Spalt zwischen dem Substrat und dem Fügepartner ausgebildet wird;
2. b) Dosieren der Unterfüllmasse auf das Substrat;
3. c) Unterfließenlassen des Fügepartners durch Kapillarwirkung, wobei der Spalt beim Unterfließen wenigstens teilweise mit der Unterfüllmasse befüllt wird; und
4. d) Härten der Unterfüllmasse durch Wärmeeintrag.

The invention further relates to a method for bonding a substrate to at least one joining partner using the underfill material as previously described, wherein the method comprises the following steps:

1. a) Providing the substrate and placing the at least one joining partner on the substrate, whereby a gap is formed between the substrate and the joining partner;
2. b) Dosing the underlayment onto the substrate;
3. c) Flowing the joining partner underneath by capillary action, whereby the gap is at least partially filled with the backing material during the flow; and
4. d) Hardening of the backer rod by heat input.

The substrate and/or the at least one joining partner are in particular electronic components, for example printed circuit boards and/or chips.

The use of the curable backer rod ensures reliable flow even under larger components. The use of the reactive (meth)acrylate (B1) in component (B) as a thinner results in particularly advantageous flow properties, enabling high filler levels. The radical initiator for heat curing (E1) allows for complete curing even in shadowed areas. A further advantage of the backer rod according to the invention is the high rate of radical heat curing and the associated network formation, which counteracts the sedimentation of fillers after the dispensing process.

It is understood that the backer rod is dispensed onto the substrate in such a way that it subsequently flows into the gap by capillary action and at least partially fills it. In particular, the backer rod is dispensed onto the substrate directly next to the gap.

The underflow of the joining partner occurs primarily without any further action on the dispensed underfill material, i.e., exclusively through capillary action. In other words, after the underfill material is dispensed onto the substrate, a waiting period is observed during which the underfill material flows under the joining partner.

It is understood that multiple gaps can form between the substrate and the joining partner, and that one or more gaps can form between the substrate and different joining partners. In this case, the backing material is applied to several points on the substrate, and the gaps are then at least partially filled by the backing material.

In particular, the gap is completely filled with the backing material during the flow process. This results in a particularly stable bond between the substrate and the joining partner.

To improve the flow behavior, the substrate and/or at least one joining partner can be preheated before flowing.

Preheating can be carried out before, during and/or after the underlayment has been dosed onto the substrate.

The substrate and/or the joining partner are heated to a temperature of at least 50 °C. The maximum temperature used for preheating depends in particular on the temperature sensitivity of the substrate and the at least one joining partner, as well as on the temperature at which the curable underfill begins to harden.

The temperature used for preheating must not exceed the temperature used for curing the mass according to the invention.

By heating the substrate and/or at least one joining partner, the flow rate can be increased and particularly long flow paths can be achieved. In comparison to the prior art, the particularly advantageous effect of the additional (meth)acrylate (B1) of component (B) becomes apparent. Due to the rapid network formation in step (d) during the curing of the underfill material by heat input, this counteracts the settling of fillers and additives and ensures a homogeneous distribution of these components in the cured underfill material.

The curable underfill material can further comprise a radical photoinitiator (E2), wherein the underfill material is fixed by actinic radiation after inflow.

In other words, the backer rod can be fixed after flowing in by means of an additional irradiation step, in particular by suitable exposure of the fillet welds formed by the backer rod in the edge area of the gap.

This allows for a rapid increase in the strength of the backer rod.

In the context of the inventive method, “fixation” or “fixing” refers to the development of a strength in the underfill material from which no further flow of the underfill material can occur, or the degree of strength from which joined parts, in particular substrates, can be handled in subsequent processes without the joint being destroyed.

Depending on the formulation of the curable underfill, the radical photoinitiator (E2) can react with the (meth)acrylate (B1) of component (B) and an optionally added further radically curable component (B2) to fix the underfill.

Furthermore, the invention relates to the use of the previously described heat-curable underfill material as underfill, preferably as underfill on electronic components such as, in particular, printed circuit boards and chips.

BRIEF DESCRIPTION OF THE DRAWINGS

• - 1 ein REM-Bild einer erfindungsgemäßen Unterfüllmasse nach der Aushärtung; und
• - 2 ein REM-Bild einer Vergleichsmasse nach der Aushärtung.

The figures show:

• - 1 a SEM image of a backer rod according to the invention after hardening; and
• - 2 A SEM image of a reference mass after hardening.

DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS

• „Einkomponentig“ oder „Einkomponentenmasse“ bedeutet im Sinne der Erfindung, dass die genannten Komponenten der Unterfüllmasse zusammen in einer Packungseinheit vorliegen.

The invention is described in detail below, using examples of preferred embodiments, which, however, should not be understood in a limiting sense. The following definitions are used in the description:

• “Single-component” or “single-component compound” means, within the meaning of the invention, that the aforementioned components of the underfill compound are present together in one packaging unit.

The underfill material is considered "processable" if the viscosity of the respective ready-mixed underfill material increases by less than 25% during storage at room temperature over the specified period.

For the purposes of the invention, ‘liquid’ means that at 23 °C the loss modulus G'' determined by viscosity measurement is greater than the storage modulus G' of the relevant sub-filling material.

“At least difunctional” means that each molecule contains two or more units of the respective functional group.

Insofar as the indefinite article “ein” or “eine” is used, this also includes the plural form “ein oder mehr”, unless this is expressly excluded.

All weight percentages listed below refer to the total weight of the hardenable underlayment, unless otherwise stated.

The individual components of the hardenable underlay are described in more detail below.

Component (A): Epoxy

The epoxy (A) serves as the resin component of the backer rod and is not further restricted in its chemical structure. The epoxy (A) comprises aromatic or aliphatic compounds with at least one epoxide group in the molecule, such as cycloaliphatic epoxides, glycidyl ethers, glycidylamines, and mixtures thereof.

The epoxy (A) can be mono- or higher-functional.

Preferably the epoxy (A) comprises at least one di- or higher-functional epoxy.

Examples of monofunctional epoxides (A) include butyl glycidyl ether, (2-ethylhexyl)glycidyl ether, phenyl glycidyl ether, 2,3-epoxypropyl o-tolyl ether, 4-tert-butylphenyl glycidyl ether, styrene oxide, α-pinene oxide, cresol glycidyl ether, polyethylene glycol monoglycidyl ether, polypropylene glycol monoglycidyl ether, polytetramethylene monoglycidyl ether, fatty acid glycidyl esters, norbornene oxide, glycidyl ether of cardanol and glycidyl neodecanoate.

Cycloaliphatic epoxides (A) are known in the prior art and include compounds that bear both a cycloaliphatic group and an oxirane ring.

Examples include 3-cyclohexenylmethyl-3-cyclohexylcarboxylate diepoxide, 3,4-epoxycyclohexylalkyl-3',4'-epoxycyclohexanecarboxylate, 3,4-epoxy-6-methylcyclohexylmethyl-3',4'-epoxy-6-methylcyclohexanecarboxylate, vinylcyclohexene dioxide, bis(3,4-epoxycyclohexylmethyl)adipate, dicyclopentadiene dioxide, limonene dioxide, and 1,2-epoxy-6-(2,3-epoxypropoxy)hexahydro-4,7-methanindane.

Aromatic epoxides (A) can also be used in the underfill material according to the invention.

Examples of aromatic epoxides (A) are bisphenol-A epoxy resins, bisphenol-F epoxy resins, phenol-novolak epoxy resins, cresol-novolak epoxy resins, biphenyl epoxy resins, 4,4'-biphenyl epoxy resins, divinylbenzene dioxide, 2-glycidylphenyl glycidyl ethers, naphthalenediol diglycidyl ethers, glycidyl ethers of tris(hydroxyphenyl)methane, glycidyl ethers of tris(hydroxyphenyl)ethane and glycidyl ethers of allylphenol novolaks.

Furthermore, all fully or partially hydrogenated analogues of aromatic epoxides (A) can also be used. Halogen-free or low-halogen bisphenol-A and bisphenol-F epoxy resins are preferred.

Isocyanurates substituted with epoxy-containing groups and other heterocyclic compounds can also be used as component (A) in the underfill material according to the invention. Triglycidyl isocyanurate and monoallyldiglycidyl isocyanurate are examples.

Furthermore, polyfunctional epoxy resins of all the resin groups mentioned, tough elasticized epoxy resins and mixtures of different epoxy compounds can also be used in the underfill material according to the invention.

A combination of several epoxy-containing compounds, at least one of which is di- or higher-functional, is also within the scope of the invention.

Suitable epoxides (A) are commercially available under the trade names CELLOXIDE™ 2021P, CELLOXIDE™ 8000 from Daicel Corporation, Japan, or EPlKOTE™ RESIN 828 LVEL, EPlKOTE™ RESIN 166, EPlKOTE™ RESIN 169 from Westlake Epoxy, or Epilox™ resins of product lines A, T and AF from Leuna Harze, Germany, or EPICLON™ 840, 840-S, 850, 850-S, EXA850CRP, 850-LC from DIC K.K., Japan.

Component (A) can be present in the curable underfill material according to the invention, based on the total weight of the underfill material, in a proportion of 5 to 50 wt.%, preferably in a proportion of 10 to 30 wt.%.

Component (B): Radically curable component

The curable underfill material comprises a radically curable component (B) which includes or consists of a (meth)acrylate (B1).

Component (B) can be present in the underfill material in a proportion of at least 4 wt.%, preferably at least 8 wt.%, in each case based on the total weight of the curable underfill material.

The proportion of component (B) in the curable underfill material according to the invention can be from 4 to 40 wt.%, preferably from 8 to 30 wt.%, in each case based on the total weight of the curable underfill material.

(B1): (Meth)acrylate

The term "(meth)acrylates" here and in the following refers to derivatives of acrylic acid and methacrylic acid, as well as combinations and mixtures thereof. The (meth)acrylate (B1) is not further structurally restricted.

The (meth)acrylate (B1) can be mono- or higher-functionalized.

To ensure a high glass transition temperature of the cured underfill, the (meth)acrylate (B1) can have a homopolymerization glass transition temperature (homopolymerization _Tg ) of 60 °C or higher, preferably 70 °C or higher, particularly preferably 80 °C or higher.

The homopolymerization glass transition temperature of a given (meth)acrylate as a monomer refers to the glass transition temperature of a homopolymer of that monomer. The homopolymer must have a sufficiently high molecular weight so that its glass transition temperature reaches a limit. It is generally known that the glass transition temperature of a homopolymer increases with increasing molecular weight up to a certain limit. Furthermore, the homopolymer must be essentially free of moisture, residual monomer, solvents, and other impurities that could affect the glass transition temperature.

The homopolymer of the respective monomer can be obtained by adding a thermal initiator and subsequent heat curing, or alternatively by adding a photoinitiator and curing using light.

The glass transition temperature of the homopolymer can then be measured, for example using differential scanning calorimetry (DSC) or mechanical analysis (DMA). Of these two methods, values obtained from DSC measurements are preferred.

Furthermore, the homopolymerization glass transition temperature is usually available as a material property in data sheets provided by the commercial suppliers of the respective (meth)acrylate.

By using a (meth)acrylate (B1) with a high homopolymerization glass transition temperature, the glass transition temperature of the cured underfill can also be increased, so that a particularly temperature-stable adhesive bond can be achieved when using such an underfill.

Furthermore, the use of a (meth)acrylate (B1) with a high homopolymerization glass transition temperature can limit the thermal expansion of the backer rod, resulting in lower coefficients of thermal expansion.

The viscosity of the (meth)acrylate (B1) is preferably below 1,000 mPa·s, particularly preferably below 500 mPa·s. In this way, the flow behavior of the backer rod can be further improved, in particular the achievable flow distances can be increased.

The viscosity of (meth)acrylate (B1) can be determined using a rheometer at room temperature and a shear rate of 10/s.

Both aliphatic and aromatic (meth)acrylates (B1) can be used.

Suitable examples include the following (meth)acrylates (B1): isobornyl acrylate, stearyl acrylate, tetrahydrofurfuryl acrylate, cyclohexyl acrylate, 3,3,5-trimethylcyclohexanol acrylate, behenyl acrylate, 2-methoxyethyl acrylate and other single- or multiply alkoxylated alkyl acrylates, isobutyl acrylate, isooctyl acrylate, lauryl acrylate, tridecyl acrylate, isostearyl acrylate, 2-(o-phenylphenoxy)ethyl acrylate, acryloylmorpholine, N,N-dimethylacrylamide, 4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,10-decanediol diacrylate, tricyclodecanedimethanol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, polybutadiene diacrylate, cyclohexanedimethanol diacrylate, diurethane acrylates of monomeric, oligomeric or polymeric forms Diols and polyols, trimethylolpropane triacrylate (TMPTA), and dipentaerythritol hexaacrylate (DPHA), and combinations thereof.

Higher-functional (meth)acrylates (B1) derived from multiply branched or dendrimeric alcohols can also be used to advantage.

The analogous methacrylates are also within the scope of the invention.

Preferably, the molecular weight of the (meth)acrylate (B1) is below 600 g/mol. Such (meth)acrylates exhibit particularly good flow properties and/or a low viscosity, so that more flowable underfill materials can be obtained.

A combination of several (meth)acrylates (B1) is also within the scope of the invention.

Furthermore, hybrid compounds which, in addition to at least one epoxy function, have at least one (meth)acrylate function, can be advantageously used in the curable mass.

The proportion of (meth)acrylate (B1) in component (B) is in particular at least 80 wt.%, preferably at least 90 wt.%, in each case based on the total weight of component (B).

(B2): Another radically curable component

In the underfill material according to the invention, in addition to the (meth)acrylate (B1), a further radically curable component (B2) can optionally be used.

The further radically curable component (B2) is not structurally restricted as long as it contains an ethylene unsaturated double or triple bond and is not a (meth)acrylate.

Suitable examples include bismaleimides, allyl and methallyl compounds, such as 1,3,5-triallyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, which is commercially available as TAICROS®, as well as isoprene, butadiene, and pro- pargyles and vinyl compounds, including N-vinyl compounds such as vinylmethyloxazolidinone (VMOX), N-vinylcaprolactam, N-vinylpyrrolidone and N-vinylimidazole.

Unhydrogenated polybutadienes with free double bonds, such as the Poly BD® types, can also be used as another radiation-curable compound.

A combination of several other radically hardenable components (B2) is also within the scope of the invention.

The further radically curable component (B2) is present in the underfill materials according to the invention in a proportion of 0 to 20 wt.%, preferably 0 to 10 wt.%, in each case based on the total weight of the component (B).

Component (B) can consist of the (meth)acrylate (B1) and optionally the further radically curable component (B2). Preferably, component (B) consists of the (meth)acrylate (B1).

Component (C): Cyclic carbodiimide

The curable underfill material comprises a cyclic carbodiimide (C).

Carbodiimides possess a wide range of reactivity and can therefore be used in diverse roles in curable underfill materials. They exhibit particularly high reactivity towards carboxyl groups, but epoxides, amines, and hydroxyl groups can also react with carbodiimides.

Carbodiimides can react into the polymer network or further crosslink it. Furthermore, polymers with hydrolysis-sensitive groups, especially polyesters, can be stabilized by capping or recrosslinking their hydrolysis products.

The addition of cyclic carbodiimide (C) increases the temperature and humidity resistance as well as the glass transition temperature of the cured underfill. Furthermore, it reduces the thermal expansion of the cured underfill.

Furthermore, the cyclic structure of cyclic carbodiimide (C) prevents the formation of free isocyanate compounds during the hardening of the backer rod, thus reducing the risk of contamination by mostly toxic by-products.

The cyclic carbodiimide (C) of the present invention has a ring structure comprising a carbodiimide group whose nitrogen atoms are linked together via a linker.

Exemplary cyclic carbodiimides that can be used in the underfill material according to the invention are described in EP 3 127 933 A1 and the EP 2 380 883 A1 described.

The number of atoms comprising the ring structure, i.e., the structure consisting of the linker and the carbodiimide group, is not further restricted and ranges, for example, from 8 to 50 atoms. Preferably, the ring structure consists of 9 to 30 atoms, and more preferably of 10 to 20 atoms.

The ring structure is monofunctional with respect to the carbodiimide group. This means that the ring structure contains only a single carbodiimide group.

The linker can be an aliphatic, alicyclic or aromatic hydrocarbon residue, or a combination of such hydrocarbon residues.

The linker may also contain one or more heteroatoms, such as sulfur, oxygen, nitrogen and/or phosphorus.

In one variant, the linker is an aromatic hydrocarbon residue of the formula -Ar ¹ -OXO-Ar ² -, where Ar ¹ and Ar ² each independently denote at least divalent aromatic hydrocarbon residues, in particular an aromatic hydrocarbon residue with 5 to 15 carbon atoms, which is optionally heteroatom-containing and/or substituted, and where X is at least divalent alipha aliphatic or aromatic hydrocarbon residue, in particular an aliphatic or aromatic hydrocarbon residue with 1 to 20 carbon atoms, for example 2 to 20 carbon atoms or 5 to 20 carbon atoms, which is optionally heteroatom-containing and/or substituted.

In another variant, the linker is an aliphatic hydrocarbon residue of the formula -R ¹ -OXOR ² -, where R ¹ and R ² each independently denote an at least divalent aliphatic hydrocarbon residue, in particular an aliphatic hydrocarbon residue with 1 to 20 carbon atoms, which is optionally heteroatom-containing and/or substituted, where X denotes an at least divalent aliphatic or aromatic hydrocarbon residue, in particular an aliphatic or aromatic hydrocarbon residue with 1 to 20 carbon atoms, for example 2 to 20 carbon atoms or 5 to 20 carbon atoms, which is optionally heteroatom-containing and/or substituted.

The cyclic carbodiimide (C) can have a single or multiple ring structures, wherein the cyclic carbodiimide (C) preferably comprises at least two ring structures linked together.

The ring structures can be linked via one or more atoms as multicyclic compounds, for example, bicyclic or tricyclic. It is also possible for the ring structures to be linked in the form of a spiro compound.

The linkers of the linked ring structures are each designed as previously described for a single linker, whereby the atoms used to link the ring structures are added to the number of atoms for each of the linked ring structures.

For example, the ring structures are linked to each other via one of the aromatic residues Ar ¹ or Ar ² , the aliphatic residues R ¹ or R ² or the aliphatic residue X.

The linkers of the linked ring structures can be the same or different from each other.

Dinitro compounds, for example, can be used as starting materials for the synthesis of cyclic carbodiimides (C). The nitro groups can first be converted to the amine, then to the isocyanate, and subsequently linked to the cyclic carbodiimide (C) by decarboxylation. Synthesis examples and exemplary components (C) can be found in the EP 2 380 883 A1 can be taken.

The cyclic carbodiimide (C) preferably exists in solid form as particles at room temperature. The volumetric particle diameter d98 is particularly in the range of 1 to 50 µm, more preferably in the range of 2 to 40 µm, and most preferably in the range of 5 to 30 µm. The volumetric particle diameter is determined particularly by laser diffraction.

An example of a commercially available cyclic carbodiimide (C) is TCC-FP10M from Teijin Limited, Japan.

In the curable underfill material according to the invention, the cyclic carbodiimide (C) can be present in a proportion of 1 to 20 wt.%, preferably 2 to 15 wt.%, in each case based on the total weight of the underfill material.

Component (D): Nitrogen-based hardener

The underfill material according to the invention contains at least one nitrogen-based hardener (D) comprising a latent nitrogen-based hardener (D1).

In the context of the present invention, a “latent hardener” is understood to be a hardener which remains inactive under storage conditions for a certain period of time, but becomes active when heated to a temperature customary for hot curing, for example a temperature in the range between 60 and 160 °C, and triggers a polyaddition and crosslinking reaction between the latent hardener (D1) and the epoxy (A).

In the hardenable underfill material according to the invention, the nitrogen-based hardener (D) is present in particular in a proportion of 0.5 wt.% to 30 wt.%, preferably in a proportion of 1 wt.% to 25 wt.%, in each case based on the total weight of the underfill material.

However, component (D) does not include the cyclic carbodiimide of component (C). Therefore, the proportions of the nitrogen-based hardener (D) are calculated without the cyclic carbodiimide of component (C).

Component (D1): Nitrogen-based latent hardener

The component (D1) is not further restricted in its chemical structure and preferably comprises a latent hardener from the group of primary, secondary and tertiary amines, cyanamides, imidazoles, hydrazides and/or epoxy adducts of the aforementioned compounds.

Preferably, the latent hardener (D1) is present in the backer rod in solid form.

Commercial examples of suitable solid amine hardeners containing primary, secondary and/or tertiary amine groups are Ancamine 2014 AS (Evonik), Ancamine 2337S (Evonik), Ajicure PN-23J (Ajinomoto), Aradur 9506 (Huntsman), FUJICURE FXR-1121 (Sanho), FUJICURE FXR-1020 (Sanho), Hardener XB 3123 (Huntsman), Versalink 740M (Evonik), Lonzacure M-CDEA, Primacure M-DEA, Aradur 9664-1 (Huntsman) and Aradur 9719-1 (Huntsman).

The latent hardeners mentioned (D1) each preferably comprise at least two nitrogen-containing groups per molecule suitable for epoxy hardening.

Commercial examples of suitable latent hardeners (D1) from the cyanamide group are Dyhard 100 SF from Alzchem, DICYANEX 1200 from Evonik and EPIKURE Curing Agent P-104 from Westlake.

Commercial examples of suitable solid imidazole hardeners are Adeka Hardener EH-5011S, Adeka Hardener EH-5046S and Curezol 2P4MZ (Shikoku).

Commercial examples of suitable hydrazides are Technicure ADH-J (ACCI Specialty Materials), Technicure IDH-J (ACCI Specialty Materials), Ajicure UDH-J (Ajinomoto) and Ajicure VDH-J (Ajinomoto).

Encapsulated hardeners such as Novacure types from Asahi Kasei, including HX-3722, HX3742, HX-3088, HXA3922HP, HXA3932HP, HX-3941HP, HXA3927HP, HXA3937HP, HXA9322HP, HXA9382HP, HXA4922HP, HXA4982HP, HXA5911HP, HXA5945HP, HXA5934HP or Technicure types from ACCI Specialty Materials, can also be advantageously used in the underfill material according to the invention.

By combining different latent nitrogen-based hardeners (D1), an acceleration of epoxy curing can be achieved even without the addition of an accelerator.

Commercial examples of nitrogen-containing compounds that have an accelerating effect on hardening, especially in combination with another latent hardener (D1), are Ancamine 2014 AS (Evonik), Ancamine 2337S (Evonik), Ajicure PN-23J (Ajinomoto), Aradur 9506 (Huntsman), FUJICURE FXR-1121 (Sanho), FUJICURE FXR-1020 (Sanho), Hardener XB 3123 (Huntsman), Adeka Hardener EH-5011S, Adeka Hardener EH-5046S and Technicure LC-80 (ACCI Specialty Materials).

The preceding lists are to be considered exemplary and not exhaustive. Mixtures of the mentioned latent hardeners (D1) are also within the scope of the invention.

The proportion of the nitrogen-based latent hardener (D1) in the component (D) is in particular at least 30 wt.%, preferably at least 70 wt.%, in each case based on the total weight of the component (D).

(D2): Accelerator

The underfill material according to the invention can optionally include at least one accelerator (D2) which also contains nitrogen. This accelerator is, for example, selected from the group consisting of urea derivatives, amines, and imidazoles.

Examples of suitable accelerators (D2) based on ureas and their derivatives are urones. Commercial examples include Dyhard UR 500 (Alzchem), Dyhard UR 300 (Alzchem), and EPICURE Catalyst. 116 (Westlake), Technicure MDU-11M (ACCI Specialty Materials), Technicure IPDU-8 (ACCI Specialty Materials), Technicure MDU-11M (ACCI Specialty Materials), Dyhard UR 800 (Alzchem), URAcc 57 (Alzchem), UR 700 (Alzchem), UR 500 (Alzchem), EPICURE Catalyst 116 (Westlake).

The accelerators (D2) listed here are explicitly to be understood as additional accelerating compounds that are not included by component (D1) and its combinations.

The use of multiple accelerators (D2) is also in line with the invention.

The accelerator (D2) is present in the masses according to the invention, in particular in a proportion of 0 to 70 wt.%, preferably 0 to 30 wt.%, in each case based on the total weight of the component (D).

Component (D) can consist of the nitrogen-based latent hardener (D1) and optionally the accelerator (D2). Alternatively, component (D) can consist solely of the nitrogen-based latent hardener (D1).

Component (E): Radical initiator component

The curable underfill material also includes a radical initiator component (E).

The radical initiator component (E) enables the radically curable components contained in the inventive underfill material, in particular the (meth)acrylate (B1) and optionally the further radically curable component (B2), to be cured. Curing can be carried out by means of heat and optionally by means of actinic radiation.

The radical initiator component (E) includes at least one radical initiator for heat curing (E1) and optionally one radical photoinitiator (E2).

The radical initiator component (E) can be present in a proportion of 0.01 to 15 wt.%, preferably in a proportion of 0.1 to 8 wt.%, in each case based on the total weight of the backing material.

Radical initiator for heat curing (E1)

The radical initiator for heat curing (E1) can be selected from peroxo compounds and/or benzpinacols.

Suitable peroxo compounds include, for example, peroxo(di)esters, hydroperoxides, (di)alkyl peroxides, ketone peroxides, perketals, peracids, and peroxomonocarbonates. Furthermore, peroxodicarbonates, such as those found in the WO 2023/030 763 A1 as described, are advantageously used in the underfill material according to the invention.

Examples of suitable peroxoesters include cumene peroxyneodecanoate, 1,1,3,3-tetramethylbutylperoxyneodecanoate, tert-amylperoxyneodecanoate, tert-butylperoxyneodecanoate, 1,1,3,3-tetramethylbutylperoxypivalate, tert-amylperoxypivalate, tert-butylperoxypivalate, Didecanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)-hexane, 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, tert-amyl peroxy-2-ethylhexanoate, dibenzoyl peroxide, tert-butyl peroxy-2-ethylhexanoate, tert-Butylperoxyisobutyrate, tert-Butylperoxy-3,5,5-trimethylhexanoate, tert-Butyl peroxyacetate and tert-Butyl peroxybenzoate.

Examples of suitable hydroperoxides include diisopropylbenzene monohydroperoxide, p-menthane hydroperoxide, cumene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, tert-butyl hydroperoxide and tert-amyl hydroperoxide.

Examples of suitable peroxomonocarbonates include tert-amyl-peroxy-2-ethylhexyl carbonate, tert-butyl-peroxyisopropyl carbonate, and tert-butyl-peroxy-2-ethylhexyl carbonate.

Examples of suitable peroxodicarbonates include di-(4-tert-butyl-cyclohexyl)-peroxodicarbonate, di-(2-ethylhexyl)-peroxodicarbonate, di-n-butylperoxodicarbonate, dicetylperoxodicarbonate and dimyristilperoxodicarbonate.

Benzpinacol and derivatives thereof can also be used as radical initiators for hot curing (E1), as are found, for example, in the US 4 288 527 A are described. These include in particular halogenated, alkylated and methoxy-substituted benzopinacols.

Furthermore, protected benzopinacols can be used as component (E1). These can be either singly or doubly protected and include silyl-protected, in particular singly and doubly trialkylsilyl-protected, benzopinacols.

Examples of benzopinacols that can be used in the curable underfill material according to the invention are 1,1,2,2-tetraphenyl-1,2-ethanediol, 1,1,2,2-tetrakis(4-methylphenyl)-1,2-ethanediol, 1,1,2,2-tetrakis(4-methoxyphenyl)-1,2-ethanediol, 1,1,2,2-tetrakis(4-chlorophenyl)-1,2-ethanediol, 1-hydroxy-2-trimethylsiloxy-1,1,2,2-tetraphenylethane, 1,1,2,2-tetraphenyl-1,2-bis-(trimethylsiloxy)ethane and 1,1,2,2-tetrakis(4-methylphenyl)-1,2-bis-(trimethylsiloxy)ethane.

The radical initiator for heat curing (E1) can be present in the underfill material according to the invention in a proportion of 0.01 to 10 wt.%, preferably 0.1 to 5 wt.%, in each case based on the total weight of the underfill material.

Radical photoinitiator (E2)

The radical photoinitiator (E2) optionally enables light fixation of the curable underfill material.

Any common, commercially available compound can be used as a radical photoinitiator (E2), such as α-hydroxyketones, benzophenone, α,α'-diethoxyacetophenone, 4,4-diethylaminobenzophenone, 2,2-dimethoxy-2-phenylacetophenone, 4-isopropylphenyl-2-hydroxy-2-propylketone, 1-hydroxycyclohexylphenylketone, isoamyl para-dimethylaminobenzoate, methyl 4-dimethylaminobenzoate, methyl orthobenzoylbenzoate, benzoin, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, 2-hydroxy-2-methyl-1-phenylpropan-1-one, 2-isopropylthioxanthone, dibenzosuberone, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, and bisacylphosphine oxides, wherein the aforementioned compounds can be used alone or in combination with two or more of the aforementioned compounds. can be used as a radical photoinitiator (E2).

For example, the Omnirad™ types from IGM Resins can be used as radical photoinitiators (E2) that can be activated by UV radiation, including the types Omnirad 184, Omnirad 500, Omnirad 1173, Omnirad 2959, Omnirad 754, Omnirad BDK, Omnirad 369, Omnirad 907, Omnirad 2022, Omnirad 2100, Omnirad 784, Omnirad 250, Omnirad TPO, Omnirad TPO-L, Omnirad 819, Omnirad 819 DW, Omnirad MBF, Omnirad BMS, Omnirad 4265.

The preceding lists are to be seen as exemplary for the radical photoinitiator (E2) and are by no means to be understood as limiting.

The radical photoinitiator used as component (E2) in the masses according to the invention is preferably activatable by actinic radiation of a wavelength in the range of 200 to 460 nm, particularly preferably from 250 to 400 nm.

If necessary, the radical photoinitiator (E2) can be combined with a suitable sensitizing agent.

The radical photoinitiator (E2) can be present in the curable underfill material in a proportion of 0 to 5 wt.%, preferably 0 to 3 wt.%, in each case based on the total weight of the underfill material.

Component (F): Filler

In addition to components (A) to (E), fillers (F) are used in the underfill. These allow, among other things, the chemical resistance, media absorption, mechanical properties, and coefficient of thermal expansion of the hardened underfill to be influenced.

The filler (F) can be selected from the following group: oxides, nitrides, borides, carbides, sulfides and silicides of metals and semimetals, including mixed compounds of several metals and/or semimetals; carbon modifications such as diamond, graphite and carbon nanotubes; silicates and borates of metals and semimetals; all types of glasses; metals and semimetals in elemental form, in the form of alloys or intermetallic phases; inorganic or organic salts insoluble in the resin matrix; particles of polymeric materials, such as silicone, polyamide, polyethylene and PTFE; and combinations thereof.

For example, the filler can include aluminum oxide, for example in the form of corundum.

To achieve a particularly low coefficient of thermal expansion of the backer rod, silicon dioxide can preferably be used as a filler (F), for example in the form of quartz and/or quartz glass.

Materials with a negative coefficient of thermal expansion, such as zirconium tungstate, can also be used as filler (F).

The selection of fillers (F) is in no way limited with regard to particle shapes (such as angular, spherical, plate-shaped or needle-shaped, hollow forms) and particle sizes (macroscopic, microscopic, nanoscale). It is also known that different particle shapes or particle sizes or particle size distributions can be used in combination to achieve, for example, a lower viscosity or a higher maximum filler content.

The volumetric particle diameter d98 of the filler (F) is preferably in the range of 10 nm to 50 µm, more preferably in the range of 1 µm to 30 µm, and particularly preferably in the range of 3 to 15 µm. The volumetric particle diameter can be determined by laser diffraction.

The filler (F) is preferably present in the curable underfill mass in a proportion of 30 to 80 wt.%, particularly preferably in a proportion of 40 to 70 wt.%, in each case based on the total weight of the underfill mass.

Component (G): Additive

In addition to components (A) to (F), the curable underfill material according to the invention may contain further additives (G). By way of example, but not limited to, catalysts, toughness modifiers such as core-shell particles or block copolymers, dyes, pigments, fluorescent agents, thixotropic agents, thickeners, thermostabilizers, antioxidants, plasticizers, flame retardants, thermally and/or electrically conductive particles, corrosion inhibitors, water scavengers, thinning agents, leveling and wetting additives, adhesion promoters and combinations thereof, which may be used as additives (G), are mentioned.

The additives (G) are present in the hardenable mass, in particular in a proportion of 0 to 15 wt.%, preferably in a proportion of 0.01 to 10 wt.%, in each case based on the total weight of the underfill mass.

Formulation of the curable underfill

One formulation of the curable underfill material according to the invention, in particular for use in the method according to the invention, comprises the components (A) to (F) described above.

1. (A) 5 bis 50 Gew.-% des Epoxids;
2. (B) 4 bis 40 Gew.-% der radikalisch härtbaren Komponente, die das (Meth)acrylat (B1) umfasst;
3. (C) 1 bis 20 Gew.-% des cyclischen Carbodiimids;
4. (D) 0,5 bis 30 Gew.-% des Härters auf Stickstoffbasis, der den latenten Härter auf Stickstoffbasis (D1) umfasst;
5. (E) 0,01 bis 15 Gew.-% der radikalischen Initiatorkomponente, die den radikalischen Initiator für die Warmhärtung (E1) umfasst; und
6. (F) 30 bis 80 Gew.-% des Füllstoffs.

According to a first embodiment, the underfill material comprises or consists of the following components, each based on the total weight of the underfill material:

1. (A) 5 to 50 wt.% of the epoxide;
2. (B) 4 to 40 wt.% of the radically curable component comprising the (meth)acrylate (B1);
3. (C) 1 to 20 wt.% of cyclic carbodiimide;
4. (D) 0.5 to 30 wt.% of the nitrogen-based hardener comprising the latent nitrogen-based hardener (D1);
5. (E) 0.01 to 15 wt.% of the radical initiator component comprising the radical initiator for heat curing (E1); and
6. (F) 30 to 80 wt.% of the filler.

1. (A) 5 bis 50 Gew.-% des Epoxids;
2. (B) 4 bis 40 Gew.-% der radikalisch härtbaren Komponente, die das (Meth)acrylat (B1) umfasst;
3. (C) 1 bis 20 Gew.-% des cyclischen Carbodiimids;
4. (D) 0,5 bis 30 Gew.-% des Härters auf Stickstoffbasis, der den latenten Härter auf Stickstoffbasis (D1) umfasst;
5. (E) 0,01 bis 15 Gew.-% der radikalischen Initiatorkomponente, die den radikalischen Initiator für die Warmhärtung (E1) und den radikalischen Photoinitiator (E2) umfasst; und
6. (F) 30 bis 80 Gew.-% des Füllstoffs.

According to a second embodiment, the underfill material comprises or consists of the following components, each based on the total weight of the underfill material:

1. (A) 5 to 50 wt.% of the epoxide;
2. (B) 4 to 40 wt.% of the radically curable component comprising the (meth)acrylate (B1);
3. (C) 1 to 20 wt.% of cyclic carbodiimide;
4. (D) 0.5 to 30 wt.% of the nitrogen-based hardener comprising the latent nitrogen-based hardener (D1);
5. (E) 0.01 to 15 wt.% of the radical initiator component comprising the radical initiator for heat curing (E1) and the radical photoinitiator (E2); and
6. (F) 30 to 80 wt.% of the filler.

• (A) 5 bis 50 Gew.-% des Epoxids;
• (B) 4 bis 40 Gew.-% der radikalisch härtbaren Komponente, die das (Meth)acrylat (B1) umfasst;
• (C) 1 bis 20 Gew.-% des cyclischen Carbodiimids;
• (D) 0,5 bis 30 Gew.-% des Härters auf Stickstoffbasis, der den latenten Härter auf Stickstoffbasis (D1) umfasst;
• (E) 0,01 bis 15 Gew.-% der radikalischen Initiatorkomponente, die den radikalischen Initiator für die Warmhärtung (E1) umfasst; und
• (F) 30 bis 80 Gew.-% des Füllstoffs, wobei der Füllstoff Siliciumdioxid und/oder Aluminiumoxid umfasst.

According to a third embodiment, the underfill material comprises or consists of the following components, each based on the total weight of the underfill material:

• (A) 5 to 50 wt.% of the epoxide;
• (B) 4 to 40 wt.% of the radically curable component comprising the (meth)acrylate (B1);
• (C) 1 to 20 wt.% of cyclic carbodiimide;
• (D) 0.5 to 30 wt.% of the nitrogen-based hardener comprising the latent nitrogen-based hardener (D1);
• (E) 0.01 to 15 wt.% of the radical initiator component comprising the radical initiator for heat curing (E1); and
• (F) 30 to 80 wt.% of the filler, wherein the filler comprises silicon dioxide and/or aluminium oxide.

Optionally, the underfilling materials of the embodiments described above may contain, in addition to components (A) to (F), 0 to 15 wt.% of additives (G), based on the total weight of the underfilling material.

The hardenable underfill material according to the invention in all embodiments is preferably provided as a single-component material.

Properties and uses of the hardenable underfill

The addition of (meth)acrylate (B1) gives the underfill material according to the invention a particularly low viscosity and increases its flowability. However, low viscosity does not necessarily result in high flowability. Therefore, flowability must always be considered separately from viscosity.

The viscosity of the underfill material is, for example, below 50 Pa·s, and in particular below 30 Pa·s.

The filler material can flow under gaps with a height of less than 150 µm by capillary action. Subject to the maximum filler size limits, gaps with a height of less than 50 µm down to less than 30 µm are also possible.

To ensure reliable flow, the flow distance of the backer rod for the glass/glass substrate combination is at least 10 mm, preferably at least 15 mm, and particularly preferably at least 20 mm, measured at 90 °C and a flow time of 60 s for a 130 µm high gap. Flow distances of less than 10 mm are generally insufficient to meet the requirements of typical applications.

It is understood that the flow distance depends on the specific material combination. For example, a flow distance of 5 mm may be sufficient for reliable underflow with the substrate combination FR4/FR4. However, such an underfill material according to the present invention would also meet the values previously specified for the substrate combination glass/glass.

To aid the flow of the underfill material, the substrate and/or at least one of the joining partners can be additionally heated. Temperatures of at least 50 °C are preferred. Temperatures of 70 °C are further preferred, and temperatures of 90 °C are particularly preferred, but not exceeding 150 °C or 130 °C, to prevent premature hardening of the underfill material.

It is understood that the temperature used to preheat the substrate and/or the at least one joining partner must not be so high that the backing material hardens before a sufficient flow distance has been reached. The temperature used for preheating must therefore not exceed the temperature used to cure the material according to the invention.

Provided that the temperature used for preheating corresponds to the temperature used for curing the mass according to the invention, a sufficient flow distance can still be achieved, since the nitrogen-based latent hardener (D1) used enables delayed curing and thus a sudden curing of the backer rod upon contact with the preheated substrate and/or at least one joining partner can be avoided.

Optionally, the backer rod can be additionally cured with light after being poured. In this case, the curable backer rod comprises component (E2), the radical photoinitiator. Irradiation is carried out in particular by irradiating the fillet weld that forms at the edge of the poured gap.

After fixation with actinic radiation, the backing material acquires a so-called "green strength" or handling strength. This means that no further shrinkage occurs after fixation. The substrate and the respective joining partner remain fixed to each other and can thus be fed into further production steps. Light fixation is particularly suitable for processes where short cycle times are required.

The underfill material according to the invention is characterized by high reactivity combined with a long working time at room temperature. The underfill material can be cured within a short time at temperatures below 180 °C, preferably below 150 °C. Typically, the material is completely cured in less than 60 minutes at a temperature of 130 °C.

In particular, the temperature at which the backing material begins to harden is above 80 °C. If the substrate and/or the at least one joining partner are additionally preheated, the temperature at which the backing material begins to harden is preferably above the preheating temperature.

At the same time, the hardenable underfill material has a processing time of at least 72 hours at room temperature.

The cured backer material has a glass transition temperature of at least 120 °C, preferably at least 130 °C, and more preferably at least 140 °C. A high glass transition temperature is necessary to prevent undesirable softening of the cured backer material and to ensure reliable bonding of the substrate and joining partner(s), such as electronic components, even at high temperatures. A glass transition temperature of less than 120 °C can lead to stresses and consequently to delamination and cracking in the joint.

Electronic components are often exposed to high thermal stresses throughout their service life. Due to its low coefficient of thermal expansion (CTE), the hardened backer rod can compensate for these stresses. This counteracts the stress input into components and/or substrates caused, for example, by thermal expansion. The backer rod according to the invention, in its cured state, has a coefficient of thermal expansion of less than 40 ppm/K, preferably less than 35 ppm/K.

The hardened underfill material retains high adhesion. The compressive shear strength for the substrate combination FR4/FR4 is, for example, at least 10 MPa, preferably over 15 MPa, and particularly preferably over 20 MPa.

The joints are particularly well protected against moisture penetration thanks to the use of the curable underfill. Especially high resistance is also observed against simultaneous exposure to temperature and humidity.

Pure epoxy-amine systems, as frequently used in underfill applications, also tend to segregate during heat curing. The sedimentation of fillers is further intensified, in particular, by the (pre-)heating of the substrate and/or component that is standard practice. The use of (meth)acrylate (B1) in the radically curable component (B) of the underfill material according to the invention enables the rapid formation of an initial polymer network during heat curing. This network counteracts the sedimentation of solid components, especially the fillers (F) and the optional solid additives (G). The cured underfill material thus exhibits high homogeneity throughout its entire volume and enables a reliable bond.

The previously described hardenable underfill material is particularly suitable for use as a so-called "capillary underfill".

Measurement methods and definitions used

room temperature

Room temperature is defined as 23 °C ± 2 °C.

Curing

"Curing" is defined as a polymerization or addition reaction beyond the gel point. The gel point is the point at which the storage modulus G' equals the loss modulus G''.

Flow test and homogeneity test

To assemble the test specimens, two 75 mm x 25 mm x 1 mm glass slides were joined together with a 65 mm overlap and a 130 µm gap. Two drops of a UV-curing adhesive containing 130 µm spacer particles were dispensed onto the corners of each slide using a 580 µm conical dispensing needle. The slides were then joined with an offset, creating an approximately 10 mm wide rim. A 50 g weight was placed on the specimen to ensure a defined contact pressure, and the adhesive was cured using a UV lamp.

A hot plate was preheated to 90 °C, and the glass specimens were placed on it for 2 minutes. The flow test was also performed on the hot plate. For this test, the adhesive was manually applied along an open side of the preheated glass specimen using a 580 µm conical needle.

The flow front was marked after 60 seconds and the flow path was measured.

Subsequently, selected samples were examined for potential sedimentation. For this purpose, the samples described above were first embedded using an investment material. CaldoFix-2 Resin was mixed at a ratio of 25 parts to 7 parts CaldoFix-2 Hardener. The samples were placed in this mixture and cured for 24 hours at room temperature. A cross-section was then prepared from these samples using the ATA SAPHIR 520 grinding machine. These cross-sections were examined under a SEM microscope (see [reference]). 1 and 2 ).

Coefficient of thermal expansion (CTE) and glass transition temperature ( _Tg )

A 2 mm thick polyoxymethylene (POM) plate with an elongated hole approximately 25 mm long and 4 mm wide served as the mold for producing the test specimen. The mold was placed on a plastic film (biaxially oriented polyester, Hostaphan RN, 75 µm thick, siliconized on one side), filled with the reactive backing compound without air bubbles, and covered with another protective film.

The heat curing takes place in a preheated convection oven at 150 °C for 50 min, whereby the test specimen shape including cover films is clamped between two aluminium plates (3 mm thick each) using spring clamps.

After the test specimen mold has cooled, the rod-shaped sample of the hardened underfill material is demolded and a material sample of approximately 4 mm width, 4 mm length and 2 mm thickness is cut off, deburred with 600 grit sandpaper and fed to a thermomechanical analyzer (TMA) from Mettler Toledo (type TMA A840e or TMA A841 Ee).

The measurement comprises the following segments: (1) isothermal, 23 °C, 5 min; (2) dynamic 23-240 °C, 2 K/min. The contact force is 0.1 N in all segments. The heating segment (2) is evaluated.

The mean coefficient of thermal expansion α was evaluated over the temperature range 30-100 °C.

The glass transition temperature was also determined using the TMA measurement (segment 2). This is recognizable by a distinct rise in the measurement curve due to greater expansion of the sample, which creates a kink in the curve. The evaluation is performed by drawing two tangents to this kink. The glass transition is the point of intersection of the tangents.

compressive shear strength (DSF)

Two test specimens (dimensions 20 mm x 20 mm x 5 mm) made of tempered FR4 were bonded with a 5 mm overlap using the respective backing compound. A bead of backing compound was applied to the first test specimen, and then a second specimen was joined. The adhesive layer thickness of 0.1 mm and the overlap were adjusted using spacers and a bonding device. The joined specimens were cured in a preheated convection oven for 60 minutes at 130 °C.

The measurement was performed at room temperature on a Zwick Roell "AllRoundLine" testing machine with a deformation rate of 10 mm/min.

Viscosity and processing time

To determine the processing time, the viscosity of the curable underfill material was measured at a shear rate of 10⁻⁶/s using an Anton Paar MCR-302 rheometer. A PP20 punch was used with a gap of 500 µm and a temperature of 23 ± 1 °C. Sufficient processing time is achieved if, during storage at room temperature, the viscosity increase over a period of 72 hours is less than 25%.

Production examples

To produce the curable masses according to the invention, the components (A), (B) and optionally (G) were first mixed together using a dissolver (PC Laborsystem GmbH) until a homogeneous mass was obtained.

After subsequent cooling to room temperature, component (F) was added and stirred at room temperature.

Finally, components (C), (D) and (E) were added and the mixture was stirred for 30 minutes at room temperature.

Component (A) - Epoxy:

• A-1: Epikote Resin 169 from the company Westlake Epoxy
• A-2: jER YX-8000D from Mitsubishi Chemical
• A-3: Tactix 742 from Huntsman
• A-4: Epotec YDM-441 from Aditya Birla COMPANY

Component (B) - Radically curable component:

• B1-1: Sartomer SR834 from Arkema (M _w = 332.4 g/mol, viscosity = 110 mPa·s, homo-T _g = 211 °C (manufacturer's specification))
• B1-2: Sartomer SR833S from Arkema (M _w = 304.0 g/mol, viscosity = 140 mPa·s, homo-T _g = 185 °C (manufacturer's specification))
• B1-3: Laromer HDDA from BTC Europe GmbH (M _w = 226.3 g/mol, viscosity = 7 mPa·s, homo-T _g = 45 °C (manufacturer's specification))

Component (C) - Cyclic carbodiimide:

• C-1: TCC-FP10M from Teijin Ltd
• C-2: DCC from the company Sigma Aldrich

Component (D) - Nitrogen-based hardener:

• D1-1: Dyhard 100SF from the company Alzchem Group
• D1-2: HXA9322 from the company Asahi Kasei
• D2-1: Dyhard UR 500 from the COMPANY Alzchem Group

Component (E) - Radical initiator component:

• E1-1: Peroxan BEC from Pergan
• E1-2: Peroxan APO from Pergan
• E2-1: TPO-L from the company IGM-Resins

Component (F) - Filler:

• F-1: Denka Fused Silica FB-3SDC from Denka
• F-2: Denka Spherical Alumina DAW-03DC from the company Denka

Component (G) - Additive:

• G-1: Glymo von der FIRMA Evonik Industries
• G-2: Jonol von der FIRMA Sigma Aldrich
• G-3: HQMME von der FIRMA Eastman
• G-4: Cab-O-sil TS 720 von der FIRMA Cabot
• G-5: Macrolex Blau RR von der FIRMA Lanxess
• G-6: Tinuvin 292 von der FIRMA BASF

Tabelle 1: Zusammensetzung und Eigenschaften erfindungsgemäßer härtbarer Unterfüllmassen. Komponente E1 E2 E3 E4 E5 E6 E7 A-1 Epikote Resin 169 27,1 27,1 27,1 9,1 20 25,4 A-2 jER YX-8000D 27,1 B1-1 Sartomer SR834 18 18 15,39 18 13,3 18 B1-2 Sartomer SR833S 18 C-1 TCC-FP10M 4,3 4,3 4,3 3,73 4,3 3,3 8 D1-1 Dyhard 100SF 3 3 3 3 2,2 3 D1-2 HXA9322 20,99 D2-1 Dyhard UR 500 0,8 0,8 0,8 0,8 0,6 0,8 E1-1 Peroxan BEC 0,5 0,5 0,5 0,5 0,4 0,5 E1-2 Peroxan APO 0,5 E2-1 TPO-L 0,3 F-1 Denka Fused Silica FB-3SDC 46 45 46 49,91 46 44 F-2 Denka Spherical Alumina DAW-03DC 60 G-1 Glymo 0,3 0,3 0,3 0,2 0,3 0,2 0,3 G-2 Jonol 0,1 0,02 G-3 HQMME 0,1 0,02 G-4 Cab-O-sil TS 720 0,3 0,14 G-5 Macrolex Blau RR 0,1 G-6 Tinuvin 292 0,1 CTE [ppm/K], 30-100 °C - TMA 33 33 33 22 33 29 35 Tg [°C] - TMA 140 135 138 135 131 152 159 Viskosität [mPa·s], 10 s ^-1 12.586 18.953 14.636 26.414 8.194 12.869 25.394 Stabilität - Viskositätsveränderung [%] 5,8 -5,1 -2,8 15 2,2 -0,3 Fließtest [mm], 60 s, 90 °C, 130 µm 20,5 17 24 13 11,5 10 15 DSF [MPa], FR4-FR4, 1 h, 130 °C 30,8 16,1 66,6 81,3 20,5 45,3 34,2 Tabelle 2: Zusammensetzung und Eigenschaften der Vergleichsmassen. Komponente V1 V2 V3 V4 V5 V6 V7 A-1 Epikote Resin 169 27,6 30,7 50,4 42,9 23,1 23,1 27,1 A-3 Tactix 742 8 A-4 Epotec YDM-441 8 B1-1 Sartomer SR834 18 18 33 18 18 18 B1-3 HDDA C-1 TCC-FP10M 4,3 8 4,3 C-2 DCC 4,3 D1-1 Dyhard 100SF 3 3,5 5,5 4,75 3,3 3,3 3 D2-1 Dyhard UR 500 0,8 1 1,6 1,25 0,8 0,8 0,8 E1-1 Peroxan BEC 0,5 1 0,5 0,5 0,5 0,5 F-1 Denka Fused Silica FB-3SDC 46 46 46 46 46 46 G-1 Glymo 0,3 0,3 0,5 0,3 0,3 0,3 0,3 CTE [ppm/K], 30-100 °C - TMA 48 32 50 36 35 33 32 Tg [°C] - TMA 102 116 145 133 133 136 113 Viskosität [mPa·s], 10 s ^-1 14.427 6.687 1.487 69.021 12.059 9.970 3.418 Stabilität - Viskositätsveränderung [%] 7,4 4,0 -3,4 -5,9 1,5 11,0 -11,0 Fließtest [mm], 60 s, 90 °C, 130 µm 23 25 39 12 7 8 25 DSF FR4-FR4, 1 h, 130 °C 33,0 27,7 28,3 73,4 33,6 26,4 17,1

• G-1: Glymo from the company Evonik Industries
• G-2: Jonol from the company Sigma Aldrich
• G-3: HQMME from the company Eastman
• G-4: Cab-O-sil TS 720 from the company Cabot
• G-5: Macrolex Blue RR from Lanxess
• G-6: Tinuvin 292 from BASF

Table 1: Composition and properties of the hardenable underfill materials according to the invention. component E1 E2 E3 E4 E5 E6 E7 A-1 Epikote Resin 169 27.1 27.1 27.1 9.1 20 25.4 A-2 jER YX-8000D 27.1 B1-1 Sartomer SR834 18 18 15.39 18 13.3 18 B1-2 Sartomer SR833S 18 C-1 TCC-FP10M 4.3 4.3 4.3 3.73 4.3 3.3 8 D1-1 Dyhard 100SF 3 3 3 3 2.2 3 D1-2 HXA9322 20.99 D2-1 Dyhard UR 500 0.8 0.8 0.8 0.8 0.6 0.8 E1-1 Peroxan BEC 0.5 0.5 0.5 0.5 0.4 0.5 E1-2 Peroxan APO 0.5 E2-1 TPO-L 0.3 F-1 Denka Fused Silica FB-3SDC 46 45 46 49.91 46 44 F-2 Denka Spherical Alumina DAW-03DC 60 G-1 Glymo 0.3 0.3 0.3 0.2 0.3 0.2 0.3 G-2 Jonol 0.1 0.02 G-3 HQMME 0.1 0.02 G-4 Cab-O-sil TS 720 0.3 0.14 G-5 Macrolex Blue RR 0.1 G-6 Tinuvin 292 0.1 CTE [ppm/K], 30-100 °C - TMA 33 33 33 22 33 29 35 Tg [°C] - TMA 140 135 138 135 131 152 159 Viscosity [mPa·s], 10 s ^-1 12,586 18,953 14,636 26,414 8,194 12,869 25,394 Stability - viscosity change [%] 5.8 -5.1 -2.8 15 2.2 -0.3 Flow test [mm], 60 s, 90 °C, 130 µm 20.5 17 24 13 11.5 10 15 DSF [MPa], FR4-FR4, 1 h, 130 °C 30.8 16.1 66.6 81.3 20.5 45.3 34.2 Table 2: Composition and properties of the comparison masses. component V1 V2 V3 V4 V5 V6 V7 A-1 Epikote Resin 169 27.6 30.7 50.4 42.9 23.1 23.1 27.1 A-3 Tactix 742 8 A-4 Epotec YDM-441 8 B1-1 Sartomer SR834 18 18 33 18 18 18 B1-3 HDDA C-1 TCC-FP10M 4.3 8 4.3 C-2 DCC 4.3 D1-1 Dyhard 100SF 3 3.5 5.5 4.75 3.3 3.3 3 D2-1 Dyhard UR 500 0.8 1 1.6 1.25 0.8 0.8 0.8 E1-1 Peroxan BEC 0.5 1 0.5 0.5 0.5 0.5 F-1 Denka Fused Silica FB-3SDC 46 46 46 46 46 46 G-1 Glymo 0.3 0.3 0.5 0.3 0.3 0.3 0.3 CTE [ppm/K], 30-100 °C - TMA 48 32 50 36 35 33 32 Tg [°C] - TMA 102 116 145 133 133 136 113 Viscosity [mPa·s], 10 s ^-1 14,427 6,687 1,487 69,021 12,059 9,970 3,418 Stability - viscosity change [%] 7.4 4.0 -3.4 -5.9 1.5 11.0 -11.0 Flow test [mm], 60 s, 90 °C, 130 µm 23 25 39 12 7 8 25 DSF FR4-FR4, 1 h, 130 °C 33.0 27.7 28.3 73.4 33.6 26.4 17.1

Examples E1 to E7 according to the invention each contain an epoxy (A), a (meth)acrylate (B1), a cyclic carbodiimide (C), a latent nitrogen-based hardener (D1), an initiator component (E) comprising a radical initiator for hot curing (E1), and a filler (F). The liquid masses exhibit good flowability and possess sufficient storage stability over 3 days. In the cured state, the masses achieve a glass transition temperature of over 120 °C and a coefficient of thermal expansion (CTE) of less than 40 ppm/K.

The liquid underfill material in example E1 achieves a flow distance of over 20 mm. The CTE of the hardened material is 33 ppm/K at a glass transition temperature of 140 °C.

Example E2 shows the addition of further additives. Both the liquid and the hardened underfill material meet all technical requirements, albeit with a slightly reduced compressive shear strength.

Examples E3 to E5 show variations of the epoxy (A), the (meth)acrylate (B1), the latent nitrogen-based hardener (D1) and the radical initiator for hot curing (E1).

The curable underfill from example E3 contains an alternative (meth)acrylate component (B1). The cured underfill exhibits a significantly increased compressive shear strength at a flow depth of 24 mm.

Example E4 uses an encapsulated hardener (D1) with an alternative radical initiator for heat curing (E1). The cured underfill material has a particularly low CTE of 22 ppm/K.

The hydrogenated epoxy (A) in Example E6 allows for particularly low-viscosity formulations, and the use of corundum as a filler (F), as shown in Example E7, is also possible. The flow distances of the underfill materials are sufficiently high to ensure reliable flow.

Furthermore, the glass transition temperature of the hardened underfill materials can be increased to 150 °C and more by using corundum as a filler (F) (Example E7) or larger quantities of cyclic carbodiimide (C), as shown in Example E8.

In the comparison masses V1 to V3, individual components essential to the invention are omitted.

Without the cyclic carbodiimide (C) (reference compound V2), the glass transition temperature of the cured reference compound is below 120 °C, so that such underfill compounds are only conditionally suitable for use on electronic components.

The omission of a filler (F) (reference mass V3) results in particularly flowable underfill materials, which, however, exhibit excessive thermal expansion of over 50 ppm/K in the hardened state.

If, as shown in comparison mass V1, no initiator for radical hot curing (E1) is added, both the CTE and the glass transition temperature of the cured underfill materials are outside the ranges desirable for application on electronic components.

If the underfill material does not contain (meth)acrylate (B1), as shown in comparison material V4, this results in increased base viscosity and disadvantages in the underfill's workability. The liquid underfill material still has a sufficient flowability of 12 mm and, in the cured state, exhibits adequate values for CTE and glass transition temperature. However, due to the absence of the (meth)acrylate component (B1), the polymer network formation during heat curing is slow. This leads to sedimentation of fillers and, consequently, to delamination of the underfill (see [reference]). 1 and 2 This is derived from the 1 and 2 clearly, whereby 1 a SEM image of a sample according to example E4 and 2 A SEM image of a sample according to reference material V4 is shown. The sample according to example E4 shows a significantly more homogeneous distribution of the fillers than is the case for reference material V4. Furthermore, the relatively high viscosity of the reference material, combined with its just-adequate flow properties, suggests that a deviation from the specified composition, for example with regard to the type and proportion of the filler or other components, will lead to underfill materials that are not suitable for practical use.

Selected tri- and higher-functionality epoxides (A) can lead to a denser polymer network and increase the glass transition temperature of the cured underfill material, as well as reduce the CTE (reference materials V5 and V6). However, the flowability of the liquid underfill materials is too low for use as underfill. The missing component (C) therefore cannot be compensated for.

The use of a non-cyclic carbodiimide (C), as in comparison mass 7, results in a reduced glass transition temperature, below the required range starting at 120 °C.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• WO 2010/071211 A1 [0007]
• EP 2 948 506 B1 [0010]
• EP 2 380 883 A1 [0102, 0114]
• WO 2023/030 763 A1 [0145]
• US 4 288 527 A [0150]

Claims (14)

A heat-curable backer rod comprising: (A) an epoxy; (B) a radical-curable component comprising a (meth)acrylate (B1); (C) a cyclic carbodiimide, wherein the cyclic carbodiimide has a ring structure comprising a carbodiimide group whose nitrogen atoms are linked together via a linker; (D) a nitrogen-based hardener comprising a latent nitrogen-based hardener (D1); (E) a radical initiator component comprising a radical initiator for heat curing (E1); and (F) a filler. Underfill material according to Claim 1 , wherein the hardened underfill material has a glass transition temperature of 120 °C or higher. Underfill material according to Claim 1 or 2 , wherein the hardened backer rod has a coefficient of thermal expansion of less than 40 ppm/K. Underfill material according to one of the preceding claims, wherein the component (B) is present in the underfill material in a proportion of at least 4 wt.%, based on the total weight of the underfill material. Underfill material according to one of the preceding claims, wherein the component (B1) has a viscosity of less than 1,000 mPa·s. Underfill material according to one of the preceding claims, wherein the component (C) comprises at least a difunctional cyclic carbodiimide. Underfill material according to Claim 6 , wherein the at least difunctional cyclic carbodiimide (C) comprises at least two ring structures, each with a carbodiimide group, wherein the ring structures are linked together. Underfill material according to any of the preceding claims, wherein the filler (F) comprises silicon dioxide and/or aluminium oxide. A backer rod according to any of the preceding claims, wherein the backer rod comprises the following components, each based on the total weight of the backer rod: (A) 5 to 50 wt.% of the epoxy; (B) 4 to 40 wt.% of the radical curing component; (C) 1 to 20 wt.% of the cyclic carbodiimide; (D) 0.5 to 30 wt.% of the nitrogen-based hardener; (E) 0.01 to 15 wt.% of the radical initiator component, comprising the radical initiator for hot curing (E1); (F) 30 to 80 wt.% of the filler; and (G) 0 to 15 wt.% of an additive. A method for bonding a substrate to at least one joining partner using a backing compound according to one of the preceding claims, the method comprising the following steps: a) providing the substrate and placing the at least one joining partner on the substrate, forming a gap between the substrate and the joining partner; b) dispensing the backing compound onto the substrate; c) allowing the joining partner to be drawn under the backing compound by capillary action, the gap being at least partially filled with the backing compound as it flows under; and d) curing the backing compound by applying heat. Procedure according to Claim 10 , whereby the gap is completely filled with the underfill material during the flow process. Procedure according to Claim 10 or 11 , wherein the substrate and/or the at least one joining partner is preheated before being poured underneath. Procedure according to one of the Claims 10 until 12 , wherein the radical initiator (E) comprises a radical photoinitiator (E2), and wherein the backfill material is fixed by actinic radiation after being poured under. Use of a heat-curing underlay compound according to one of the Claims 1 until 9 as underfill.