EP0865653A2

EP0865653A2 - Electrically conducting reaction resin mixture

Info

Publication number: EP0865653A2
Application number: EP96946140A
Authority: EP
Inventors: Heiner Bayer; Wilhelm Hekele; Erich Kattner
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1995-12-06
Filing date: 1996-12-04
Publication date: 1998-09-23
Also published as: JP2000501551A; WO1997021229A3; WO1997021229A2

Abstract

To create electrically conducting adhesive compounds and electrically conducting structures, an epoxy-based reaction resin mixture is proposed which can be hardened by a selected initiator system, UV radiation or heat. The single-component reaction resin mixture which stores well can be applied by, for example, screen printing, hardened rapidly with UV light and fully hardened with heat.

Description

description

Electrically conductive reaction resin mixture.

Reaction resins filled with electrically conductive particles are increasingly being used in electronic technology to produce electrically conductive connections. They are used for chip assembly, with semiconductor chips being glued onto the back in an electrically conductive manner, for bonding, for producing conductor tracks on rigid and flexible substrates and increasingly also for assembling components as a replacement or alternative to soldering.

Reactive resin-based conductive adhesives with chemical systems based on epoxies, silicones and acrylates are available on the market. In the case of thermally hardening conductive adhesives, however, there is a conflict for the user between the possible service life and the time required for the hardening. Such conductive adhesives either have a too short useful life or a too long thermal curing time, which inadmissibly increases the throughput time in production and processing processes. The user of the commercially available conductive adhesives is usually given the choice of using either two-component systems or only one-component systems which can be stored in a refrigerated state. While the two-component systems require an increased mixing effort and have a limited service life, with the resins that can only be stored in a refrigerated state, increased logistical effort and uncertainty during transport and storage have to be accepted. In addition, the cooled products usually also require a large increase in temperature for curing. However, this is usually not desired and, in the case of materials with different coefficients of expansion, causes different thermal expansions which arise during heating and lead to stresses after cooling. These tensions can affect the durability of the adhesive bonds or the service life. he shorten the components manufactured or fastened by the gluing or in particular also impair the function of sensitive components.

It has therefore already been proposed to use UV-curing, electrically conductive reactive resins. These should enable rapid hardening at moderate temperatures and, as one-component systems, enable simple and safe production as well as high storage stability.

From US 3 968 056 electrically conductive inks based on polyester, acrylates or epoxides are known. The photosensitivity is guaranteed by radical or cationic photoinitiators. Silver-coated glass spheres are included to provide electrical conductivity. As an application, it is proposed to use this conductive mixture to produce conductive layers, in particular on printed circuit boards, the reaction resin layers to be cured being irradiated over the entire surface. These reactive resins are not suitable for bondings in which no full-area irradiation is possible. Because of the shadowing caused by the parts to be glued, they cannot be completely hardened.

Additional problems with electrically conductive reactive resins arise from the conductive particles themselves, which make the resins non-transparent due to their metallic nature and thus greatly reduce the penetration depth of the light required for curing. However, electrically conductive reaction resin systems which, in addition to their UV curability, can also be thermally cured at usable temperatures are not known.

The problem of the present invention is therefore to provide a UV-curable reactive resin system which is simple and safe to process and which is sufficiently short Time leads to completely hardened and electrically conductive resin layers, which can be used as an adhesive and which can be produced in a purity sufficient for microelectronic applications.

This problem is solved according to the invention with a reaction mixture according to claim 1. Further embodiments of the invention and preferred uses of the reaction resin mixture can be found in the subclaims.

The reaction resin mixture according to the invention is a one-component system which can be stored for several months at room temperature. Neither use properties such as viscosity or curing profile nor the final properties of the cured resin, such as for example, change during the storage period

Conductivity or liability. The reaction resin mixtures can only be cured by UV and additionally have a thermal curing process which leads to fully cured, electrically conductive resins at moderate temperatures of 80 to 150 ° C. With suitable electrically conductive particles as fillers, sufficient electrical conductivity is obtained which corresponds to a low specific resistance value of, for example, 10 ^~ ohm x cm. Without any sedimentation of the conductive particles being observed, the reactive resin mixture according to the invention can be used to produce electrically conductive layers up to a thickness of, for example, 50 μm. However, even thin adhesive joints from a thickness of approx. 2 μm can be easily cured with UV and fully cured thermally.

The electrically conductive layers or structures produced with the reaction resin mixture show good adhesion to metals, semiconductor surfaces or other organic and inorganic layers. Electrically conductive adhesive connections produced with the reactive resin mixture have a high shear strength of, for example, 10 to 20 N / mm ² . Due to the fast and at any temperature bare UV curing, it is possible to perform demanding adhesive tasks with the reactive resin mixture, which for example require exact and quick adjustment or a high dimensional stability of the applied adhesive layer during the curing process. Due to the thermal

Post-curing can also produce large-area electrically conductive bonds which are only accessible to UV radiation at the edge of the adhesive joint. Due to the UV curing, the adhesive points are fixed in such a way that during thermal post-curing there is no running of the reaction resin layer and no slipping of the fixed parts to be bonded.

The components of the reaction resin mixture can be selected so that they are low in corrosion-promoting ingredients such as free chlorides. They are therefore well suited for electronic applications.

The reaction resin mixture according to the invention has a resin matrix with components A to E in the following proportions:

A) 90 to 40 percent by weight of a cationically curable solvent-free epoxy resin

B) 10 to 60 percent by weight of a compound containing hydroxyl groups

00.01 to 5 weight percent of a latent thermal initiator for cationic epoxy polymerization

D) 0.01 to 5 percent by weight of a thiolanium salt with a non-nucleophilic complex anion

E) optional additives, if appropriate.

To induce the electrical conductivity are in the

Reaction resin mixture additionally as component F) 10 to 40 Volume percent, preferably 15 to 30 volume percent contain electrically conductive particles.

Cycloaliphatic epoxides are preferred for component A). These are largely harmless with regard to environmental compatibility and occupational safety. They can be represented in a highly pure and therefore low in corrosion-promoting ingredients. Commercial cycloaliphatic epoxides also include mixtures of several different cycloaliphatic epoxides. Suitable compounds obey, for example, the following structural formulas

However, it is also possible to use different epoxy resins or components containing epoxy groups. Other suitable epoxy resins are, for example, the glycidyl ethers and glycidyl esters widely used in industry. Suitable diluents are, for example, epoxidized olefins and polyolefins.

Component B) contains a compound containing hydroxyl groups, preferably a polyhydric, aliphatic or cycloaliphatic alcohol. For example, glycols and other aliphatic diols, trifunctional or tetrafunctional alcohols such as trimethylolpropane or the ethers of glycols with phenols or bisphenols can be used. More Suitable compounds are polymer polyols which are used in the production of polyurethanes. The polyols offered by Dow Chemical under the names NIAX ® and TONE ® may serve as examples.

However, it is also possible to use as component B) those hydroxyl-containing compounds which are obtained by reacting epoxy compounds with alcohols or phenols. The addition product of a cycloaliphatic is, for example, very suitable as component B)

Epoxy with a polyphenol, preferably with a bisphenol such as bisphenol A. Such addition products can be obtained under base conditions under mild conditions. A preferred addition product is the 2: 1 adduct, in which one equivalent of phenolic groups is reacted with two equivalents of epoxy groups. Starting from a diepoxide and a bisphenol, the 2: 1 adduct is also a diepoxide. The reaction mixture has practically no free phenolic OH groups if the reaction leading to the addition product is carried out in a molar ratio greater than 2: 1 to 20: 1 and preferably from 3: 1 to 10: 1. In particular with a higher proportion of cycloaliphatic epoxide, an increasing proportion of unreacted and therefore unchanged cycloaliphatic epoxide remains after the reaction in addition to the addition product. If this molar ratio is chosen to be sufficiently high, the addition product may contain notable proportions of cycloaliphatic epoxide and thus unreacted epoxides corresponding to component A).

To induce cationic curing, a cationic photoinitiator or a cationic photoinitiator system is contained as component C).

When UV irradiation occurs, these photoinitiators release reactive cations, for example protons, which initiate the cationic curing process of the epoxy resin. The photoinitiators are derived from stable organic onium salts, especially with nitrogen, phosphorus, oxygen, sulfur, selenium or iodine as the central atom of the cation. Aromatic sulfonium and iodonium salts with complex anions have proven to be particularly advantageous. A photoinitiator which releases a Lewis acid and is implemented, for example, as a pi-donor transition metal complex is also possible. Phenacylsulfonium salts, hydroxyphenylsulfonium salts and sulfoxonium salts should also be mentioned. Onium salts can also be used, which are not stimulated directly by acid sensitization but rather by a sensitizer. Organic silicon compounds which release a silicon on exposure to UV radiation in the presence of aluminum-organic compounds can also be used as photoinitiators for the cationic curing process.

The following sulfonium salt, for example, is highly suitable as a photoinitiator. It is a main component of Cyracure ® UVI 6974 (Union Carbide):

A thiolanium salt is present as component D), which acts as a thermally activatable initiator. Benzylthiolanium salts of the general structure are preferred:

where Rl is hydrogen, alkyl, aryl, alkoxy, thioether, halogen, CN or N0 ₂ . R2 is hydrogen, alkyl or aryl; R3 is hydrogen, alkyl or aryl or an aromatic system condensed to the thiolane ring; X ^~ = PF ₆ ^~ , AsF 6 "or Sb F ^~

Unsubstituted benzylthiolanium salts are preferably used, in particular benzylthiolaniumhexafluoroantimonate.

Components E) which may contain customary additives, such as leveling agents, adhesion promoters, thixotropic agents, etc., are optionally present.

Component F) preferably contains metallic or at least metallic coated fillers. Well-suited fillers are, for example, silver flakes or silver powder. Metal-coated fillers, such as glass spheres coated with silver, are suitable to a limited extent. With the fillers mentioned, the desired high electrical conductivity is already obtained with a filler content of component F) which is far below the maximum filler content at which the reaction resin mixture can still be processed reasonably. The percolation threshold, ie the filler content, above which a rapid increase in electrical conductivity is observed, is approximately 15 to 20 percent by volume for the specified electrically conductive particles. The reaction resin mixture according to the invention has a particle content above the percolation threshold, which can be achieved for different particles with different filler contents.

The type and external shape of the electrically conductive particles are in principle of less importance than expected with regard to their suitability for the reactive resin mixture according to the invention. The size and the shape of the filler particles influence the layer thickness that can be produced with the resin, but not the hardening properties. For example, flakes show little sedimentation, that is to say more flat and flake-shaped particles, while with powdery and more compact particles, comparable conductivity can only be achieved with a higher filler content.

Not trivial and of decisive influence on the hardness

However, the surface properties of the filler particles, which the manufacturer has already provided with various unspecified coatings, e.g. with soaps. Such coatings can be residues of solvents used in the manufacture or intentionally

■ • -5 applied coatings to support free-flowing properties or to protect against oxidation. In preliminary experiments, the inventors were able to show that the type of this coating has a significant influence on both thermal and UV-cationic curing properties of the reaction compositions according to the invention.

Have 20 mix. This interaction, which has not yet been elucidated, has the effect that different fillers (from different manufacturers) which, according to their external appearance, e.g. are extremely similar in size and shape, and do not influence the hardenability of the reaction resin matrix at all

25 and another time lead to complete inhibition of the hardening process.

FIG. 1 shows this finding on the basis of DSC measurements of two reaction resin mixtures. With the same resin matrix

30 shows the mixture A with a first filler having good thermal curability, which can be seen on the steep flank of the peak in the region of the curing temperature. The DSC curve is comparable to the spectrum of the pure resin matrix. Mix B with equal proportions of a second

35 filler but otherwise unchanged reaction resin matrix has a thermally almost completely inhibited hardening, which can be seen from the absence of a hardening peak in the region of the hardening temperature.

This inhibition has apparently hitherto led to the fact that no usable UV-curable epoxy resin-based reactive resins with electrical conductivity could be produced. The invention now shows that with the initiator components C) and D) with a suitable choice of the electrically conductive ¬ against particles, an electrically conductive and UV-curable reaction resin with the desired properties already mentioned can be obtained.

The invention is explained in more detail below with reference to exemplary embodiments and the associated two figures.

Figure 1 shows the hardening behavior of two already mentioned

Reaction resin mixtures depending on the chosen one

Filler based on a DSC diagram.

FIG. 2 shows an adhesive connection produced with the reaction resin mixture according to the invention in a schematic cross section.

FIG. 3 shows a multilayer conductor track structure which is also produced using the reaction resin mixture according to the invention.

Preliminary tests: selection of suitable electrically conductive particles.

To test different commercially available conductive particles for the reactive resin mixtures according to the invention, resin components A to D according to the invention are first used homogeneously mixed together. The constituents of the resin matrix VO thus obtained are given in Table 1.

Table 1:

The resin matrix VO is now mixed in seven preliminary tests, each with one of seven different fillers. 70 percent by weight of filler are chosen. Table 2 shows the fillers examined.

Table 2

Trial name Filler type Supplier suitability

VI PUAG307 Ag-Flakes Metalor, CH yes

V2 IGV powder Ag powder IFAM, Bremen yes

V3 D25 Ag-Flakes due to Demetron

V4 Bailotini Ag-coated Potters- S3000-S3E glass balls Ballotini

V5 Metalite Ag-coated PQ-Corp. USA requires Silver SF20 glass balls

V6 PUAG250 Ag powder Metalor, CH no

V7 AgClad TW Ag-coated PQ-Corp. USA no thick-walled

Hollow spheres

Here: ADOH120 is the addition product of a cycloaliphatic epoxide CY177 and bisphenol A (Merck) in a ratio of 10: 2; CY177: cycloaliphatic epoxy; Cyracure UVI6974: cationic photoinitiator, triarylsulfonium salt.

With the mixtures according to preliminary experiments VI and V7, two experiments are carried out for thermal and for UV curing. To study UV curing, a flat layer of the corresponding preliminary test mixture is irradiated with a UV lamp for a period of 10 to 120 seconds. The adhesive layer is then checked for complete hardening, for example with a grindometer, from which a hardened skin can only be removed until the hardened layer reaches the surface. To check the thermal hardenability, a DSC measurement is carried out at a heating rate of 10 ° C. per minute. Samples hardened in a convection oven are checked for residual reaction with DSC. It shows that only the fillers according to preliminary experiments VI to V5 are suitable for the reaction resin mixture according to the invention, V3 to V5 being only suitable to a limited extent. VI and V2 show good curability both under UV and thermally. Hardened layers of adhesive have good electrical conductivity. With V3, only a small penetration depth of the UV radiation is observed, so that in the case of thicker layers only skin formation is achieved by pure UV curing. Mixture V3 is therefore not suitable for UV curing thick layers. The mixtures according to V4 and V5 can be hardened well, but then have only a low electrical conductivity, which makes them unsuitable for various applications. The mixtures according to preliminary tests V6 and V7 show a massive inhibition of thermal curing, so that they are not suitable for the process according to the invention.

Execution examples:

According to Table 3, two ready-to-use reaction resin mixtures RH1 and RH2 are produced by homogeneously mixing the components shown in Table 3.

Table 3

Product Manufacturer Components RH1 RH2

-Assignment by weight- parts by weight

ADOH120 Siemens B 29.8 28.0

CY177 Ciba-Geigy A 59.4 60.0

TCD alcohol Hoechst B 7.5 7.5

Silane A187 UCC E 0.3

BYK A500 BYK-Chemie E 0.4

Cyracure UCC C 3.0 1.0

UVI6974

Benzylthio-Aldrich D 1.0 0.75 lanium hexafluoroanti- month

PUAG307 Metalor F 225.0

D25 Demetron F 208.0

The silane A187 is an adhesion promoter, BYK A500 a defoamer and TCD alcohol a cycloaliphatic alcohol.

The reaction resin mixture RH1 is used for the production of conductor tracks. Modified stencil printing is used to print conductor tracks with a cross section of 1.0 × 0.06 mm 2 and a length of 2 cm onto an insulating substrate (for example glass, aluminum oxide ceramic or polystyrene). The curing takes place by UV radiation (for example 1.5 min with 50 mW / cm ² ) and / or by increasing the temperature (for example 30 to 90 minutes to 120 to 150 ° C.). Shorter UV curing is also possible with thinner layers.

After hardening, the conductor tracks have a resistance of 1.2 ohms, over which a specific resistance of 4 x 10 ^~ 4 ohms x cm calculated. Further properties are a glass transition temperature of approx. 80 ° C, an expansion coefficient of 100 ppm below Tg and an elastic modulus of 3900 N / mm ² .

Eε shows itself as a major advantage that the printed structures remain dimensionally stable in the production process. The resin itself is stable to viscosity in screen or stencil printing. As a preferred application, this enables the production of multilayer metallizations by multilayer printing.

FIG. 3: Resin structures of a first conductor track level L 1 are printed on the electrically insulating substrate S and hardened by means of UV. In the next step, an insulation layer I is printed, for which a reaction resin with the same reaction resin matrix is used, but without the electrically conductive particles (without component F). The insulation layer I can be applied over the entire surface or in a structured manner in such a way that regions of the first conductor track level L1 which are provided for contacting the second conductor track level L2 remain uncovered. The insulation layer I is also hardened with UV. Then conductor track structures L2 of the second conductor track level are printed on and hardened with UV. In this case, the electrical contact between the first and second interconnect levels is produced in the areas not covered by the insulation layer I. Finally, a further insulation layer can be applied over the entire surface (not shown in the figure). The multilayered layer stack can then be cured together in a thermal process.

UV-curable resins for insulation layers are commercially available and can be used instead of the resin matrix according to the invention. The reaction resin mixture RH2 is tested for fastening semiconductor chips. After curing, it has a specific resistance of less than 6 x 10 ^" ohms x cm, the resistance being measured on printed strips.

With a chip with an area of approx. 8 mm2 one finds

Transfer resistances of approx. 1 m Ω.

Figure 2 shows a semiconductor chip C, which was glued with the aid of an adhesive layer KS with its metallized back to a substrate S '. The adhesive layer KS is applied in such a way that after the semiconductor chip C has been placed on its edge or on portions of the edge, a bead remains which is readily accessible to UV radiation. At the adhesive joint or at the bead, UV radiation is used for hardening and the entire system is thermally hardened. Adhesive joints between 2 and 50 μm thick can be realized in this way.

If required, further additives such as deaerators, flow control agents, adhesion promoters, thixotropic agents are added to the resin systems, these additives being able to vary depending on the desired application.

Further applications for the reactive resin mixture according to the invention can generally be seen where high conductivity is required. It is particularly suitable as an adhesive application for fastening chips (the bonds), for fastening components on printed circuit boards and hybrids, for producing printed conductors on printed circuit boards or ceramics and in particular for producing complex conductive geometries. It is applied using planar technology, preferably with screen printing or stamp printing or by dispenser application, for example with a robot-guided dispensing needle.

Claims

claims

1. Electrically conductive reactive resin mixture with thermal and UV initiated cationic curing and the following resin components:

A) 90 to 40 percent by weight of a cationically curable, solvent-free epoxy resin

B) 10 to 60 percent by weight of a compound containing hydroxyl groups

C) 0.01 to 5 percent by weight of a cationic photo initiator

D) 0.01 to 5 percent by weight of a latent thermal initiator for the cationic epoxy polymerization

E) optional additives, if appropriate,

and, based on the entire reaction resin mixture:

F) 10 to 40% by volume of electrically conductive particles as filler.

2. Reaction resin mixture according to claim 1, in which component A) is a cycloaliphatic epoxide.

3. Reaction resin mixture according to claim 1 or 2, in which component B) is a reaction product of an epoxy and a preferably polyhydric phenol.

4. Reaction resin mixture according to one of claims 1 to 3, wherein the latent thermal initiator is a thiolanium salt with a non-nucleophilic complex anion.

5. Reaction resin mixture according to one of claims 1 to 4, in which component E) is a metallic or at least metallic coated filler.

6. Reaction resin mixture according to one of claims 1 to 5, in which component E) comprises silver flakes or silver powder.

7. A method for producing an electrically conductive layer or structure with a reaction resin mixture according to claim 1,

in which the reaction resin mixture is printed onto a substrate as a whole-surface layer or in the form of a first pattern,

- in which the layer or the pattern is hardened by means of UV radiation, and

- in which the layer or the pattern is thermally cured, if necessary.

8. The method according to claim 7, in which a structured insulation layer I is applied over the first pattern and a further pattern of the electrically conductive reactive resin mixture is applied over it, two interconnect levels being obtained which are partially contacted.

9. The method according to claim 8, in which the same reaction resin mixture as for the pattern, but without electrically conductive particles (component F), is used to produce the insulation layer.

10. Use of the method according to claim 7 for generating electrically conductive bonds, in particular in the case of electronic and microelectronic components.

11. Use of the method according to claim 7 for generating electrically conductive bonds with adhesive joints between 3 and 60 microns thick.

12. Use according to claim 11, in which a component to be bonded is placed on the reaction resin layer in such a way that an adhesive bead is formed, in which this bead is hardened with UV radiation and in which the entire reaction resin layer is then thermally cured.