Epoxy resins are widely used in many important fields, such as the automotive, semiconductor and aerospace industries, as castings, adhesives, composite matrix, electric laminates, electronic encapsulates, insulating materials and high-performance structure composites. However, their applicability is often limited by the presence of residual stresses, especially the interfacial residual stresses, which are always induced during the curing, cooling and other temperature cycles due to shrinkages and mismatch of thermal expansion. These residual stresses could deform the fine gold lines, damage the passivation layers, and allow moisture to permeate to the underlying metals. In the worst case the die or even the epoxy package itself could crack due to stress corrosion. With the increasing in chip integration and the decreasing in chip size in very-large-scale-integrated (VLSI) chip packages, the effects of such residual stresses in epoxy moulding compounds on product reliability has become more and more critical.

Numerous different methods have been used in an attempt to reduce the residual stress in epoxy resins. They include: (a) selecting new epoxy resin structure or modified resins, (b) selecting new curing agents or modifiers, (c) using surface-treated fillers, and (d) incorporation of thermoplastic polymers or rubber particles. These methods work generally by reducing the modulus and/or thermal expansion of epoxy resins, or by dispersing the stress uniformly. None of the methods has, however, been able to reduce the residual stress in epoxy resins without compromising other important properties of the resins, as shown in the following summary of the prior art.

U. S. Pat. 3908040 describes a method of encapsulation using epoxy resins containing discontinuous grafite and/or boron nitride fibers. This encapsulant has a low coefficient of linear thermal expansion and low stress.

Japan Pat. 04325513 and 04120161 describe a method of preparing low stress epoxy resins by blending the resins with modified silicones or silicone rubbers and butadiene-styrene block copolymers. A secondary amine (with an epoxy group-containing siloxane) is used as curing catalyst.

In U. S. Pat. 5284938 and Japan Pat. 02191659,02038413, a polysiloxane-modified epoxy- based encapsulation composition is suggested. One of the following: polysiloxane, soft vinyl silicone polymers, di-methyl siloxane oligomer or polylactone-polysiloxane block copolymer, is used as a modifier in the epoxy composition to provide a low stress encapsulant without significantly sacrificing Tg.

S. U. Pat. 1216191 and Japan Pat. 03066722,63295626,04209648,63273627 describe a method for modification of epoxy resins using a modifier from the following materials: liquid carboxyl-terminated butadiene rubber, a polyaminomaleimide or a liquid carboxyl- terminated 1,2-polybutadiene, trans-1,4-polybutadiene rubber, spiro ortho ester compound, or functional group-terminated polyisoprene and isoprene-styrene copolymer. The modified resins show good moisture-resistant, and have high glass transition temperature and low stress.

In E. Pat. 0302097, a low stress method using core-shell rubber particles is described. The core-shell particles (polymer soft rubbery core with harder polymer shell) are added to epoxy resins to give the compounds having spiral flow of 138.43 cm, gel time 19.5s, and stress (ASTM F-100) 1766.

U. S. Pat. 5418266,5298548 and Japan Pat. 01217030,02032120,02032118 describe a modified epoxy resin composition using a specific phenolic resin, such as the naphthalene ring-containing phenolic resins or phenolic OH-containing silicones. The composition has low modulus of elastisity, low coefficient of thermal expansion and high Tg.

Japan Pat. 01101363,03258852,62210655 and U. S. Pat. 4972008 describe a low thermal stress method for epoxy sealing materials by the incorporation of surface-treated fillers, spherical fillers or low stress agent-treated fillers. As an example, the filler (e. g. fused silica) is first treated with silane, titanate and/or Zr aluminate coupling agents, then treated with low-modulus elastomers (e. g. organopolysiloxanes) with molecular weight >5000.

The sealing materials have low linear expansion and low stress.

U. S. Pat. 5494950 describes an epoxy resin composition containing alkylated hydroquinone epoxide. The epoxy composition, when used as a moulding material, is low in water absorption rate and in flexural modulus (low stress).

Japan Pat. 04202217,05070756 and U. S. Pat. 5082880,5015674 report a method to modify epoxy resins by using epoxy resins of specific structure together with a silicone polymer in an oily state or as particles, or with a special polymaleimide. The resin composition exhibits low thermal expansion, low elastic modulus and low stress.

E. Pat. 263237 and Japan. Pat. 05105739,03119022 describe a low stress method of modification for epoxy resins using (meth) acrylic acid-modified polyfunctional resins, epoxy-terminated silicone oils, silicone oil-modified epoxy resins or an amine-terminated siloxane. When the modified resin is moulded onto an integrated circuit, it shows no cracks after impregnated with solder bath at 260 degree for 10 seconds.

In Japan Pat. 62022823, a method to modify epoxy resins using a crystal nylon 12 is described. The modified epoxy material has low stress and good laser markability.

THE OBJECTIVES OF INVENTION The present invention is developed in view of the above mentioned background in order to overcome the disadvantages associated with those low-stress methods.

It is a primary objective of the present invention to provide a method to reduce the interfacial residual stress in epoxy resins to ultra low level (i. e., to near zero or zero level).

It is a further objective of the present invention to provide a method to reduce the interfacial residual stress in epoxy resins to ultra low level without compromising other important properties of the resins such as the good process flow property, adhesion strength and gelation time, etc.

It is also a further objective of the present invention to provide a method which can be used to reduce the interfacial residual stress in epoxy resins to different levels according to

application requirements.

Other objectives and advantages of the present invention will be apparent from the following description.

SUMMARY OF THE INVENTION The present invention provides a method for reducing the interfacial residual stress in epoxy resins by using an unique type of modifiers. The method is based on a new concept of stress-transfer which is radically different from the principle by which all the other known methods have worked by reducing the modulus and/or thermal expansion of epoxy resins, or by dispersing the stress uniformly.

In this method, the modifiers used must have a continuous or semi-continuous structure (i. e., a non-particle structure) such as sheet, film, plate, etc. The modifiers are prepared from the following materials: metals, ceramic materials or thermoplastic polymers, etc, with or without a surface coating with elastomers.

The method is able to reduce the stress in the resins to an ultra low level, i. e., to a near zero or zero level, without compromising other important properties of the resins such as the good process flow property, adhesion strength and gelation time, etc. All the other methods used so far have not been able to reduce the stress in the resins to such a level.

The method can also be applied in a controllable way, i. e., to reduce the interfacial residual stress in epoxy resins to different levels according to the application requirements.

Although the method is developed primary for reducing residual stress in epoxy resins, it is also useful for reducing residual stress in other polymers, compositions and resins.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The following detailed description of the present invention illustrates the preferred embodiment.

The present invention provides a method to reduce residual stress based on a new concept

of stress-transfer. According to this concept, the residual stress in epoxy resins (or other polymers) can be transferred to a modifier which is placed in contact with the resins (or other polymers). The residual stress in epoxy resins, as a result, can be reduced to different levels or even to zero, depending on the extent of transfer which in turn depends on the nature of the modifier used.

The modifiers used in this method must be in the form of a continuous or semi-continuous structure (i. e., a non-particle structure) such as sheet, film, plate, rod, tube, wire, mesh, etc.

They can be prepared from materials such as ceramic materials, metals, thermoplastic polymers, etc, with or without a surface coating with elastomers.

A detailed preparation of the modifiers is as follows. 50 to 150 parts (by weight) of a ceramic material, metal or thermoplastic polymer is washed and cleaned with 100 to 200 parts of a suitable solvent such as isopropyl alcohol or acetone. For the ceramic material or metal, 100 to 200 parts of a 0.5% surface coupling agent (y-aminopropyl triethoxysilane) solution in toluene is then added, and stirred while refluxing for about 0.5 to 2 hour. The treated material is then, after filtration, washed at least three times with warm acetone, and dried for more than 96 hours at 120°C in a vacuum oven. The treatment with surface coupling agent is not necessary for the thermoplastic polymer. The material is then mixed with 5 to 30 parts of a melting elastomer for surface coating if it is considered necessary.

The stress-transfer modifier is obtained after heating the material, with or without surface coating, in an oven at 180°C for 1 to 8 hours.

The preferred ceramic materials used in the present invention are silica, alumina, boria, aluminium nitride, quartz, silicon carbide, silicon nitride, titanium dioxide, and so on. The ceramic materials must have a continuous or semi-continuous structure such as sheet, fibre, rod, tube, plate, membrane, foil, wire, mesh or foam, etc. They must be dried at 200°C for 3 hours before used.

The metals used here are aluminium (Al), copper (Cu), nickel (Ni), iron (Fe), cobalt (Co), lead (Pb), silicon (Si), Tin (Sn), zinc (Zn) and steel, Fi/Ni alloy, Cu/Zn alloy Fi/Ni/Co alloy etc. The metals must have any of the following continuous or semi-continuous struture such as sheet, fibre, rod, tube, plate, membrane, foil, wire, mesh or foam, etc. They must also be dried at 250°C for 3 hours before used.

The thermoplastic polymers used are polytetrafloroethylene (PTFE), polyimide (PI), polyetherimide (PEI), polyethersulphone (PES), polyamide nylon 6 (PA 6), and so on.

They must also have a continuous or semi-continuous structures such as sheet, fibre, rod, tube, plate, membrane, foil, wire, mesh or foam etc.

The elastomers (rubbers) used for surface coating are selected from the following materials: butadiene acrylonitrile (BN), carboxyl-terminated butadiene acrylonitrile (CTBN), amino-terminated butadiene acrylonitrile (ATBN), poly (dimethylsiloxane) (PDMS), epoxy-terminated poly (n-butylacrylate) (ETPnBA) or a carboxyl-terminated poly (n-butylacrylate) (CTPnBA), etc. They have an average molecular weight in the range of 1,000 to 50,000.

The final heat-treatment of the materials is an important step in the preparation of modifiers. The length of heat-treatment depends on the type of materials used and the level to which the residual stress is to be reduced. Generally, a longer heat-treatment will produce a more effective stress-transfer modifier able to reduce the stress to a lower level.

The stress-transfer modifier obtained from the above preparation process is added to a known epoxy resin composition to produce the desired resin composition, or added to the resin in the final curing process.

The amount of stress-transfer modifier used is generally less than 5% (by weight) of the total resin composition. The effects of modifier on other important properties of the resins will, therefore, be minimum, if there is any.

Although the modifiers of the present invention are developed primary for use in epoxy resins, it is understood that they can also be used to reduce stress in other polymers, compositions and resins.

In order to further illustrate the benefits of the present invention, the following specific examples are provided. It will be understood that these examples are provided for illustrative purposes only, and are not to be construed as limiting the scope of the present invention as herein disclosed and as set forth in the appended claims.

EXAMPLES Example 1 (i) Constituents of stress-transfer modifier Ceramic Metal Thermoplastic Elastomer Material Alumina------CTBN Ratio by weight (%) 100------20 CTBN: carboxyl-terminated butadiene acrylonitrile (ii) Preparation of modifier 100 parts (by weight) of alumina fibre (15mm in length and 0.02mm in diameter) was washed and cleaned using 100 parts of isopropyl alcohol. It was then mixed with 150 parts of a 0.5% surface coupling agent (y-aminopropyl triethoxysilane) solution in toluene, and the mixture was stirred while refluxing for about 1 hour. The alumina was then, after filtration, washed at least three times with warm acetone, and dried for more than 96 hours at 120°C in a vacuum oven. Finally the treated alumina was mixed with 20 parts of a melting CTBN elastomer, and the stress-transfer modifier was obtained after heating the mixture in an oven at 180°C for 3 hours.

(iii) Interfacial residual stress of epoxy resin with the stress-transfer modifier (STM) STM content by weight (%) 0 0.8 2.1 3.3 4.5 5.0 Residual stress (MPa) 1188 1075 769 638 469 500 Epoxy resin: a liquid diglycidyl ether of bisphenol A (DGEBA) Curing agent 4,4'-diaminodiphenylmethane (DDM)

Curing cycle: cured at 90°C/3h, post-cured at 180°C/lh and annealed at 170°C/lh Residual stress: measured by the strain gauge method Example 2 (i) Constituents of stress-transfer modifier Ceramic Metal Thermoplastic Elastomer Material---Cu---PDMS Ratio by weight (%)---100---22 Cu: copper PDMS: polydimethyl siloxane (ii) Preparation of modifier 100 parts (by weight) of copper wire (lOmm in length and 0.3mm in diameter) was washed and cleaned using 100 parts of isopropyl alcohol. It was then mixed with 150 parts of a 0.5% surface coupling agent (y-aminopropyl triethoxysilane) solution in toluene, and the mixture was stirred while refluxing for about 1 hour. The copper was then, after filtration, washed at least three times with warm acetone, and dried for more than 96 hours at 120°C in a vacuum oven. Finally the treated copper was mixed with 22 parts of a melting PDMS elastomer, and the stress-transfer modifier was obtained after heating the mixture in an oven at 180°C for 4 hours.

(iii) Interfacial residual stress of epoxy resin with the stress-transfer modifier (STM) STM content by weight (%) 0 0.4 0.9 2.1 4.3 4.9 Residual stress (MPa) 1188 1150 1125 738 369 313 Epoxy resin: a liquid diglycidyl ether of bisphenol A (DGEBA)

Curing agent: 4,4'-diaminodiphenylmethane (DDM) Curing cycle: cured at 90°C/3h, post-cured at 180°C/lh and annealed at 170°C/lh Residual stress: measured by the strain gauge method Example 3 (i) Constituents of stress-transfer modifier Ceramic Metal Thermoplastic Elastomer Material------PEI PDMS Ratio by weight (%)------100 15 PEI: polyetherimide PDMS: polydimethyl siloxane (ii) Preparation of modifier 100 parts (by weight) of PEI sheet (5mm x 2.5mm x 0.5mm) was washed and cleaned using 150 parts of acetone. It was then mixed with 15 parts of a melting PDMS elastomer, and the stress-transfer modifier was obtained after heating the mixture in an oven at 180°C for 1 to 5 hours.

(iii) Interfacial residual stress of epoxy resin with the stress-transfer modifier (STM) STM content by weight (%) 0 0.6 2.3 3.6 4.0 4.8 Residual stress (MPa) 1188 1113 569 263 250 194 Heat-treatment: 3 hours Epoxy resin: a liquid diglycidyl ether of bisphenol A (DGEBA) Curing agent: 4,4'-diaminodiphenylmethane (DDM) Curing cycle: cured at 90°C/3h, post-cured at 180°C/lh and annealed at 170°C/lh

Residual stress: measured by the strain gauge method (iv) Interfacial residual stress of epoxy resin with the modifier heat-treated for various length of time Heatingtime (h) 1 2 3 4 5 Residual stress (MPa) 847 788 569 313 0 Content (by weight) of stress-transfer modifier was 2.3% Epoxy resin: a liquid diglycidyl ether of bisphenol A (DGEBA) Curing agent: 4,4'-diaminodiphenylmethane (DDM) Curing cycle: cured at 90°C/3h, post-cured at 180°C/lh and annealed at 170°C/lh Residual stress: measured by the strain gauge method Example 4 (i) Constituents of stress-transfer modifier Ceramic Metal Thermoplastic Elastomer Material Silica------CTBN Ratio by weight (%) 100------20 CTBN: carboxyl-terminated butadiene acrylonitrile (ii) Preparation of modifier 100 parts (by weight) of silica sheet (8mm x 5mm x 3.2mm) was washed and cleaned using 100 parts of isopropyl alcohol. It was then mixed with 150 parts of a 0.5% surface coupling agent (y-aminopropyl triethoxysilane) solution in toluene, and the mixture was stirred while refluxing for about 1 hour. The silica was then, after filtration, washed at least three times with warm acetone, and dried for more than 96 hours at 120°C in a vacuum oven.

The treated silica was then mixed with 20 parts of a melting CTBN elastomer, and the stress-transfer modifier was obtained after heating the mixture in an oven at 180°C for 6 hours.

(iii) Interfacial residual stress of epoxy resin with the stress-transfer modifier (STM) STM content by weight (%) 0 1.1 2.5 3.2 4.6 5.0 Residual stress (MPa) 1188 1000 644 406 13 0 Epoxy resin: a liquid diglycidyl ether of bisphenol A (DGEBA) Curing agent: 4,4'-diaminodiphenylmethane (DDM) Curing cycle: cured at 90°C/3h, post-cured at 180°C/lh and annealed at 170°C/lh Residual stress: measured by the strain gauge method Example 5 (i) Constituents of stress-transfer modifier Ceramic Metal Thermoplastic Elastomer Material------PI PDMS Ratio by weight (%)------100 18 PI: polyimide PDMS: polydimethyl siloxane (ii) Preparation of modifier 100 parts (by weight) of PI film (10mm x 5mm x 0.3mm) was washed and cleaned using 150 parts of acetone. It was then mixed with 18 parts of a melting PDMS elastomer, and the stress-transfer modifier was obtained after heating the mixture in an oven at 180°C for 5 hours.

(iii) Interfacial residual stress of epoxy resin with the stress-transfer modifier (STM) STM content by weight (%) 0 0.4 1.8 2.7 3.2 4.9 Residual stress (MPa) 1188 1094 594 256 206 0 Epoxy resin: a liquid diglycidyl ether of bisphenol A (DGEBA) Curing agent: 4,4'-diaminodiphenylmethane (DDM) Curing cycle: cured at 90°C/3h, post-cured at 180°C/lh and annealed at 170°C/lh Residual stress: measured by the strain gauge method