Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-09-27
2004-05-25
Sellers, Robert (Department: 1712)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C523S458000, C525S399000, C525S407000, C525S410000, C525S423000, C525S438000, C525S454000, C525S463000
Reexamination Certificate
active
06740192
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to novel electrically conductive adhesives for solder replacement in electronics packaging applications. More specifically, the present invention relates to novel electrically conductive adhesives which are fabricated using epoxide-modified polyurethane (EPU) resins and show superior impact performance and very stable contact resistance with non-noble metal finished components.
2. Description of the Related Art
Soldering processes with tin/lead solders (Sn/Pb) are standard interconnection technologies of electronic components on printed circuit boards (PCBs). The most common reflow soldering process for the surface mount technology (SMT) uses tin/lead solder pastes. However, because of the lead in the tin/lead solder pastes, this procedure is considered to be environmentally harmful. Lead is well-known hazard to human health. Even small quantities can damage the brain, nervous system, liver, and kidneys when ingested. When Sn/Pb solders are disposed in landfills, lead can leach into soils and pollute ground water. Accordingly, pressure to remove or minimize the use of lead is steadily building. Most European communities, in fact, have proposed a ban on the landfill disposal of electronic products containing leaded printed circuit boards, as well as on the sale of products containing the metal. In the United States, consumer electronics are identified as the second largest source of lead (30 percent) in the municipal solid waste stream, after lead-acid batteries (65 percent) which are already being separated from t prior to disposal. Therefore, lead-free and environmentally sound interconnect bonding processes are urgently needed. Among the possibilities are electrically conductive adhesives.
A great deal of effort has been made to use electrically conductive adhesives to replace conventional tin/lead solders in both rigid and flexible substrate applications in order to reduce the adverse effects of lead and the organic solvents which are used to clean the flux. Compared to traditional lead-bearing solders, electrically conductive adhesives offer many advantages: low processing temperature, fine pitch capability, lower sensitivity to thermomechanical stresses, environmentally friendly, and simple processing. Even though electrically conductive adhesives do not have the conductivity of metals and solders, their conductivity is adequate for many electrical circuits. However, limitations and concerns do exist.
Surface mount electronic packages are subjected to significant shocks during assembly, handling and throughout product life. A package can not survive such an impact without desirable impact performance. Therefore, impact strength is one of critical properties of electrically conductive adhesives. Existing conductive adhesives typically have poor impact performance. When they are used in surface mount technology, the surface mount components fall apart from the substrate when the package experiences a sudden impact.
In 1996, the National Center of Manufacturing and Science (NCMS) evaluated 25 commercial electrically conductive adhesives as solder replacement in terms of contact resistance and impact strength. It found that none of the commercial conductive adhesives tested have adequate impact strength. Therefore, there exists a need in the art for the development of new conductive adhesives with desirable impact performance.
Polyurethane materials exhibit high toughness and good adhesion, but difficulties in working with polyurethane resins has rendered their use impractical. Therefore, there exists a need in the art for a polymeric material having high toughness and good adhesion, while still being practical for use in polyurethane adhesives.
U.S. Pat. No. 3,624,178 to Allscheil, et al., discloses epoxide-modified polyurethanes for use as coating agents, laminates, paints, adhesives and as insulating compounds for the electrical industry. The reference does not suggest using the polyurethanes in electrically conductive adhesives.
Accordingly, it is an object of the present invention to provide electrically conductive adhesives with superior impact performance and stable electrical contact resistance with non-noble metals.
It is another object of the present invention to provide electrically conductive adhesives with superior impact performance and stable electrical contact resistance with non-noble metals comprising an epoxide-modified polyurethane resin.
SUMMARY OF THE INVENTION
The present invention relates to novel electrically conductive adhesives made with epoxide-modified polyurethane resins. The novel adhesives comprise (a) a novel epoxide-modified polyurethane resin; (b) a cross-linking agent; (c) an adhesion promoter; and (d) a conductive filler. The electrically conductive adhesive is applied to at least one electrically conductive material(s) which is joined to a substrate and cured. The epoxide-modified polyurethanes useful in the electrically conductive adhesives of the present invention have a structure selected from:
where m is 2 or 3; n is one or greater, R
1
is a substituted or unsubstituted aliphatic hydrocarbon radical, a substituted or unsubstituted cycloaliphatic hydrocarbon radical, a substituted or unsubstituted aromatic hydrocarbon radical, or a substituted or unsubstituted araliphatic hydrocarbon radical; R
2
is a substituted or unsubstituted aliphatic hydrocarbon radical, a substituted or unsubstituted cycloaliphatic hydrocarbon radical, a substituted or unsubstituted alkoxy radical, a substituted or unsubstituted polyester; or a substituted or unsubstituted polyether; R
4
is either:
R
3
is a substituted or unsubstituted aliphatic hydrocarbon radical, a substituted or unsubstituted cycloaliphatic hydrocarbon radical, a substituted or unsubstituted alkoxy radical, a substituted or unsubstituted polyester, or a substituted or unsubstituted polyether; and X
1
, X′
1
and X
2
are either a single bond, —O—; —COO—; —NH—; or —S—.
The cross-linking agent is any compound suitable for hardening the composition, but is preferably a carboxylic acid anhydride cross-linker.
The adhesion promoter is a material which promotes the adhesion between the substrate and the adhesive. Exemplary adhesion promoters include organofinctional silane adhesion promoters. The adhesion promoter material is present in the electrically conductive adhesive composition in an amount between 0.02 to 10 weight percent. Preferably the composition contains between 0.1 to 2 weight percent.
The conductive filler is any solid powder which has a high electrical conductivity. The fillers include, but are not limited to silver, nickel, copper, aluminum, palladium, platinum, gold and other alloys, or carbon black, carbon fiber and graphite. The filler can be in any form capable of being incorporated into the adhesive, including powder or flake form. The preferred powder is silver flakes. The conductive adhesive composition of the present invention comprises 5 to 95 weight percent of a conductive filler. Preferably, the composition comprises 50 to 80 percent of metal fillers.
The adhesive of the present invention may optionally include one or more of (e) an epoxy resin; (f) a catalyst; and (g) a diluent.
Epoxy resins include, but are not limited to, bisphenol A, bisphenol F, cycloaliphatic, biphenyl, naphthalene and novolac type epoxies. A preferable epoxy resin is bisphenol F. The epoxy resin is preferably present in the adhesive composition from about 0 to 80 weight percent.
Exemplary catalysts include, but are not limited to, imidazoles, tertiary amines and ureas. The formulation of the present invention further may contain from 0 to 10 weight percent, preferably 0 to 2 weight percent of a catalyst.
A reactive or nonreactive diluent may be used for some formulations with high viscosity. Examples of reactive diluents include, but are not limited to, glycidyl ethers. The composition of the present invention may contain between 0 to 50 weight percent of a diluent.
DETAILED DESCRIPTION OF THE INVENTION
1. D
Lu Daoqiang
Wong Ching-Ping
Deveau Todd
Georgia Tech Research Corp.
Sellers Robert
Thomas Kayden Horstemeyer & Risley LLP
Vorndran Charles
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