Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2002-08-20
2004-10-05
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
Reexamination Certificate
active
06800223
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to thermosetting electroconductive paste suitable for formation of conductive bumps at predetermined locations on a printed circuit layer for the purpose of making a connection by throughhole-type conductive bumps between circuit layers. It is especially useful in printed circuit boards or multi-layer printed circuit boards.
BACKGROUND OF THE INVENTION
A simple manufacturing method for printed circuit boards is disclosed in Japanese Patent Application Disclosure (Kokai) H6-342977A (1994), wherein substantially-conical electroconductive bumps connect upper and lower circuit layers constituting a structure on the printed circuit board. According to this method, conical-like electroconductive bumps are formed by a printing method on electroconductive metallic foil with an electroconductive composition comprising a mixture of electroconductive powder with thermosetting binder resin or thermoplastic resin by penetrating the insulation sheet comprising a synthetic resin sheet, superposing electroconductive metallic foil and compressing to laminate the layers as one unit (laminate body is compressed without modification, or it is heated and compressed), the tip of the bump undergoes plastic deformation and has satisfactory adhesion and bonding with opposite electroconductive metallic foil with which it is in contact, thus enabling formation of an electrical connection between the upper and lower circuit layers. Consequently, with respect to the bump, after compression of the laminate layers, a highly reliable electrical contact and strong adhesive strength with the electroconductive metallic foil are required. The electroconductive composition is prepared by mixing electroconductive powder, e.g. gold, silver, copper, solder powder, alloy powder, or mixtures thereof, with a thermosetting binder, e.g. polycarbonate resin, polysulfone resin, polyester resin, phenoxy resin, phenolic resin, and polyimide resin.
An improved electroconductive paste composition for electroconductive bump use is sought which would exhibit a highly reliable electrical contact and a strong adhesive strength with an electroconductive metallic foil. This invention fills such a need.
SUMMARY OF THE INVENTION
The invention is directed to a thermosetting electroconductive paste for forming electroconductive bumps at predetermined locations on at least one circuit layer that is laminated to an insulating layer wherein upon lamination the electroconductive bumps penetrate the insulating layer forming an electrical connection to a second circuit layer wherein the paste comprises, based on total composition, 80 to 90 wt % electroconductive powders comprising at least a first and second electroconductive metal powder of which packing densities are in the range of 20% or less of the average density (sp. gr.) of metal for the first powder and 20 to 40% of the average density (sp. gr.) of metal for the second powder; and 10 to 20 wt % epoxy resin, curing agent, and solvent.
The invention is further directed to a thermosetting electroconductive paste, wherein the electroconductive powders described above comprise 25 to 75 wt % flake silver powders having a packing density of 3 to 3.5 g/ml, and an average particle size of 2~4.5 &mgr;m and 25~75 wt % silver powders being aggregate of spherical silver particles, having packing density of 0.7~1.7 g/ml and average particle size of 1.5~3.5 &mgr;m.
The invention is still further directed to a thermosetting electroconductive paste as described above, wherein its viscosity measured by a Brookfield viscometer at rotation of 0.5 rpm and 25° C. is in the range of 2200~3200 Pa.s.
DETAILED DESCRIPTION OF THE INVENTION
The objective of this invention is to provide a thermosetting electroconductive paste for electroconductive bump use which is able to form bumps possessing a very reliable electrical contact and a very strong adhesive strength to an electroconductive metallic foil. The thermosetting electroconductive paste used for the bump formation is suitable for a method of electrically connecting upper and lower circuit layers and forming a throughhole-type conductive member connection between the upper and lower circuit layers by forming substantially conical electroconductive bumps by a printing method on the electroconductive metallic foil. For example, this is achieved by using a penetrating insulation sheet comprising a synthetic resin sheet, further superposing an electroconductive metallic foil with electroconductive bumps formed thereon and compressing the sheets, usually by lamination, as a unit.
In order to satisfy the required properties, a thermosetting electroconductive paste was developed for electroconductive bumps for penetrating the insulating layers separated by a predetermined distance in a printed circuit board. The thick film composition contains about 80 to about 90 wt % electroconductive component comprising at least two kinds of electroconductive powder comprising a first electroconductive powder having a packing density of not more than 20% with respect to the average density (sp.gr.) and a second powder having a packing density in the range of about 20 to about 40% with respect to the average density (sp.gr.), wherein the remainder of components is at least a epoxy resin, curing agent, and solvent. A connection is provided by the substantially conical electroconductive bumps forming conductive member connections by penetrating the circuit layers constituting the structure of the printed circuit board. The bump, after penetration, is able to satisfy the requisites of highly reliable electrical contact with a very strong adhesive strength with the electroconductive metallic foil. This is accomplished by providing easier deformation of the electroconductive bumps at the time of compressing the layers during lamination to form one unit so that the contact surface area between the substantially conical electroconductive bump and the electroconductive metallic foil is increased.
In general, it is possible to use known electroconductive powders, such as gold, platinum, palladium, silver, copper, aluminum, nickel, tungsten, or alloy powder containing powder. Gold, platinum, and palladium powders are expensive. Copper and nickel powders have concerns regarding increase in surface resistance of the conductive powder by surface oxidation and the temperature dependence of the process increases because of the low melting point. For these reasons, use of silver powder in this invention is most desirable. It is possible to use silver powder of different shapes such as spherical (hereinafter termed aggregate powder) and flake. Such as an embodiment may comprise a combination of 25~75 wt % powder in flake form having packing density of 3~3.5 g/ml and average particle size of 2~4.5 &mgr;m, and 25~75 wt % aggregate powder having average particle size of 1.5~3.5 &mgr;m, packing density of 0.7~1.7 g/ml, being an aggregate of fine substantially-spherical particles. The symbol “~” herein means “to about”.
Epoxy resin is preferred as a thermosetting binder component of the composition. Examples include bisphenol A epoxy resin, phenol novolak-type epoxy resin, and cresol novolak-type epoxy resin. Resins other than epoxy resins can be suitably blended therein according to need.
Curing agents can be used with no particular restrictions. Preferable curing agents include, for example, amine curing agents such as dicyandiamine, carboxylhydrazide, imidazole curing agents such as heptadecylimidazole, latent curing agents represented by acid anhydride, and modified phenolic resin.
Suggested solvents are, for example, aliphatic alcohol, e.g. ethanol, i-propanol, n-propanol, butanol, esters thereof, e.g. acetate, propionate and the like, Carbitol® solvent,,e.g. methyl carbitol, ethyl carbitol, butyl carbitol, butyl carbitol acetate and the like, cellosolve solvent, e.g. cellosolve, butyl cellosolve, isoamyl cellosolve, hexyl cellosolve, butyl cellosolve acetate and the like, ketone solvent, e.g. acetone, methyl ethyl ketone, 2-pentanone, 3
Kojo Hiroki
Matsuno Hisashi
E. I. du Pont de Nemours and Company
Kopec Mark
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