powder and the solder powder. For this reason, the In general terms, the electrically conductive adhesive composition provides superior electrical conductivity compositions comprise two primary components: a low with little opportunity for conductivity deterioration melting point metal or metal alloy powder and crossdue to oxidation, corrosion or thermal expansion and linking agent mixture comprising a protected curing contraction. 5 agent, which also acts as a primary fluxing agent. De
BRIEF DESCRIPTION OF THE DRAWINGS P0** uP°*the mte+nded end USe'the Preferred c°m"
positions of the invention contain three or more of the
The invention may be better understood with refer- following:
ence to the accompanying drawings, in which: (1) Optionally, a high melting point metal or metal
FIG. 1 illustrates the effects of varying alloys on 10 alloy powder;
conductivity of the final composition; (2) A low reciting point metal or metal alloy powder
FIGS. 2A and 2B shows the effects of relative con- (solder);
centration changes on resistivities for preferred sample (3) Optionally, a resin;
compositions; (4) ^ cross-linking agent mixture comprising a pro
FIG. 3 illustrates the effect of adding a third metal or 15 tected curing agent> which £so acts ^ a primary
metal alloy powder incorporating high melting point fluxing agent- and
metals which readily dissolve in the molten solder pow- (5) Optionally,' a reactive monomer or polymer
de£T ,. , . . „ which can be cross-linked by the curing agent
FIG. 4 is a schematic diagram of a dc-to-dc converter (hereinafter referred to as the "monomer").
^SO^ , . „ 20 The compositions frequently also contain at least one
FIG. 5 is a printed circuit pattern for the converter ^ th ^ ... Qtner tQ ^
circuit illustrated m FIG. 4. '. '. , .
prove certain properties such as adhesion or solderabil
DETAILED DESCRIPTION OF THE ity. The composition must either contain (3) and/or (5),
INVENTION 25 or alternatively (3) and/or (5) may be combined with
~ .. .. j • j * i * t. • (4) into a single species, as in the case of a protected
The present invention was designed not only to obvi- ^' 1 . JT .' r . .
ate the shortcomings of the heretofore known composi- ... j.^-v , ,e 'x , m"
tions, but also to provide a conductive adhesive compo- (4) and/or (5) are halogenated.
sition with the following advantageous properties: In PreP*ruig *e composition, the proportions of
(a) A bulk electrical conductivity approaching that of 30 components (l)-(5) plus a solvent may be varied over a solid copper (never achieved with the previous considerable range and still yield an electrically concompositions)- ductive material once cured. Measurable electrical con
(b) Good solde'rability of the cured compositions ductivity can be achieved with component (1) compriswithout need to plate the resultant cured composi- from °"65% of the volume of the composition (0% tions. 35 being the case of a composition useful as a solder paste).
(c) Adhesive strengths comparable to copper clad Similarly, component (2) comprises from 6-65% by FR4 epoxy printed circuit board laminates; and volume of the composition. Component (3) comprises
(d) Highly corrosion resistant final products with from °~45% of the composition by volume (0% being resistance to degradation at high temperatures and the case of a solder which requires no adhesive), relative humidities. 40 Component (4) comprises 0.01-61% of the composition
It had not heretofore been possible to achieve this com- by volume. Component (5) comprises 0-50% of the
bination of properties in a single composition. composition by volume. Some of the compositions
As a conductive adhesive, the compositions of the within these ranges exhibit some shrinkage and cracking
present invention offer many desirable characteristics. or balling when cured; however, they remain useful as
The electrical conductivity of the inventive composi- 45 conductive adhesives in applications where these char
tions are superior to the known conductive polymer acteristics are harmless, such as in attaching compo
thick films. Unlike prior art polymer thick films, more- nents to electrical circuits.
over, the inventive compositions are generally solder- Preferably, the composition after curing has a bulk
able. The compositions may be screen printed with electrical resistivity of less than 10~4 Ohm-cm. This
higher precision than current solder pastes. Unlike cur- 50 electrical conductivity range can be satisfied by numer
rent solder pastes, they exhibit less of a tendency ous formulations having components (l)-(5) within the
towards formation of solder "satellites" and bridging. following ranges:
After curing, no cleaning or washing for flux removal is Component (1): 0-38% of the volume of the composi
required. tion (values near 0% being compositions useful as
The adhesion properties, mechanical integrity, and 55 solder pastes); corrosion resistance of the compositions of the inven- Component (2): 6-37% by volume of the composition are far superior to those of previous compositions, tion;
because there is no need to add aggressive fluxing Component (3): 0-42% by volume (values near 0%
agents. The compositions are fully cross-linked, and all being compositions useful as solder pastes);
components thereof are chemically immobilized upon 60 Component (4): 7-60% by volume;
curing. Even the reaction by-products of the flux de- Component (5): 0-47% by volume,
oxidation of the metals seem to be chemically bound in Some of the compositions within these ranges may ex
the polymer matrix. The presence of the reactive mono- hibit shrinkage and cracking or bailing when cured, but
mer in the composition controls the rate of curing of the they remain useful as conductive adhesives in applica
resin ensuring strong adhesion of the composition to the 65 tions where these characteristics are harmless,
circuit board substrate upon curing. With certain for- Most preferably, the composition has a bulk electrical
mulations, soldering to the cured composition is possi- resistivity of approximately 3X10-5 or less. These
ble. characteristics can be satisfied by numerous formula