CA1254323A

CA1254323A - Conductive compositions that are directly solderable and flexible and that can be bonded directly to substrates

Info

Publication number: CA1254323A
Application number: CA000485268A
Authority: CA
Inventors: Frank W. Martin; Samson Shahbazi
Original assignee: Electro Materials Corp of America
Current assignee: Vibrantz Corp
Priority date: 1984-07-18
Filing date: 1985-06-26
Publication date: 1989-05-16
Also published as: KR930000776B1; DE3564742D1; KR860001362A; CN85105454A; US4595604A; ATE36918T1; JPS6143644A; CN85105454B; EP0169059A2; SG86888G; EP0169059A3; EP0169059B1; PH22389A

Abstract

- i -Patent Application of F. Wayne Martin and Samson Shahbazi for CONDUCTIVE COMPOSITIONS THAT ARE DIRECTLY
SOLDERABLE AND FLEXIBLE AND THAT CAN BE
BONDED DIRECTLY TO SUBSTRATES
ABSTRACT OF THE DISCLOSURE
This invention provides conductive compositions that are solderable and flexible and that can be bonded directly to substrates. These compositions are made up of a combination of silver exclusively in the form of flake and resin system, said resin system comprising of vinyl chloride/vinyl acetate copolymer, epoxy resin and epoxy hardener. Also provided is a method of making these compositions. Another embodiment of this invention is a method of applying conductive compositions directly to substrates by bonding the conductive compositions directly to the substrate. Once cured, the compositions demonstrate good adhesion directly to the substrate in addition to excellent conductivity, solderability and flexibility characteristics.

Description

l;~S43~3 BACKGROUND OF THE INVENTION
l. Fleld of the Inventlon This lnventlon relates to conductlve composltlons whlch a-e solderable and flexlble, and to thelr method of productlon. More particularly, thls lnventlon relates to conductlve compositlons that exhibit good flexibility and solderability and that can be bonded dlrectly to substrates.

2. Descri~tion of the Prior Art An electrically conductive composition applled to a supportlng or base structure must be sufficlently conductlve to carry electrlc current and must be fi-mly adhered o- bonded to the support or base. Furthermore, it is desi-able that the conductive composition be dlrectly solderable and flexible.
U. S. Patent 3,030,237 dlscloses an lmproved coatlng material essentially comprised of o-ganlc resin carrier and a suitable metallic plgment. The lnventlon lles ln the discovery that a coatlng havlng a metallic pigment component consistlng of a mlxture of ball and flake partlcles having particular dimenslons and used in ce-tain proportions, ln combinatlon wlth an organlc resln carrler, can exhlbit good adheslon and electrlcal conductlvity.

iZS43Z3 Also dlsclosed ls that conventlonal wl~e leadq may be sllver-soldered to the cured coatlng uslng entl-ely common silver qolde-lng technlques.
U.S. Patent 2,959,498 dlscloseR a method of ~ormlng conductlve sllver clrcults by flrst applylng to a -eqlnous dlelectrlc surrace a layer of an lncomplet~ly cured the~mosettlng resln, and then applylng on sald laye-, ln the outline of the deslred conductlve clrcult, a layer of rlnely dlvlded sllver admlxed wlth a thermoplastlc -esln contalnlng a solvent for both of sald reslns, and then heatlng to cure both layers of applled re~lns. It ls essentlal that the reslnous panel flrst be coated wlth a thermosetting resln and that thls resln is only partlally cured before appllcatlon of the conductlve composltion.
The conductlve clrcult ls applled over the undercoatlngJ
preferably by squeezlng a sllver paste ln the deslred electrlcal clrcult pattern. The sllve- paste ls composed of flnely dlvlded qllver partlcles ln a thermoplastlc resln ln a ratlo of between 4:1 and lO:l sllve- to resln. The resultlng sllver clrcult pattern ls fl~mly adhered to the reslnous substrate and the sllve- lineq -eadlly accepts solder.
U.S. Patent 4,371,459 dlscloses a screen printable conductor composltlon that 18 flexlble comprlslng (a) a conductlve pha~e contalnlng sllver and base metal powde-s dlspersed ln a solutlon of (b) a multlpolyme- prepared by copolymerlzatlon of vlnyl acetate, vlnyl chlorlde, and an ethylenlcally unsaturated dlca-boxyllc acld and a llnear aromatlc polyester resln dlssolved ln (c) a volatile nonhydrocarbon solvent. These composltlons a-e partlcularly useful for appllcatlon to membrane touch swltche~.
United States Patent No. 4,564,563 of F.W. M~in et al, issued January 14, 1986, discloses a solderable electrically conductive ~osition comprising metallic sllver partlcles embedded ln a matrlx formed from acryllc, 125~323 carboxylated vlnyl and an epoxy. The composition is formed by dissolving acryllc powder and vlnyl powder in respective solvents to form a flr~t solutlon and a second solutlon. The solutlons are then mlxed with metalllc sllver particles and an epoxy to form an lnk whlch ls applled to a substrate to form a fllm thereon. The fllm ls cured to evaporate the solvents and allow polymerization to occur thereby having a solde-able electrlcally conductlve fllm.
Heretofore, conductlve composltlons developed for use ln connectlon with supportlng or base structures or other electrlcal equlpment have been characterlzed by unacceptably hlgh electrlcal reslstlvlty or unacceptably low adheslon to the supportlng or base materlal.
Furthermore, many prlor art compositlons suffer from the deficlency of not being dlrectly solderable; that ls, the tlme consumlng and expenslve process of electroless plating or the llke must be employed ln order to apply solder to the conductlve composltlon. Yet another deficiency lies in the fact that many of the prior art conductlve composltions are not flexlble and can therefore not be applled to flexible substrates. Addltlonally, many of the prlor art composltlons cannot be bonded dlectly to the substrate on whlch they are applied.
Whlle some prlor art compositlons do solve some of these deflclencies they are deflclent ln that compllcated formulatlons employlng addltlonal compounds must be used and addltlonal processlng steps must be conducted.
Furthermore, none of the prlor art composltlons exhibit all of the characteristics of sufficlent conductivlty, flexlbllity and solderablllty whlle at the same tlme being capable of di-ect bondlng to substrates.
It has been a long sought goal to provlde conductlve composltlons that have good conductlvlty, adhesion and flexibility and whlch are dlrectly solde-able wlthout havlng to resort to the need for compllcated procedureS or lZS~3~

formulations. It has also been desired to have a conductlve composltlon that has sufficient conductivlty and flexlbillty to make lt capable of replacing mechanlcal end connectors on flexlble polymer thick fllm cl-cuits.

SUMMARY OF THE INVENTION
It ls therefore an ob~ect of the lnventlon to provide conductlve composltions that adhere directly to the subst~ate upon which they are applled, are ~lexlble and dlrectly solderable.
It is another obJect o~ the lnventlon to provlde conductlve composltlons havlng good electrlcal characterlstics.
It ls another ob~ect o~ the lnvention to provlde conductlve compositlons which are solderable without the need to use compllcated procedu-es such as electroless platlng.
It is yet another ob~ect to provide conductive compositlons that have sufflclent conductlvlty and flexlbllty to enable lt to replace mechanlcal end connectors on flexible polymer thlck fllm clrcults.
Another obJect of the lnventlon ls to provlde a method of maklng conductlve composltlons that exhlbit the above mentloned characterlstlcs.
These ob~ects and other ob~ects that wlll become apparent, are achleved by the present lnventlon whlch comprlses, ln one aspect, a dl-ectly solderable and flexible conductive composition comprising from about 88% to about 95% by weight silver and a resin syste~
comprising, based on total weight of said composition:
(a) from about 4% to about 7% by weight vinyl chloride/vinyl acetate copolymer;
(b) from about 0.25~ to about 2% by weight epoxy resin; and (c) from about 0.5~ to about 3% by weight epoxy hardener;

~`
~, ~2S43Z3 - 4a -said silver being 100~ in the form of flake having an average particle size from about 0.5 to about 50 microns, said vinyl chloride/vinyl acetate copolymer having a number average molecular weight from about 14,000 to 35,000 and containing no carboxylic acid groups, and said epoxy resin being non-aqueous-based and heat curable.
Another aspect of the invention is a method of making a conductive composltion comprlslng dlssolvlng vinyl chlorlde/vlnyl acetate copolymer ln ketone solvent to form a rlr~t solutlon, dlssolvlng epoxy resln and epoxy hardener in ester solvent to form a second solution and then mlxlng ~j i~54;~Z3 the rlrst and second ~olutions wlth sllver rlake to form a mixture such that the silver flake ls ~ubstantlaily wetted and dlspersed. Yet another aspect of the lnventlon 18 a method of applylng a conductlve composltlon onto a substrate comprlslng bondlng the conductlve composltlon dlrectly to the substrate.

DETAILED DESCRIPTION OF THE INVENTION
The composltlon o~ the lnstant lnventlon comprlses sllver ~laXe and a resln system, sald resln system comprislng vlnyl chlorlde/vlnyl acetate copolymer, epoxy resln and epoxy hardener. In practlce, the sllver ~lake 18 dlstrlbuted throughout the resln system.
Thls resln system not only holds the sllver ~lake ln suspenslon but also provldes adheslon o~ the sllver rlake to the substrate, holds the sllver flake together such that a conductlve path ls formed and provldes flexlblllty. Furthermore, on curlng the resln pulls away from the sllver towards the substrate thus leavlng a hlghly enrlched sllver layer on top.
The composltlons o~ the lnstant lnventlon are applled to substrates whlle in the llquld state, that ls, the composltlons are mlxed wlth a solvent whlch acts as a vehlcle. Once applled to the substrate the compositlon ls cured and the solvent evaporates leavlng a drled, cured composltlon that contalns only traces of solvent. In the followlng dlscusslon, ratlos Or certaln elements employed wlll be dlsclosed; lt 18 lmportant to polnt out that these ratlo~ relate to the composltlon ln lts drled and cured state, not to the composltion ln its liquld state durlng the process o~ manufacture, that 18, when contalnlng a solvent, unless othe-wlse lndlcated.
It 18 lmportant to note that for the purpose of thls lnventlon the sllver employed must be 100% sllver flake.
Sllver ln other ~orms such as, for example, ball or mlxtures o~ ball and flake wlll not lead to the productlon lZ54323 Or a composltlon that demonstrates all of the advantages Or the compo~lt~ons produced according to the lnstant lnventlon. We consider a partlcle to be in flake form when the dlqtance across the partlcle ls at least flve S times greater than the partlcle ls thlck. Preferably, the partlcle should be ten tlme~ g~eater acro~s than lt ls thlck. The presence Or only sllver flake has a further advantage ln that lt (the sllver flake) ~eems to contrlbute to reduclng or ellmlnatlng sllver mlgratlon problems seen ln the prlor art compo~ltlon~. The average partlcle slze Or the ~llver flake may be rrom about 0.5 mlcrons to about 50 mlcrons, preferably ln the range of about 2 mlcrons to about 4 m~crons. It ls understood that sllver flake partlcles may be present havlng a partlcle slze Or less than 0.5 mlcrons or more than 50 mlcrons; as long as the average partlcle slze of the sllver flake ls between 0.5 mlcrons and 50 mlcrons, the composltlons of the lnventlon can be prepared. If the average particle slze Or the sllver flake ls less than 0.5 mlcrons, the~e wlll be dlfflculty ln gettlng the composltlons to load thls quantity Or sllver. IP the~sllver ls too fine, the oonsistency Or the compo~ition will be too clay-llke. If the average partlcle slze Or the sllver rlake 1~ above about 50 mlcrons, then the composltlon wlll be very dlrflcult to apply. For example, the composltlon wlll tend to clog up du-lng a screenlng proce~s or the llke.
The cured composltlons Or the lnventlon contaln between about 88S and 93%, by welght, sllver flake.
The vlnyl chlorlde/vlnyl acetate copolymer that may be u~ed ln the composltlons Or the lnstant lnventlon have a number average molecular welght range Or between about 14,000 and 35,000. Prererably, the number average molecular welght Or the copolymer 18 about 20,000. The hlgher molecular welght vlnyl chlorlde/vlnyl acetate copolymer has better durablllty and toughness characterlstlcs than the lower molecular welght vlnyl lZS43Z3 chlorlde/vlnyl acetate copolymers. We have found that other types of vlnyl chlorlde/vlnyl acetate polymers do not functlon as the vlnyl chlorlde/vlnyl acetate copolymer does o~ the lnstant lnventlon. E.I. DuPont de Nemours and ~r~any 3ells a carboxyl modlried vlnyl chlorlde/vinyl acetate copolymer deslgnated V.M.C.C. Thls product has a weight -atlo Or vlnyl chlorlde to vlnyl acetate to malelc acid of 83:16:1. Composltlons made wlth thl3 product deteriorate as soon as solder 18 applled. Apparently, the presence Or the dlcarboxyllc acld ls detrlmental to the end uses dl~closed ln thls lnventlon. A polyvlnyl butyral was also trled. This is a modlfled polyvlnyl alcohol. The composltlon made wlth thls resln also dlslntegrated a~
soon as solder was applied. The compositions of the ln~tant lnvention requi~e a high perrormance vlnyl chlorlde/vlnyl acetate copolymer; low molecular weight vlnyl chlorlde!vlnyl acetate copolymers or modlfied vinyl chlorlde/vlnyl acetate copolymers do not work. The welght ratlo of vlnyl chlorlde to vlnyl acetate can be from about 80:20 to about 90:10. Prefe-ably, the welght ratlo 1~
from about 86:16 to about 88:12. The vlnyl chlorlde/vlnyl acetate copolymer havlng a number average molecular welght of' 35,000 had a welght ratio Or vlnyl chlorlde to vlnyl acetate Or about 90:10; the vlnyl chlorlde/vlnyl acetate copolymer havlng a number average molecular welght of 14,000 had a welght ratlo of vlnyl chlorlde to vlnyl acetate Or about 86:14.
Although the examples contalned ln this speclrlcatlon were all car~led out by a vinyl chlorlde/vinyl acetate copolymer designated V~ supplied by ~ion Carbide, other types Or vlnyl chlorlde/vlnyl acetate copolymer may be employed as long as the copolymer conrormQ to the spec~rtcatlonQ herelnberore desc-lbed.
The epoxy resln employed as part Or the resln system can be any type Or epoxy resln that ls commerclally avallable and that conrorms to the followlng 12543Z;~

speclflcatlons. We have used an epoxy esln sold by Ciba Gelgy deslgnated Araldite 6010 This 18 an Immodlfied llquld Or medlum vlscoslty whlch 18 based on bl~phenol A
and eplchlorohydrln. The epoxy hardener employed 18 supplled by Clba Gelgy and deslgnated HY 940. The purpose Or the epoxy hardener 18 to crossllnk wlth the epoxy resln to rOrm a solid under curlng condltlons; the epoxy -esln and epoxy hardener employed ln the lnstant lnventlon must not crossllnk at room temperature. It must therefore be a one step heat curable system and be compatlble wlth the other lngredlent~ such as the solvent. The flnal st~pulatlon 18 that the epoxy resln can't be a water based epo xy resln.
The welght ratlo used of silver flake to resln system 1~ lmportant. If these elements are not used wlthln the ranges Or ratlos speclfled, the composltions produ¢ed wlll not possess all of the characte-lstics herelnbefore descrlbed. In the broadest ~ense the welght ratlo of sllver flake to resln system should be f-om about 88:12 to about 95:5 and preferably from about 89:11 to about 91:9.
Wlthln the resln system, the welght ratlo of vlnyl chlorlde/vlnyl acetate copolymer to epoxy resln and har~en~ 1~ from about 100:1 to about 2:1, preferably fran about 2:1 to about 4:1. The welght ratlo of epoxy resln to epoxy hardener is from about 30:70 to about 40:60, preferably from about 34: 66 to sbout 38: 62. Addltlonally, the welght ratlo of vlnyl chlo~lde/vlnyl acetate copolymer to epoxy resin and epoxy hardener 1~ from about 100:1 to ab out 2: 1.
The composltlons of thls lnventlon should have sllver flake present ln an amount of from about 88~ to about 95 of the total composltlon, by welght, and the resln system 19 present ln an amount of from about 7~ to about 12% of the total composltlon, by welght. Fu--thermore, the vinyl chlorlde/vlnyl acetate copolymer 18 present ln an amount from about 4~ to about 7%, by welght, of the total * Trade Mark ** l~ade Mark composltlon, the epoxy hardener ls pre~ent ln an amount Or from about .5% to about 3%, by welght, of the total c xposltlon and the epoxy resln 15 ~-esent ln an amount of rrom about .25~ to about 2%, by welght, of the total c x posltlon.
The composltlon Or thls lnventlon, a mentloned berore, demonstrate advantages over the p~ior art composltlons. For example, the composltlons are flexlble such that they can be applled to a ~lexlble subst-ate ~uch as, ror example, membrane key boards. When we say that the composltlons are flexlble, we mean that the composltlons applled to substrates can be subJected to derormatlve rorces and wlll not release from the substrate or crack or break. The composltlon, ln its deformed state, wlll functlon as well as the compositlon ln lts underormed state. The most severe form Or deformation would be to put a c-ea~e ln the compositlon. Even under this condltion the composltion functlons well. If the composltion cracks or breaks as a ~esult of belng dlstorted the composltlon would not be conslde-ed Mexlble; cracks ln the compositlon cause a severe decrease ln conductlvlty and other electrical and mechanlcal propertles.
Another advantage Or the composltions o~ the lnstant lnventlon ls that they are dlrectly solderable. Thls means that they accept solde- dl-ectly wlthout the englneer havlng to resort to the use of tlme consumlng and expenslve procedures such as electroless platlng. The composltlons can be soldered by any one of a number of methods such a~, for example, dlp or reflow solderlng technlques, as long as the solder tempe-ature does not exceed 205C. or course, the need for the use Or a solder flux ls not obvlated by the composltlons of the lnstant lnventlon. Solder rlux 18 a resln based materlal that alds the wettability Or composltlons. Solder flux ls non-conductive, ls wldely u~ed and ls generally a llquld. The r ~.

l~S43Z3 flux can be applled ln a varlety of ways. Pre-fluxlng can be employed whereln the composltlon to be soldered ls fl-st dlpped lnto llquld flux and then the solder ls applled when the compos1tlon ls stlll wet. Alternatlvely, the flux can be a component of a solder-alloy mlxture. In thls case, the vehlcle 1~ the flux and pre-fluxlng ls not required. Addltlonally, the flux may also be actlvated or not actlvate. ~ctlvated flux clean~ the surf~ce to be soldered.
The composltlons of the lnventlon can be applled to a varlety of ~ubstrates whether rlgld or flexlble and that are made from a variety Or materlalQ. Speclflcally, the substrates may be ceramlc or reslnous ln nature. An example of a typlcal substrate ls a prlnted cl-cult board. The only llmltatlon on the 3ubstrate i9 that lt must be able to wlthstand the condltlons of the solder both wlthout belng deformed or destroyed. The composltlons are applled to substrates ln llquld form and a-e then cured.
The composltlons of the lnventlon are made by dlssolvlng vinyl chloride/vlnyl acetate copolymer ln ketone solvent to form a flrst solutlon, dissolvlng epoxy resln and epoxy hardener ln ester solvent to fo~m a second solutlon,and then mlxlng the flrst and second solutlons with sllver flake to form a mlxture such that the sllve-flake ls thoroughly dlspersed and wetted. The welght ratlo of sllver flake to vlnyl chloride/vlnyl acetate copolymer to epoxy resln ls from about 68:4:4 to about 75:6:2. The welght ratlo of epoxy resln to epoxy hardener ls from about 40:60 to about 36:63. The welght ratlo of vlnyl chorlde/vlnyl acetate to ketone solvent is from about 15:85 to about 25:75 and the welght ratlo of the epoxy resln and epoxy hardener to ester solvent ls f~om about 100:1 to about 75:25. The welght ratlo of the fi-st solutlon to the second solutlon ls from about 100:1 to about 88:12. The vlnyl chlorlde/vlnyl scetate copolymer t~l 3Z~

ls present ln an amount of from about 15~ to about 25%, of the flrst solutlon, by welght. The epoxy resln and epoxy hardener ls present ln an amount from 70% to about 99% of the second solution, by welght, and the sllver flake is present in an amount of from about 68% to about 75% of the total mlxture, by welght.
' There are no speclal technlques lnvolved ln carrylng out the method of thls lnvention other than the vlnyl chlorlde/vlnyl acetate copolymer must be dlssolved ln ketone solvent and the epoxy resln and epoxy har~ener must be dlssolved ln an ester solvent. The sllver flake may be added by any conventlonal technlque. The ketone solvent may be any ketone solvent such as~ for example, gamma butyrolactone, acetone or cyclohexane, or the llke. The ester solvent can be any ester solvent such as, for example, butyl'~a~bltoI'~ acetate and the like; butyl "Carbltol~ acetate ls an ester solvent supplled by Unlon Carblde. Prlor to the appllcatlon of the composltion to the substrate the flneness of grlnd must be checked and ~ound to be 25 mlcrons or less. Flneness of grlnd can be checked uslng any standa-d technlque employlng a flneness of grlnd gauge. If the flneness of g-lnd ls not 25 mlcrons or less, then the composltlon should be passed through a 3 roll mlll untll the proper flneness of grlnd ls obtalned. Uslng feeler gauges, the roll gaps should be set as follows: .002" gap between apron roll and mlddle roll and .015" gap between mlddle roll and back -oll. The composltion should not be passed through the 3 roll mlll more than three tlmes. Addltlonally, the vlscoslty of the composltlon should range from about 30,000 cps to about 100,000 cps at 25C when meaqured uslng a standard Brookfleld vl~cometer. Preferably, the vlscoslty ls between about 30,000 cps and about 70,000 cps.
The mlxture so formed can be applled to a substrate by any one of a number of technlques such as by sllk screenlng, spraylng, or ~rushing. The substrate may be 12~43Z3 flexlble or rlgld and may be of a varlety of types such as resinous or ceramlc. Typlcal examples of substrates are prlnted circuit boards and membrane key board~. The mixtu.e can be applied at a thickness of 1 mll or more.
Once applied to the ~ubst-ate, the mlxture ls subsequently dried and cured at a temperature Or from about 100C to about 170C or about 15 to about 60 mlnuteq. The drylng and curlng can be carrled out by any one of a number of conventlonal technique~ such as baklng in an oven.
The compositlons of thls lnventlon may be bonded dlrectly to substrates. The method 1~ slmply applylng a conductlve compo31tlon onto a subst-ate comprislng bondlng the conductlve composltlon comprl~lng silver excluslvely ln the form of flake and a resln system conslstlng essentlally Or vlnyl chloride/vlnyl acetate copolymer, epoxy resln and epoxy hardener directly to the subst-ate.
The subgtrate does not need to have been pretreated or precoated wlth any partlcular composltlon or by any proce~s. Once drled and cured the composltlons demonstrate good adheslon to the substrate. The average force requl-ed to remove the composltlon rrom a rlgld substrate ls greater than 500 p.s.l. Examples of fle~lble substrates that can be coated wlth the composltlons of the ~ instant invention are polyimide substrates such as that made by E.I. DuPont de Nemours and ~ny and sold under the trademark "Kapton" ~ and polyester substrates such as t~at made by E.I.
DuPont de~1~rs and Oo~ny and sold under the-tra~rk "~ylar".
In measu-lng the force requlred to -emove (or destroy? the composltlon from a rlgld substrate, a pad of composltlonal material ls coated on the substrate one tenth Or an lnch square. Thls pad ls then soldered. A 22 gauge tln coated copper wlre is then soldered lnto the pad perpendlcular to the substrate. The force requ~-ed to tear the pad off Or the substrate ls then measured by conventlonal means. Any fallure 18 ln the ablllty of the pad to adhere to the sub~trate, not in the wlre adhe-lng lZ543Z3 to the pad. Any removal of the composltlon or deterloratlon of the composltlon 18 considered a ~allure. In measurlng the adheslon Or the composltlon on a flexlble substrate, a standard cross hatch adheslon test ls applled. A clrcult deslgn ls prlnted. A razor blade ls then u3ed to cut a cross hatch pattern ln the deslgn.
"Scotch"tape ls then applled to the pattern and pres~ure applled. Then the'~cotch~ tape 18 peeled off and observatlon8 made as to whether any composltlon ha3 been removed from the substrate. A composltlon elther passes or ralls.
In order that those skllled ln the art wlll better be able to practlce. The lnventlon, the followlng exa~ples are glven by way Or lllustratlon, and not by way of llmitatlon.

* Trade Mark of 3M C3~ny.

F~

12S~323 EXAMPLES
_ EXAMPLE I

Method o~_Makin~ a Conductlve Com~o~ltlon A flrst solution was made by dlssolvlng vlnyl chlorlde/vlnyl acetate copolymer havlng a number average molecular welght o~ 20,000 ln gamma butyrnlactone.
A second solutlon was made by dlssolvlng epoxy reQin sold by Clba Gelgy under the trade deslgnatlon~Araldlte 6010"and epoxy hardener also sold by Clba Gelgy under the trade deslgnatlon HY940*1*n butyl Carbltol~ acetate.
These two solutlons were then mixed together and sllver flake havlng an average partlcle slze Or 2 mlcrons was added such that the total composltion had the ~ollowlng makeup:
69.52~- Sllver rlake 22.0% gamma butyrolactone 5.44% vlnyl chlorlde/vlnyl acetate copolymer .66% butyl"Carbitol'~ acetate .87% "Araldlt~ 6010 1.49S epoxy hardener - HY940 The mlxture 18 blended untll the sllver flake 1 thoroughly dlsper~ed and wetted.
The composltlon 13 then passed through a 3 -oll mlll that had a .002" gap between the apron roll and the mlddle role and a .015" gap between the mlddle roll and the back roll. The composltlon was passed through the 3 roll mlll untll a ~ineness o~ graln o~ 23 mlcrons was obtalned.
The viscosity of the composltion was 60~000 cps.
EXAMPLE II
The composltlon Or Example I was applled di-ectly to a rlgld substrate by silk screenlng. It was then drled and cured at 125C ~or 30 mlnutes.
Adheslon was 500 p.s.l.
Conductivlty was 25 mlIllohmq/sqtmll.
* Trade Mark ** Trade Mark 1'~543Z3 Solderabillty was good when solder applled at 200C, and the solder had the followlng composltional make-up: 62% tln; 36% lead; 2% sllver.

EXAMPLE III
The composltlon of Example I was applled dlrectly to a flexlble substrate. It was then drled and cured at 125C for 30 mlnute~.
The compositlon passed the crosshatch adheslon test.
Resistlvlty was 25-27 mllllohm/~q/mll.
Solderablllty was good when solder was applied at 200C and the solder had the following composltlonal makeup: 62% tln; 36% lead; 2% sllver.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A directly solderable and flexible conductive composition comprising from about 88% to about 95% by weight silver and a resin system comprising, based on total weight of said composition;
(a) from about 4% to about 7% by weight vinyl chloride/vinyl acetate copolymer;
(b) from about 0.25% to about 2% by weight epoxy resin;
and (c) from about 0.5% to about 3% by weight epoxy hardener;
said silver being 100% in the form of flake having an average particle size from about 0.5 to about 50 microns, said vinyl chloride/vinyl acetate copolymer having a number average molecular weight from about 14,000 to 35,000 and containing no carboxylic acid groups, and said epoxy resin being non-aqueous-based and heat curable.

2. The composition of claim 1 wherein the ratio of (b) to (c) is from about 30:70 to about 40:60.

3. The composition of claim 1 wherein the ratio of (a) to (b) plus (c) is from about 100:1 to about 2:1.

4. The composition of claim 1 wherein the ratio of vinyl chloride to vinyl acetate is from about 80:20 to about 90:10.

5. An article comprising a substrate coated with the composition of claim 1.

6. The article of claim 5 wherein the substrate is a printed circuit board.

7. The article of claim 5 wherein the substrate is flexible.

8. A method of soldering conductive compositions comprising applying solder directly to the composition of claim 1.

9. The method of claim 8 wherein the solder has a temperature of about 205°C or less.

10. The method of soldering of claim 8 wherein the solder is applied by dip or reflow soldering techniques.

11. A method of making a conductive composition that is directly solderable and flexible comprising dissolving vinyl chloride/vinyl acetate copolymer in ketone solvent to form a first solution, dissolving epoxy resin and epoxy hardener in ester solvent to form a second solution and then mixing the first and second solutions with silver flake to form a mixture.

12. The method of claim 11 wherein the weight ratio of silver flake to vinyl chloride/vinyl acetate copolymer to epoxy resin is from about 68:4:4 to about 75:6:2.

13. The method of claim 11 wherein the weight ratio of epoxy resin to epoxy hardener is from about 40:60 to about 36:63.

14. The method of claim 11 wherein the weight ratio of vinyl chloride/vinyl acetate copolymer to ketone solvent is from about 15:85 to about 25:75.

15. The method of claim 11 wherein the weight ratio of epoxy hardener to ester solvent is from about 100:1 to about 75:25.

16. The method of claim 11 wherein the weight ratio of the first solution to the second solution is from about 100:1 to about 88:12.

17. The method of claim 11 wherein the vinyl chloride/vinyl acetate copolymer is present in an amount of from about 15% to about 25% of the first solution, by weight, the epoxy resin and epoxy hardener is present in an amount of from about 70% to about 99% of the second solution, by weight, and the silver flake is present in an amount of from about 68% to about 75% of the total mixture, by weight.

18. The method of claim 11 additionally comprising applying the mixture to a substrate, then drying and curing the mixture.

19. The method of claim 18 wherein the mixture is applied by silk screening.

20. The method of claim 18 wherein the substrate is flexible.

21. The method of claim 18 wherein the mixture is dried and cured at a temperature of from about 100°C to about 170°C from about 15 to 60 minutes.

22. A method of applying a conductive composition onto a substrate comprising bonding the conductive composition of claim 1 directly to the substrate.

23. The method fo claim 22 wherein the force required to remove the conductive composition from a rigid substrate is greater than 500 p.s.i.

24. The method of claim 22 wherein the substrate is flexible.