Solderable Conductive Compositions That Are Capable of Being Soldered by Non Silver-Bearing Solder and That Can Be Bonded Directly to Substrates and Which Are Flexible

1. Field of the Invention

This invention relates to conductive compositions which are solderable and flexible, and to their method of production. More particularly, this invention relates to conductive compositions that exhibit good flexibility and solderability and that can be bonded directly to substrates.

2. Description of the Prior Art

An electrically conductive composition applied to a supporting or base structure must be sufficiently conductive to carry electric current and must be firmly adhered or bonded to the support or base. Furthermore, it is desirable that the conductive composition be directly solderable and flexible.

U.S. Pat. No. 3,030,237 discloses an improved coating material essentially comprised of organic resin carrier and a suitable metallic pigment. The invention lies in the discovery that a coating having a metallic pigment component consisting of a mixture of ball and flake particles having particular dimensions and used in certain proportions, in combination with an organic resin carrier, can exhibit good adhesion and electrical conductivity. Also disclosed is that conventional wire leads may be silver-soldered to the cured coating using an entirely common silver-soldering technique.

U.S. Pat. No. 2,959,498 discloses a method of forming conductive silver circuits by first applying to a resinous dielectric surface a layer of an incompletely cured thermosetting resin, and then applying on said layer, in the outline of the desired conductive circuit, a layer of finely divided silver admixed with a thermoplastic resin containing a solvent for both of said resins, and then heating to cure both layers of applied resins. It is essential that the resinous panel first be coated with a thermosetting resin and that this resin be only partially cured before application of the conductive composition. The conductive circuit is applied over the undercoating, preferably by squeezing a silver paste in the desired electrical circuit pattern. The silver paste is composed of finely divided silver particles in a thermoplastic resin in a ratio of between 4:1 and 10:1 silver to resin. The resulting silver circuit pattern will be firmly adhered to the resinous substrate and the silver lines will readily accept solder.

U.S. Pat. No. 4,371,459 discloses screen printable conductor composition that is flexible comprising (a) a conductive phase containing silver and base metal powders dispersed in a solution of (b) a multipolymer prepared by copolymerization of vinyl acetate, vinyl chloride, and an ethylenically unsaturated dicarboxylic acid and a linear aromatic polyester resin dissolved in (c) volatile nonhydrocarbon solvent. These compositions are particularly useful for application to membrane touch switches.

U.S. patent application Ser. No. 556,840 filed Sept. 30, 1983, now abandoned, discloses a solderable electrically conductive composition comprising metallic silver particles embedded in a matrix formed from acrylic, carboxylated vinyl and an epoxy. The composition is formed by dissolving acrylic powder and vinyl powder in respective solvents to form a first solution and a second solution. The solutions are then mixed with metallic silver particles and an epoxy to form an ink which is applied to a substrate to form a film thereon. The film is cured to evaporate the solvents and allow polymerization to occur thereby having a solderable electrically conductive film.

Heretofore, conductive compositions developed for use in connection with supporting or base structures or other electrical equipment have been characterized by unacceptably high electrical resistivity or unacceptably low adhesion to the supporting or base material. Furthermore, many prior art compositions suffer from the deficiency of not being directly solderable; that is, the time consuming and expensive process of electroless plating or the like must be employed in order to apply solder to the conductive composition. Yet another deficiency lies in the fact that many of the prior art conductive compositions are not flexible and can therefore not be applied to flexible substrates. Still another deficiency is that typical polymer thick film conductors must be silver soldered with a silver bearing solder.

While some prior art compositions do solve some of these deficiencies they are deficient in that complicated formulations employing additional compounds must be used and additional processing steps must be conducted. Furthermore, none of the prior art compositions exhibit all of the characteristics of sufficient conductivity, flexibility and solderability while at the same time being capable of direct bonding to substrates.

It has been a long sought goal to provide conductive compositions that have good conductivity, adhesion and flexibility and which are directly solderable without having to resort to the need for complicated procedures or formulations. It is also desired to have a conductive composition that is directly solderable by a non-silver bearing solder.

It is therefore an object of the invention to provide conductive compositions that adhere directly to the substrate upon which they are applied, are flexible and directly solderable.

It is another object of the invention to provide conductive compositions with good electrical properties.

It is another object of the invention to provide conductive compositions that can be applied to flexible substrates.

A further object of the invention is to provide conductive compositions which are solderable without the need to use complicated procedures such as electroless plating.

It is yet another object to provide conductive compositions which can be soldered by non-silver bearing solder.

Another object of the invention is to provide a method of making conductive compositions that exhibit the above mentioned characteristics.

These objects and other objects that will become apparent, are achieved by the present invention which comprises, in one aspect, a directly solderable and flexible conductive composition comprising silver, exclusively in the form of flake and vinyl chloride/vinyl acetate copolymer. Another aspect of the invention is a method of making a conductive composition that is directly solderable and flexible comprising dissolving vinyl chloride/vinyl acetate copolymer in ketone solvent and mixing the resultant solution with silver flake such that the silver flake is substantially wetted and dispersed. Yet another aspect of the invention lies in a method of bonding a conductive composition onto a substrate comprising applying a conductive composition comprising silver, exclusively in the form of flake and vinyl chloride/vinyl acetate copolymer directly to the substrate.

The composition of the instant invention comprises silver exclusively in the form of flake and vinyl chloride/vinyl acetate copolymer. This vinyl chloride/vinyl acetate copolymer functions as a carrier resin in which is dispersed the silver flake. This resin not only holds the silver flake in suspension but also provides adhesion of the silver flake to a substrate, holds the silver flake together such that a conductive path is formed and provides flexibility. Furthermore, on curing the resin pulls away from the silver towards the substrate thus leaving a highly enriched silver layer on top.

It is important to note that for the purpose of this invention 100% silver flake must be employed; silver in other forms such as, for example, ball or mixtures of ball and flake will not lead to the production of a composition that demonstrates all of the advantages of the compositions produced according to the instant invention. We consider a particle to be in flake form when the distance across the particle is at least 5 times greater than the particle is thick. Preferably, the particle should be 10 times greater across than it is thick. The presence of only silver flake has a further advantage in that it (the silver flake) seems to contribute to reducing or eliminating silver migration problems seen in the prior art compositions. The average particle size of the silver flake may be from about 0.5 microns to about 50 microns, preferably in the range of about 2 microns to about 4 microns. It is understood that silver flake particles may be present having a particle size of less than 0.5 microns or more than 50 microns; as long as the average particle size of the silver flake is between 0.5 microns and 50 microns, the compositions of the invention can be prepared. If the average particle size of the silver flake is less than 0.5 microns, there will be difficulty in getting the compositions to load this quantity of silver. If the silver is too fine, the consistancy of the composition will be too clay-like. If the average particle size of the silver flake is above about 50 microns, then the composition will be very difficult to apply. For example, the composition will tend to clog up during a silk screening process or the like. The cured compositions of the invention contain between about 88% and 93%, by weight, silver flake.

The compositions of the instant invention are applied to substrates while in the liquid state, that is, the compositions are mixed with a solvent which acts as a vehicle. Once applied to the substrate the composition is cured and the solvent evaporates leaving a dried, cured composition that contains only traces of solvent. In the following discussion, ratios of certain elements employed will be disclosed; it is important to point out that these ratios relate to the composition in its dried and cured state, not to the composition in its liquid state during the process of manufacture, that is, when containing a solvent, unless otherwise indicated.

The vinyl chloride/vinyl acetate copolymer that may be used in compositions of the instant invention have a number average molecular weight range of between about 14,000 and 35,000. Preferably, the number average molecular weight of the copolymer is about 20,000. The higher molecular weight vinyl chloride/vinyl acetate copolymer has better durability and toughness characteristics than the lower molecular weight vinyl chloride/vinyl acetate copolymers. We have found that other types of vinyl chloride/vinyl acetate copolymers do not function as the vinyl chloride/vinyl acetate copolymer does of the instant invention. E.I. DuPont de Nemours sells a carboxyl modified vinyl chloride/vinyl acetate copolymer designated V.M.C.C. This product has a ratio of vinyl chloride to vinyl acetate to maleic acid of 83:16:1. Compositions made with this product deteriorate as soon as solder is applied. A polyvinyl butyral was also tried. This is a modified polyvinyl alcohol. The composition made with this resin also disintegrated as soon as solder was applied. The compositions of the instant invention require a high performance vinyl chloride/vinyl acetate copolymer; low molecular weight copolymers or modified vinyl acetate/vinyl chloride copolymers do not work. The weight ratio of vinyl chloride to vinyl acetate can be from about 80:20 to about 90:10. Preferably, the weight ratio is about 84:16 to about 88.12. The vinyl chloride/vinyl acetate copolymer having a number average molecular weight of 35,000 had a weight ratio of vinyl chloride to vinyl acetate of about 90:10; the vinyl chloride/vinyl acetate copolymer having a number average molecular weight of 14,000 had a weight ratio of vinyl chloride to vinyl acetate of about 86:14.

Although the examples contained in this specification were all carried out with a vinyl chloride/vinyl acetate copolymer designed VYHH supplied by Union Carbide, other types of vinyl chloride/vinyl acetate copolymer may be employed as long as the copolymer conforms to the specifications hereinbefore described.

The weight ratio used of silver flake to vinyl chloride/vinyl acetate copolymer is important. The weight ratio of silver flake to vinyl chloride/vinyl acetate copolymer should be from about 8:1 to about 12:1, preferably about 9:1 to about 10:1. If these elements are not used within the ranges of these ratios the composition produced will not possess all of the characteristics hereinbefore described.

The composition of this invention should have silver flake present in an amount of from about 88% to about 93% of the total composition, by weight, and the vinyl chloride/vinyl acetate copolymer in an amount of from about 7% to about 12% of the total composition, by weight.

The compositions of this invention, as mentioned before, demonstrate advantages over the prior art compositions. For example, the compositions are flexible such that they can be applied to a flexible susbstrate such as, for example, membrane key boards. When we say that the compositions are flexible, we mean that the compositions applied to substrates can be subjected to deformative forces and will not release from the substrate or crack or break. The composition, in its deformed state, will function as well as the composition in its undeformed state. The most severe form of deformation would be to put a crease in the composition. Even under this condition the composition functions well. If the composition cracks or breaks as a result of being distorted the composition would not be considered flexible; cracks in the composition cause a severe decrease in conductivity and other electrical and mechanical properties.

Another advantage of the compositions of the instant invention is that they are directly solderable. This means that they accept solder directly without the engineer having to resort to the use of time consuming and expensive procedures such as electroless plating. The compositions can be soldered by any one of a number of methods such as, for example, dip or reflow soldering techniques as long as the solder temperature does not exceed 205.degree. C. Of course, the need for the use of a solder flux is not obviated by the compositions of the instant invention. Solder flux is a resin based material that aids the wetability of compositions. Solder flux is non-conductive, is widely used and is generally a liquid. The flux can be applied in a variety of ways. Pre-fluxing can be employed wherein the composition to be soldered is first dipped into liquid flux and then the solder is applied when the composition is still wet. Alternatively, the flux can be a component of a solder-alloy mixture. In this case, the vehicle is the flux and pre-fluxing is not required. Additionally, the flux may also be activated or not activated. Activated flux cleans the surface to be soldered.

Another advantage of the compositions of this invention is that they are capable of being soldered by non-silver bearing solder. Prior art polymeric compositions must be soldered by solders containing silver. The solder employed usually contains lead-tin-silver. The compositions of the instant invention may be soldered with solder made of, for example, only lead and tin. Other examples of non-silver bearing solders that may be used are bismith-indium; tin-bismith, tin-antimony; and other indium based solders.

As previously indicated the compositions of the invention are applied to substrates in liquid form and are then cured. The article comprising the cured composition on the substrate can take any one of a number of forms. For example, the substrate may be flexible or rigid and may be made of a variety of materials such as ceramic or resinous based materials. An example of a typical article produced is a printed circuit board. The only limitation on the substrate is that it must be able to withstand the conditions of the solder bath without being deformed or destroyed.

The compositions of the instant invention are made by dissolving vinyl chloride/vinyl acetate copolymer in ketone solvent and then mixing the resultant solution with silver flake such that the silver flake is substantially dispersed and wetted in the solution. In this method the weight ratio of vinyl chloride/vinyl acetate copolymer to silver flake is from about 1:8 to about 1:12, preferably about 1:9 to about 1:10. The weight ratio of vinyl chloride/vinyl acetate copolymer to ketone solvent is from about 15:85 to about 30:70, preferably about 18:82 to about 24:76.

The vinyl chloride/vinyl acetate copolymer is present in an amount of from about 5% to about 9% of the total mixture, by weight, the ketone solvent is present in an amount of from about 20% to about 25% of the total mixture, by weight, and the silver flake is present in an amount of from 68% to about 75% of the total mixture, by weight.

There are no special techniques required in dissolving the vinyl chloride/vinyl acetate copolymer in the ketone solvent or in adding the silver flake. Entirely conventional methodology may be employed.

The ketone solvent may be any ketone solvent such as, for example, gamma butyolactone, acetone or cyclohexane or the like.

Prior to the application of the composition to the substrate, the fineness of grind of the composition must be checked and found to be 25 microns or less. Fineness of grind can be checked using any standard technique employing a fineness of grind guage. If the fineness of grind is not 25 microns or less, then the composition should be put through a 3 Roll mill until the proper fineness of grind is obtained. Using feeler guages, the roll gaps should be set as follows: 0.002" gap between apron roll and middle roll and 0.015" gap between middle roll and back roll. The composition should not be passed through the 3 Roll mill more than 3 times. Additionally, the viscosity of the composition should range from about 30,000 cps to about 100,000 cps at 25.degree. C. when measured using a standard Brookfield viscometer. Preferred viscosity range is from about 30,000 cps to about 70,000 cps.

The resultant mixture can be applied to substrates. These substrates may be flexible or rigid and may be of a variety of types such as resinous or ceramic. The mixture may be applied at a thickness of as little as one mil. Examples of typical substrates are printed circuit boards and membrane key boards. The mixture can be applied to substrates by a variety of techniques, such as by silk screening, spraying or brushing. Once the mixture has been applied to the substrate, it is then dried and cured at a temperature of from about 100.degree. C. to about 170.degree. C. for about 15 to about 60 minutes. The drying and curing step may be carried out by an conventional means such as baking in an oven.

The compositions of this invention may be bonded directly to substrates. The method is simply bonding a conductive composition consisting of silver exclusively in the form of flake and vinyl chloride/vinyl acetate copolymer directly to substrates. The substrate does not need to have been pretreated or precoated with any particular composition or by any process. Once dried and cured, the compositions demonstrate good adhesion to the substrate. The average force required to remove the composition from a rigid substrate is greater than 500 p.s.i. Examples of flexible substrates that can be coated with the compositions of the instant invention are polyimide substrates such as that made by E.I. DuPont de Nemour and sold under the trademark Kapton.RTM. and polyester substrates such as that made by E.I. DuPont de Nemour and sold under the trademark Mylar.RTM..

In measuring the force required to remove (or destroy) the composition from a rigid substrate, a pad of compositional material is coated on the substrate one tenth of an inch square. This pad is then soldered. A 22 gauge tin coated copper wire is then soldered into the pad perpendicular to the substrate. The force required to tear the pad off of the substrate is then measured by conventional means. Any failure is in the ability of the pad to adhere to the substrate, not in the wire adhering to the pad. Any removal of the composition or deterioration of the composition is considered a failure. In measuring the adhesion of the composition on a flexible substrate, a standard cross hatch adhesion test is conducted. A circuit design is printed. A razor blade is then used to cut a cross hatch pattern in the design. Scotch tape is then applied to the design and pressure applied thereto. Then the scotch tape is peeled off and observations made as to whether any composition has been removed from the substrate. A composition either passes or fails.

In order that those skilled in the art will be better able to practice the invention, the following examples are given by way of illustration, and not by way of limitation.

Method of Making Conductive Composition

A composition comprising 70% silver flake and 7.5% vinyl chloride/vinyl acetate copolymer was made as follows:

7.5 grams of vinyl chloride/vinyl acetate copolymer having a number average molecular weight of 20,000 was dissolved in 22.5 grams of gamma butylolactone. To this was added 70 grams of silver flake particles having an average particle size of 2 microns. The materials were mixed together in a clean stainless steel bowl and blended with a mixer until the silver flake was thoroughly dispersed and wetted.

The composition was then passed through a three roll mill that had a 0.002" gap between the apron roll and middle roll and a 0.015" gap between the middle roll and back roll.

The composition was passed through the 3 roll mill until a fineness of grind of 7 microns was obtained.

The composition was then screen printed onto a printed circuit board and cured at 125.degree. C. for 30 minutes.

The composition of Example I was directly applied to a printed circuit board by silk screening. It was then dried and cured at 125.degree. C. for 30 minutes.

Adhesion was greater than 550 p.s.i.

Resistivity was 20 milliohms/sq./mil.

Solderability was good when the solder applied was at 200.degree. C. and was of the following composition: 62% tin; 36% lead; 2% silver.

The composition of Example I was directly applied to a flexible substrate by silk screening. It was then dried and cured at 125.degree. C. for 30 minutes.

The composition passed the cross hatch adhesion test.

Resistivity was 20 millohms/sq/mil.

Solderability was good when the solder was applied at 200.degree. C. and was of the following composition: 62% tin; 36% lead; 2% silver.

The composition of Example I was directly applied to a printed circuit board. It was then dried and cured at 125.degree. C. for 30 minutes. It was then soldered at 200.degree. C. with a non-silver bearing solder of the following composition: 63% tin; 37 lead.

Resistivity was 2-3 milliohms/sq./mil.

Solderability as good.