The invention relates to a method of preparing articles for soldering. In particular, it relates to a method which is useful for improving the solderability of compositions comprising metal particles distributed within an organic polymer binder.

In recent years much research has been directed to the development of printed conductors employing thick film inks for use in electronic assemblies. Thick film inks are printable pastes comprising a conductive filler distributed within a curable or fusible binder. The conductive filler is generally in the form of powder or flakes of metal, for example silver or copper. When silver is used it is sometimes alloyed with palladium to inhibit electromigration, while copper may be coated with silver or a silver palladium alloy to improve the solderability of the resulting ink. Nickel, aluminium, palladium and platinum/gold alloys have also been used.

The binders which are used fall into two classes; inorganic glasses and organic polymers. Silicate, phosphate or fluoride based glasses have been used. These bind by being fused at fairly high temperatures, for example in the range 600 to 900°C. Organic binders have the advantage that they bind at lower temperatures than most glasses, for example up to 300°C They include compounds which would react together to form a polymer binder and thermoplastic. polymers which would fuse in an analogous manner to glasses or dry in a similar fashion to paints. Reactive cross-linking polymers

which, once cured, will withstand elevated temperatures are preferred. The most commonly used organic polymers are epoxies, acrylics and silicones. However, such organic polymers have 5 suffered from the disadvantage that they produce cured conductors which are difficult to solder as the organic material is non-wettable by solder unlike the metal filler. The difficulty in solderability is generally thought to arise from the fact that the

10 organic polymers are effective wetting agents for metallic fillers and thus the filler particles, on the surface are prevented from co ing into contact with solder by the presence of a film of organic binder over them.

15

Various techniques have been tried to improve the solderability of organic polymer thick films, for example by providing a copper layer on the surface of the polymer thick film by electroless copper plating,

20 and success has also been claimed for electroless nickel coatings. Other workers have reported improvements using solvent based systems with complex organic mixtures, see EP 0169059A, and by using unconventionally high silver loading of between 88

25 and 95%, see EP 0140585A.

It has now surprisingly been found that the solderability of polymer thick films can be improved by direct treatment of conventionally loaded films. ^~ Ϊ0

Thus in accordance with the present invention there is provided a method of preparing for soldering a surface of a material comprising metallic particles distributed within an organic polymer binder 35 characterised by including the step of etching at least part of the surface to be soldered by

bombardment with atomic or molecular species in a vacuum to expose metallic particles.

By a "vacuum" is meant any pressure below atmospheric. The term "atomic or molecular species" includes ordinary atoms and molecules as well as atomic or molecular ions and radicals.

The etching removes a surface layer of the organic polymer and thus exposes metallic particles comprised within the composition and previously covered by polymer binder. Compositions prepared in this way exhibit improved solderability, which is considered to be because of the presence of more exposed metallic material in the new surface than in the original surface. Conventional fluxes will not remove organic binder.

The method of the present invention can be applied to materials comprising any conventional binder for example epoxies, acrylics, polyesters, or silicones. It has been shown to be particularly effective for compositions in which the organic polymer comprises an epoxy resin. The metallic particles'distributed within the organic polymer may be, for example, silver, a silver/palladium alloy, palladium, nickel, aluminium, copper or a platinum/gold alloy. The present invention has been found to have a particularly beneficial effect on the solderability of compositions in which the metallic particles comprise silver.

The solderability generally improves as the amount of metallic particles present in the composition increases. However, for cost reasons the amount of metallic particles will be as low as

possible while still maintaining the required conductivity. Typically, the composition will comprise at least 70 wt % metallic particles.

The bombarding atomic or molecular species will preferably be produced from an etchant gas, for example by plasma etching on ion beam melting.

Preferably, the species used to bombard the surface of the material will comprise charged particles. A mixture of charged particles with ordinary atoms and molecules or radicals may be employed. The bombarding ions may act by knocking molecules or fragments of molecules from the surface of the material. Atoms, molecules and radicals which are present can also interact with the molecules in the surface of the article. The charged particles used in the method of the present invention for example may be inert gas ions, such as argon ions. Other charged particles which may be employed include oxygen ions and ions of halogenated hydrocarbons, such as CF ₄ , CCI4, CF2, CI2 and CF ₂ Cl2. The use of ions such as these has the added advantage that they may react with molecular fragments from organic polymer binder to form volatile species, for example C0 ₂ . Such volatile species will more readily be removed from the surface of the bombarded article and reduce any tendency for molecules or molecular fragments to redeposit on the surface of the article.As bombardment is continued whole molecular layers will be stripped away and metallic particles which were originally covered by organic polymer binder will be exposed.

Experiments have shown that one particuarly favourable etchant gas is a mixture of oxygen in

argon. While pure oxygen and pure argon both increase the surface metal content of materials comprising metallic particles within an organic polymer binder, argon has a greater effect than oxygen and thus materials etched with argon show greater improved solderability than those etched with oxygen. The relative effectiveness of various etchants can be ranked as follows:

improving solderability

Argon is thought to act purely by physical bombardment while oxygen can oxidise organics for example to yield H ₂ 0 and C0 ₂ . It is thought to be the combination of these effects which causes a mixture of these two gases to be more effective.

The bombardment may be effected by techniques such as plasma etching. By "plasma etching" is meant a process in which the article to be etched is placed in a vacuum chamber and arranged so that a "glow discharge" is generated and the article takes on a negative potential. The plasma is produced from a gas in the chamber and consists of a mixture of highly energetic charged and uncharged particles.

Positive ions from .the plasma are attracted to the article and bombard it. If the article is conductive, such as a metal, it may be placed at a negative potential by means of a d.c. power supply. However, a preferred technique is to use a radio frequency generator which couples capacitatively with

the article and therefore provides a dc voltage at the surface of both conductive and non-conductive articles, for example most glasses and ceramics.

Another method of bombardment which may be employed in the present invention is ion beam milling. In the case of ion beam milling a beam of energetic charged gaseous ions is generated and caused to impinge on the surface of the article to be etched, although the article itself is not maintained at a positive or negative potential. This technique has the added advantage that the ion beam energy and density can be varied independently. A variation on ion beam milling uses a "fast atom source" such that the majority, for example about 95%, of energetic species within the beam are uncharged.

Materials comprising metallic particles distributed within an organic polymer binder can be used to form conductive pathways on printed circuits and such circuits may be etched in accordance with the present invention prior to soldering. The printed circuits may be membrane circuits which consist of circuits printed onto flexible plastics material such as polyethylene as well as conventional circuits which are printed on epoxy board. Thus the present invention also provides a printed circuit prepared by the method described herein to which a component, for example a wire or capacitor, has subsequently been soldered.

The present invention will now be described by way of example with reference to the accompanying drawing and the Example described below. In the drawing. Figure 1 represents a thick film organic polymer sample prepared for bombardment while Figure

2, which is to a different scale shows several such samples mounted on a workholder.

EXAMPLE

A 1" by 4" strip of FR4 epoxy fibreglass board, supplied by Electrofoils Technology Ltd. , was cleaned using acetone and dried at 50°C for 10 minutes in an air circulated oven. The board was then allowed to cool to room temperature. Referring to Figure 1, the board, indicated generally by 1, was masked along the edge of its two longer sides using two 1/4 inch wide sticky backed plastic tapes thus leaving a 1/2 inch wide central strip along the length of the board. A thick film 2 of a composition comprising metallic particles distributed within an organic polymer binder was then cast onto the board 1 between the masking tapes using a palette knife as a coating blade to a depth in the range 0.10 to 0.15 mm. Each of the tapes was then removed to leave two uncoated strips 4 along either side of the film 2. The prepared board was then placed in an air circulated oven to cure the thick film 2. After curing a 1" by 1" section at one end of the sample was masked with aluminium foil 6 to protect the part of the film 2 beneath it from bombardment during etching thus providing a control.

Several samples were prepared using various different compositions for the thick film. Details of the compositions are given in Table 1 below. The compositions for Samples A to D were prepared in house while the compositions for E and F comprise commercially available formulations. Samples A to E were cured in an air circulated oven for 1 hour at 180°C. Sample F, which comprises a mixed two-pack

epoxy resin, was cured for only 15 minutes at 120°C.

TABLE 1

Sample B No.

weight % 70% Ag 75% Ag 80% Ag 83% Ag 72% Ag Ag (Amount metal in not known) sample

100 100

The components mentioned in Table 1 are abbreviations or trade names. A full description of those components is given below.

Abbreviation Description Supplier

828EL Bisphenol-A Epoxy Resin Shell Chemicals

DER354 Bixphenol-F Epoxy Resin DOW Chemicals

PGE Phenyl Gylcidyl Ether Anchor Chemicals

DMPF Dicyandiamide CASA Chemicals

SF282 Silver Flake Handy and Harman

A187 Gamma-glycidioxypropyl- Union Carbide trimethoxysilane

Epotek

H35-175M RS Silver Radio Spares

Epoxy

The examples were etched in a standard Edwards ESM100 Sputtering System. One of each of samples A to F was mounted on a 10" diameter water cooled copper work holder 8 using double sided tape in the arrangement shown in Figure 2.

The chamber of the Sputtering System was pumped to a base pressure of less than 5 x 10~ ⁵ mbar. Argon gas was then admitted to the chamber via a manual leak valve to a pressure of 5 x 10~ ³ mbar.

The through put was controlled by throttling the high vacuum pump with a butterfly valve and maintained at 10 Sccιrf ⁺ 10 percent for all experiments (Seem stands for standard cubic centimeters per minute) . Power was ^" applied to the work holder by a 500 Watt radio frequency generator operating at 13.56 MHz.

This procedure was repeated for samples having the composition of samples A to F under different etching conditions. The radio frequency power density and etch duration were varied. The procedure

was also repeated using different etch gases. The conditions employed are shown in Table 2 below.

TABLE 2

After etching solderability tests were carried out on the samples. Both the areas which had been exposed to bombardment and the areas which had been masked were tested. The solderability tests were carried out using a 62:36:2 tin:lead:silver wire solder having a central core of rosin flux. The cored wire was wound around a former and then cut into rings of approximately 4mm diameter. A ring was placed onto the material to be tested for solderability and heated using a controlled temperature soldering iron set at 215°C until the solder melted. The area of spread of the melted solder and the contact angle between the solder and

substrate were noted and those from different samples compared. A larger area of spread indicates improved solderability as does a decrease in contact angle.

None of the control areas of samples A to F which had been masked and thus not exposed to bombardment were solderable.

The samples which had been exposed to an argon etch showed improved solderability both for the circuit boards and the membrane circuit, and particularly for samples C and D of the circuit boards which had a higher silver loading. Experiments 1 and 2 which involved etching at power densities of 0.5 and 1 Watt/cm ² respectively for 5 minutes showed a more marked improvement in solderability than the samples prepared according to Experiment No. 3 which was etched for 10 minutes at the higher power density of 1 Watt/cm ² . It is possible that Experiment Nos. 1 and 2 provided sufficient bombardment to remove the surface layer of epoxy and expose adequate metallic particles to improve the solderability so that the additional bombardment of Experiment 3 did not lead to further improvement. The longer etching time may also have provided time for redeposition leading to contamination of the etched surface.

In the case of Experiments 4 and 5 in which air was used as the etching gas all of the samples showed some improvement in solderability following the etch and the results were generally better than those achieved with argon. Again, the greatest improvements in solderability were seen for the two samples having the highest silver loadings, namely samples C and D. The best results were obtained in

Experiment 6 where the etching gas was a mixture of 15% 0 ₂ in Ar.

While the Applicant does not wish to bound by theory it is believed that the results for samples etched with a mixture of oxygen and argon were better because this etchant gas can act by both a physical and a chemical process. The argon acts in a simple physical manner while oxygen can act by decomposition of organic surface molecules and chemical reaction to form volatile species which are unlikely to redeposit.

A membrane circuit consisting of a Du Pont Ink 5007 screen printed onto flexible polyester sheet and cured (sample G) was also etched and then tested for solderability in the same manner as the epoxy board circuits described above. The conditions employed are shown in Table 3 below. Chemical analysis has shown that Ink 5007 has a bulk silver content of about 82wt %.

TABLE 3

Experiment Etch RF Power DC Etch No. Gas Density Voltage Duration

(W/cm ² ) (V) (minutes)

Ar 0.5 -713

This sample also etched satisfactorily and was solderable after etching.

Sample C with an overall bulk silver content of 80 wt % was also subjected to X-ray photoelectron spectroscopy (XPS) both before and after etching with oxygen with the etch conditions being as for

Experiment 4. Sample C showed a surface silver level of about 30 wt % before compared with 60 wt % after etching. When XPS measurements for Sample C were made after treatment with 15% 0 in argon with the etch conditions being as for Experiment 6 the surface silver level increased to 78 wt %.