The invention relates to a solderable conductive polymer composition which can be used for forming the conductive paths of a printed circuit. In particular it relates to such a composition which comprises metallic 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 fusable binder. The conductive filler is generally in the form of powder or flake of metal, for example silver or copper. When silver is used it is sometimes alloyed with palladium to inhibit electro-migration, 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. Of these, organic binders have the advantage that they bind at lower temperatures than most inorganic glasses for example up to 300°C. They include compounds which would react together to form a polymer binder as well as thermoplastic polymers which would fuse in an analogous manner to inorganic glasses or dry in a similar fashion to paints. Reactive cross-linking polymers which, once cured, can withstand elevated temperatures are preferred. The most commonly used organic polymers are epoxies, acrylics, silicones and polyi ides. One of the most preferred systems is silver distributed within an epoxy binder.

In order to obtain sufficient conductivity of the composition for it to be effective in providing a conductive path for a printed circuit it has been necessary to include quite high levels of the metallic particles that is 70 to 80 wt % or more. However, the metals used in these compositions are expensive and thus it is preferable to keep the content of them as low as possible. Apart from conductivity another factor which has necessitated the use of quite high levels of metallic particles was the need to be able to solder the inks effectively to the components which are to be attached to the printed circuit. Solderability improves with an increasing content of metallic particles. Recent research has shown that solderability can also be improved in other ways, for example by etching the surface of the composition by bombardment with atomic or molecular species as described in our copending PCT patent application filed on 14th September 1989 and entitled "Preparing Articles for Soldering". However, this introduces a further stage in any manufacturing process.

Thus, it is desirable to find a way of increasing the conductivity of such compositions with a reduced proportion of metallic particles present but maintaining solderability.

Liquid polysulphide polymers have for many years been used in various industries. Those of lower molecular weight, for example molecular weights in the region of 1000 to 4000 which have comparatively low viscosities have been particularly employed as elastomeric modifiers for epoxy resins in the production of coatings, adhesives and electronic encapsulants. Recently, various modified polysulphide polymers have been developed for particular uses and these include reaction products of polysulphide polymers with epoxy compounds such as those described

in US 4689389 .

The inclusion of a polysulphide polymer compound in the conductive polymer composition can increase the conductivity of the composition by a factor of 10, as described in DE 2546568A.

However, these compositions are generally found not to be solderable. Compatibility between metal-filled conducting epoxies and solders used in the semi-conductor industry is of great interest because of the possibility of reducing production cost. The solderable silver-filled epoxy adhesives which have been reported are in general solvent-based systems that contain at least 85 weight percent of metal. It is desirable to develop a range of non-solvent based solderable metal filled epoxies. These filled epoxies should desirably contain less than 80 weight percent metal, no solvents and be solderable without any surface treatment. The present invention thus consists in a solderable conductive polymer composition comprising metallic particles distributed within an organic polymer binder characterised in that it includes the reaction product of a polysulphide polymer compound with an epoxy compound.

The conductive polymer composition of the present invention can be based on any of the known conductive polymer systems, however a system based on epoxy resin is preferred. Examples of epoxy resin that may be employed as the organic polymer binder include bisphenol-A based resins, bisphenol-F based resins, epoxy novolak resins, tetraglycidyl-4,4'-diaminodiphenylmethane and triglycidyl ether of p-aminophenol. The preferred epoxy resins are bisphenol-A and bisphenol-F. While any of the conventional metallic particles such as

copper, nickel, aluminium, palladium or platinum may be employed it is preferred that the metallic particles comprise silver.

It has now surprisingly been found that incorporation of an epoxidised polysulphide into metal filled epoxies having reduced metal content can result in a non-solvent based solderable conductive composition.

In contrast addition of small quanties of unepoxidised polysulphide to known compositions reduces the resistivity of the formulation but results in a surface which is not solderable.

The conductive polymer composition of the invention may include conventional reactive diluents if desired. Examples of reactive diluents that may be used include both monofunctional and multifunctional compounds. Examples of reactive diluents containing one reactive epoxy group include alkyl glycidyl ether where the alkyl group contains between 8 and 14 carbon atoms such as butyl glycidyl ether and ethylhexyl glycidyl ether; phenyl glycidyl ether; cresol glycidyl ether; and para-tertiary-butylphenyl glycidyl ether. Examples of reactive diluents containing more than one reactive epoxy group include polyglycidyl ether of an aliphatic polyol, triglycidyl ether of trimethylolpropane, cycloaliphatic diglycidyl ether, neopentylglycol diglycidyl ether, butanedioldiglycidyl ether, 1,6-hexanediol diglycidyl ether and resorcinol diglycidyl ether. The epoxidised polysulphide polymer compound employed in the composition of the present invention may be provided by any epoxidised polysulphide polymer but the polymer will preferably be liquid and will have a viscosity similar to compounds traditionally used as reactive diluents for epoxy resin systems that is, it will preferably have a viscosity of only up to

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500 centipoise at 25°C. This is so that the compound does not have an adverse effect on the viscosity of the composition prior to curing. A further advantage may arise from the use of epoxidised polysulphide compounds in these formulations. This is because terminal mercaptan groups give the compound an unpleasant smell. If the compound is prepolymerised with an epoxy compound the amount of terminal mercaptan groups is reduced and thus the mercaptan smell is reduced or obviated. An example of an epoxidised polysulphide compound is ELP-3 marketed by Morton Thiokol Limited. The epoxidised polysulphide is preferably present in the composition of the present invention in an amount of less than 4 wt.%. The metallic particles are preferably present in the composition of the present invention in an amount of from 65 to 85 wt.%, most preferably about 77 wt.%.

As stated above, the epoxidised polysulphide polymer compound may react with the organic binder on curing. Thus, the present invention includes a solderable conductive polymer composition which has been cured and in which the epoxidised polysulphide polymer compound has reacted with other components of the composition. The composition of the present invention can be used to form the conductive pathways for a printed circuit, which may be a circuit board in which the circuit is printed on epoxy board or a membrane circuit in which the circuit is printed on flexible plastics material.

The epoxidised polysulphide compounds of the present invention can be formed according to the following reactions. Initially a polysulphide compound can be formed:

1. 2C1CH ₂ CH ₂ 0H + HCHO Ethyl ene Formaldehyde Chlorohydrin

ClCH ₂ CH ₂ OCH ₂ CH ₂ Cl+H ₂ O

2. 6NaOH + (2x+2)S ^2Na ₂ S _x + Na ₂ S ₂ 0 ₃ + 3H ₂ 0

3. C1CH ₂ CH 0CH ₂ CH ₂ C1 + 2Na ₂ Sx + Na ₂ S ₂ 0 ₃

polysulphide.

Polysulphides which may be formed in this way include, for example poly(ethylene disulphide) ,

-(SCH ₂ CH ₂ S) _n or poly(butyl ether disulphide) ,

-(SCH ₂ (CH ₂ ) ₃ CH ₂ (CH ₂ ) ₃ S) _n -.

Such polysulphide can then be epoxidised by reaction with compounds with terminal epoxy groups as shown below where HS-R ¹ - SH stands for a polysulphide.

HC-C-R-CH-CH ₂ + HS-Ri-SH + H ₂ C-CH-R-C-CH w o \/ o o

curing agent

in order to ensure that terminal epoxy groups are attained a molar excess of the epoxy compound should

be employed.

In the following Examples the solderability of two single component and one two-component die attach silver-filled epoxy adhesives (Epo-tek H35-175M, Epo-tek H31 and Epo-tek H20E) manufactured by Epoxy Technology Inc. have been tested. These tests have then been used as a standard to compare the solderability of silver-filled epoxies according to the invention and solvent-based gold-filled epoxy systems as described in DE 2546568.

In the drawings. Figure 1 is a diagram of the circuit employed for E.V.R. measurement.

EXAMPLES

Identity of chemicals

Quartex 1010 (Q1010) Bisphenol-A epoxy resin

WC 67 Butane-diσl diglycidyl ether

ERL 4221E 3,4-epoxycyclohexylmethyl-3,4- epoxy-cyclohexane-carboxylate

ELP-3 Epoxidized polysulphide

LP-3 Polysulphide

2PHZ-CN l-cyanoethyl-2-phenyl-4,5-di (cyano-ethoxymethyl) imidazole

2E4MZ-CN l-cyanoethyl-2-ethyl-4-methylim- idazole

A187 Gamma-glycidoxypropyltrimethoxy- silane

IMEO 3-4 ,5-dihydro imidazole-1-yl- propyltriethoxysilane

MMC SF25 Degussa silver flake

MMC SF53 Degussa silver flake

AY5022 Johnson Matthey gold powder

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Preparation of non-solvent based silver-filled epoxies

(i) Single component formulation method

A one component silver filled epoxy adhesive was prepared in a 60 cm ³ polypropylene container which had been cleaned in methanol and then air dried. For a 50 gram batch of material (see formulation below) , all the liquid components were weighed into the container and a palette knife with a stainless steel blade was used to stir the liquid mixture for approximately five minutes. The silver flake was then added and the material mixed until all the silver flake was dispersed uniformly into the liquid mixture. The screw top was put on the container and sealed using PVC tape. The container was thereafter stored in a refrigerator.

Sin le com one o l

The curing schedule for the single component formulations was 150°C for 1 hour.

(ii) Two component formulation method

25 grams of component-A mixture was prepared in a 30 cm ³ polypropylene container which had been cleaned using methanol and then air dried. All the liquid components were weighed into the container and a palette knife with a stainless steel blade was used to stir the mixture for five minutes. The silver flake was then added and the material mixed until all the silver flake was dispersed uniformly into the liquid mixture. The screw top was put on the container and sealed using PVC tape. The container was thereafter stored at room conditions.

Component-B was prepared by the same method as component-A.

Components A and B were mixed in a 1:1 ratio by weight or by volume.

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Two component formulations

FORMULATION

Q1010 WC 67 ELP-3 MMC 53 A187 GAMMABUTYROLACTONE

TOTAL

FORMULATION

ERL/4221E

2E4MZ-CN

MMC 53

IMECO

GAMMABUTYROLACTONE

TOTAL

The curing schedule for the two component formulations was 150°C for 30 minutes.

Comparative example: Preparation of solvent based silver and gold filled epoxies

Solvent based formulation method

These may be prepared by the method described in DE 2546568. The solvent blend is composed of the following formulation:

100

The semi-solid novalac epoxy resin was dissolved in the solvent blend to give component A. The dicyandiamide was dissolved in the solvent blend to give component B. Components A and B were then mixed together, followed by addition of the polysulphide (if required) and then by the correct concentration of gold or silver.

Solvent based formulations

The following formulations are based on the gold-filled epoxy described in DE 2,546,568.

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Formulation U V W

TOTAL 100.00 100.00 10O.00 100.00

The curing schedule for these formulations is 180°C for 1 hour. In formulations U and V the weight % of gold is 89.18 and the volume % of gold is 36.40 excluding solvent.

In formulations W and X the weight % of silver is

81.76 and the volume % of silver is 36.40 excluding solvent. The electrical volume resistivity and solderability of the gold and silver formulations are compared on an equal volume basis.

ELECTRICAL VOLUME RESISTIVITY MEASUREMENTS

Sample Preparation

The container with the adhesive was taken from the refrigerator and its temperature allowed to reach equilibrium with that of the room temperature before removal of the PVC tape. The EVR test specimen was prepared using a standard one inch by three inch glass slide pre-cleaned using methanol and air dried. Two strips of transparent adhesive tape parallel to each other and 0.3 cm apart were applied onto the glass slide. A drop of silver-filled epoxy adhesive was placed between the tapes and squeezed into a thin film using a palette knife to produce an adhesive fil 0.3 cm wide and at least 6.5 cm long.

The transparent tapes were then removed and the adhesive cured for one hour at 150°C. The cured adhesive was then tested at room temperature.

Three samples of each formulation were made using the same method as that described above.

The two component systems were premixed before use.

Test Method

Apparatus

The resistivity was measured using the following apparatus and conditions. The circuit employed is illustrated in Figure 1. The apparatus consists of a constant current source (1) which is a Keithley 228 programmable current/voltage source; a digital multimeter (2) which is a Keithley 195A multimeter

which is zeroed and auto ranging; and linear four point probe (3) consisting of spring loaded copper probes with tungsten coated tips and having a constant probe spacing of 1.5cm. Adhesive 4 mounted on a glass slide 5 is positioned beneath the probe 3.

Experimental conditions

The potential difference for driving the current was 10 volts. The delay time between each current step was 5 seconds. The programmed current was Cycle range-

Forward: -0.05 to +0.05 amps Step +0.01 amps

Reverse: +0.05 to -0.05 amps Step -0.01 amps

Test Method

The equipment was connected as shown in the circuit diagram in Figure 1. The current source was switched on to supply a constant current I to the outer probes and the voltage V between the inner probes was measured with the digital multimeter. The thickness of the adhesives was measured using a digital micrometer with a semi-spherical measuring anvil. An average value was taken of the difference between the thickness of the slide and thickness of the slide plus the adhesive measured at three different points.

Calculation

EVR = (A/L) * (V//I) ohm cm

Where - EVR is the electrical volume resistivity A is the cross sectional area of adhesive (A = width * thickness) L is the distance between the inner probes

V is the voltage

I is the current

SOLDERABILITY OF SILVER AND GOLD-FILLED EPOXIES

Sample preparation

The samples were prepared as previously described.

Test method

The equipment used comprised an RS soldering station with temperature control and a soldering iron. The solder tested was Arconium semi-conductor alloy 572B, composed of 92.5% Pb, 5% In and 2.5% Ag, having a melting point of 300°C and a soldering temperature of 310°C. The flux used was Alpha 709 containing >50% methanol and having a flash point of 16 deg.c.

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RESULTS

Table 1 Resistivity and solderability of non-solvent based silver-filled epoxies

Formulation E.V.R. Compatibility Comments (ohm cm) with solder

A (No ELP-3) 0.0002 NO Good wetting solder can be removed.

B (1.81% ELP-3) 0.00003 Yes Good wetting, strong solder joint.

C (1.81% LP-3) 0.00003 No Good wetting, solder can be removed.

D (3.61% ELP-3) 0.00003 Yes Good wetting, strong solder joint.

E (No ELP-3) 0.0002 No Good wetting, solder can be removed.

F (0.875% ELP-3) 0.00003 Yes

Good wetting, strong solder joint.

G (1.75% ELP-3) 0.00003 Yes Good wetting, strong solder joint.

Table 2 Resistivity and solderability of Epo-tek silver-filled epoxies and solvent based silver and gold-filled epoxies

Formulation E.V.R. Compatability (ohm cm) with solder

Epo-tek H31 0.0002 No

Epo-tek H35-175M >0.1 original surface, No 0.002 surface removed No

Epo-tek H20 0.0001 No

U (No LP-3) Sample 1 >0.1

2 >0.1 Yes

3 >0.1

V (0.76% LP-3) Sample 1 >0.1

2 >0.1 No

3 >0.1

W (No LP-3) Sample 1 0.0003

2 0.0003 Yes

3 0.0003

X (2.33% LP-3) Sample 1 0.00004

2 0.00004 No

3 0.00003