Environmental and technical characteristics of conductive adhesives versus soldering

9. Conclusion

It is estimated that electrically conductive adhesives will not be a drop-in replacement for solder, but be used in niche applications like low temperature substrates and possibly where heat transfer is high, i.e. for shielding applications, ceramic substrates, and possibly for low current applications. A further development of the electrically conductive adhesives and process technology may be necessary if electrically conductive adhesives should more widely replace solder.

The overall result of the evaluation is that the reliability of the adhesives is very dependent on the substrate and the component plating. The silicone variants showed unacceptable interconnection resistance and the variants with carbon showed high interconnection resistance. In general, adhesives applied on tin-lead substrates or to tin-lead component terminals showed increased interconnection resistance.

The shear strength showed acceptable values for most of the variants especially when mounted on the ceramic substrate and on the FR-4 substrate with gold over nickel plating. However, the silicone variants have shown very low shear strength values on all substrates.

The overall results from the contact resistance measurement showed that the silicone variants were unacceptable for either the short circuit or mounting of chip resistors. The variants with carbon particles have relatively high contact resistance, and will only be suitable for components with high input impedance. The silver-filled epoxy, thermoplastic and thermoset variants showed good results when used with silver palladium component finish mounted on gold and silver conductors. Tin lead-plating is not compatible with the adhesives, as high contact resistance values were measured. However, one variant, the thermoplastic, showed reasonable contact resistance values when tested with tin-lead platings.

The overall results from the shear strength test have shown that when the adhesive is bonded to tin lead surfaces the fraction will be between the adhesive and the tin lead. When the bonding material is gold, silver or silver palladium the separation will be in the adhesive joint. All variants tested on the ceramic substrate showed very high shear strength force value. The variants tested on the polyester substrate has low shear strength value, except for one epoxy variant connecting to silver-palladium finish.

The variants tested on the two different FR-4 substrates have for the two silicone adhesives shown unacceptable results. The shear strength value for all other variants was acceptable when tested on the substrate with gold finish. When tested on the substrate with tin lead finish one of these variants has unacceptable shear strength values, while the other variants (except one with the same values on both substrate finishes) have lower values on the tin lead substrate.

From an environmental point of view, it is a question of release of lead and tin, or silver to the environment, unless carbon is used as the conductive material in the adhesives. All the three metals impose environmental hazards, but the actual risks are not easily evaluated. Both for solder and adhesive, recirculation of electronic equipment will reduce environmental release.

Based on the Life Cycle Assessment, it cannot be judged whether adhesive technology is better or worse than solder technology. The available data on material consumption and emissions during the life cycle are too uncertain. Further, in relation to toxicity, which is a crucial impact category in this assessment, too little is known about the actual environmental fate of metals leaking to the environment from waste steams.

The study has lead to the following recommendations in relation to manufacturing and emission handling:

	Use components with as little silver and palladium termination as possible
	Optimise adhesive consumption (for SMT about 1/5 as compared to solder paste amount)
	Assure high recovery percentages for the metals during recycling and reduce losses during manufacturing operations
	Assure proper treatment of hazardous wastes

From a working environmental point of view, the choice of technology is equivocal. The two technologies impose different hazards (the potential hazard impact of solder is slightly higher), but the occupational risks are minimal when adequate preventive measures are implemented.

A well considered working practice, the proper use of available technical preventive measures and personal protective equipment as well as good hygiene at work all reduce the actual risk. As a basic principle, the preventive measures should be prioritised as follows:

	Designing the assembly line in a way that reduces risk as much as technical possible (avoid skin contact with solder paste, flux and adhesives and avoid inhalation of solvent vapours and lead fumes)
	Establishing technical measures (e.g. ventilation) where needed.
	Prescribing the use of personal protection equipment.

From a working environmental point of view, the choice of adhesive resin should be made according to the following preference:

	Polyester
	one-component silicone / two-component silicone
	acrylate / two-component epoxy / one-component epoxy

From an environmental point of view, the choise of adhesive conductor should be made according to the following preference:

1. Carbon
2a. Silver-plated aluminium
2b. Silver-plated copper
3. Silver

From a technical point of view the adhesive systems should be selected with due respect to the below criteria:

	electrical conductive adhesive shall be applied on noble metal surfaces, i.e. tin-lead as a substrate or component terminal finish is not recommended, due to oxidation.
	redesign of the footprints on the substrates may be necessary.
	reduction of the opens in the stencil is necessary to reduce the applied amount of electrical conductive adhesive compared to solder.
	the existing production equipment can be used i.e. the stencil printer or the dispenser, the component mounting automate and the furnace. A furnace for adhesive curing is recommended, especially if both adhesive and solder shall be used in the production.
	process optimisation with electrical conductive adhesive is more difficult than with solder, especially due to no self alignment and no contraction of the adhesive.
	curing shall be performed according to manufacturers specification, i.e. the temperature shall be controlled.
	interconnection resistance is higher with electrical conductive adhesive than with solder.
	the adhesion between component/adhesive/substrate seems to be lower for electrical conductive adhesive than for solder, this may not be correct for some SMD (ceramic chip resistors and capacitors) components mounted to thickfilm silver conductors on ceramic substrate. Depending on the electrical conductive adhesives adhesion strength and the component, components shall be mechanical fixed.
	repair may be a problem, as hand mounting in electrical conductive adhesive may give an increased interconnection resistance, and revealed silver flakes may tend to migrate.
	repair is estimated to be much more costly with electrical conductive adhesives than with solder.
	protection against uncured epoxy and epoxy vapour is required. Personal shall be trained to handle epoxy.
	for low temperature substrates there is only one alternative, which is low melting solder containing Indium.
	the price for electrical conductive adhesives is about 5 times higher than for solder.

A widespread substitution of solder with silver based adhesives would increase the silver demand by approximately 11%. However, with today’s technology the demand will increase with less than 1%. The substitution for adhesive will most obviously also increase the silver demands for component termination as Ag/Pd terminated components are highly recommended for adhesives.

A widespread - future - substitution would therefore most obviously increase silver prices - when also comparing to the fact that it is estimated that the silver supply horizon is only about 14 years! However, with the present technology level, the increase in silver demand due to adhesive application would most obviously be marginal as compared to other mechanisms affecting silver prices.