Sticking to the path of good conduct - the future for ECAs

TWI Bulletin, March/April 2000

Greg Thomas graduated from Birmingham University in Materials Engineering in 1995 and joined TWI later that year. Since then he has been widely involved in adhesive technology and composite materials including the generation of Design Guidelines for the Offshore Industry. He is currently working in the Advanced Materials, Processes and Microtechnology group at TWI.

Mounting legislation against the use of lead in electronic joining applications has triggered interest in a number of substitutes for lead soldered joints. Here Greg Thomas examines the merits of electrically conductive adhesives.

Adhesives in surface mount applications have traditionally been used to hold devices in position on circuit boards to prevent displacement during soldering. The development of thermally conductive adhesives has aided heat dissipation in electronic devices, thereby increasing component lifetime. Electrically conductive adhesives (ECAs) have been in use for fifty years, but have been restricted to niche applications. In recent years, ECAs have been considered by some for solder replacement in mainstream applications. Potentially, ECAs could be used as drop-in replacements for solder, although processing, electrical and durability issues must be addressed before successful implementation.

Driving forces for ECAs

Lead-solders possess many advantages, including provision of a metallurgical joint with high conductivity, tolerance of component and process variations, relative ease of automation and ease of rework. Furthermore, they have an impressive history, with most applications, devices and processing methods geared to their high volume use. However, there are significant disadvantages associated with their use, so alternatives are required:

• The use of lead-containing materials is seen as an ecological problem due to the toxicity of lead.
• Increasing legislation ^[1] on lead-containing products may lead to high taxation on their use and high disposal costs.
• Lead-solders require high processing temperatures, which may be damaging to sensitive components/assemblies.
• Lead-solders can suffer cyclic fatigue problems, particularly during thermal excursions.
• The use of fluxes requires masking and cleaning stages in processing.

Alternatives include lead-free solders and ECAs. Lead-free solder technology ^[2] has the advantage that printing and placement parameters for lead-solders can be used, for high volume productions. In addition, some alloys do not require cleaning for flux removal. However, alloys based on tin, indium, bismuth and silver, are still youthful, with wetting characteristics, possible formation of intermetallics and long term durability not yet fully understood. Furthermore, some lead-free solder alloys possess high melting (eg 240°C) and processing temperatures.

Silver-loaded adhesives therefore offer significant potential in electronic applications. The advantages associated with ECA technology include the following:

• No lead content.
• Reduced cost due to reduced number of processing steps (no cleaning).
• Simplified processing due to lower curing temperatures.
• Compatibility with solder processing equipment (although parameters may vary).
• Ability to accommodate reducing assembly size and component pitch.
• Able to accommodate fatigue.
• Properties of ECA can be matched to application requirements.

Types of ECA

Electrically conductive adhesives can be either isotropic (ICA) or anisotropic (ACA) in nature. Both types use a dispersion of metal particles in a polymer resin to induce conductivity. Copper, nickel, gold and silver have been used, with the latter being the most common.

A range of adhesives is available, depending on processing and performance requirements, including silicones, polyimides, pressure sensitive tapes/films and epoxies (the latter possessing the greatest market share). Both thermosetting and thermoplastic resins are available, the latter offering potential for rework.

Isotropic adhesives (ICAs)

Isotropic adhesives possess high loading of silver flakes (typically 70-80wt%; 25-30vol%) and are conductive in all directions ( Fig.1). Conductivity is dependent on touching contact between the silver flakes and the substrate/component surfaces.

Isotropic adhesives have generated significant interest as solder replacements in surface mount assembly ( Fig.2), particularly with increasing legislation regarding the use of lead. ^[1] Their paste nature means that ICAs are compatible with processing technologies currently used for solder pastes, although specific parameters will require modification to account for the differing rheological characteristics.

Anisotropic adhesives (ACAs)

Anisotropic adhesives, often known as z-axis adhesives, possess lower metal content (typically 5-20vol%) and only conduct in one direction. Conductive joints are made with ACAs by pressing component and substrate together until the metal particles (usually silver or gold-coated spheres/balls) bridge the gap ( Fig.3). A disadvantage of ACAs is that pressure and heat must be applied while the adhesive cures, otherwise the conductive path is lost. This has limited the use of ACAs to some extent, although large bond area applications in flip-chip technology and liquid crystal displays (LCD) are increasing ( Fig.4).

Processing considerations

Due to the thixotropic nature of ECAs (ie they spread under the application of pressure), the volume of adhesive deposited must be accurately controlled to prevent spreading between adjacent device terminations (which would result in a short circuit). This is achieved by controlling aperture dimensions and stencil thickness when stencil printing. As a result, less material is required to make a joint than with solders. However, ECAs do not wet and spread on metallic surfaces in the same way as solders, and because of their low surface tension, they do not possess the self-aligning and wicking characteristics of solders. These factors combine to produce a different joint geometry for ECAs to that of solders ( Fig.5) - a smaller joint area results, which has an impact on the strength of the joint. Therefore, placement accuracy, device coplanarity and device/lead geometry is more important.

Although the dispensing and processing equipment currently used with solders can be used with ECAs, care must be taken to define these specific parameters to ensure that good quality, reliable assemblies are produced. At present, there is no design guideline available in this area, due to the rheology differences between individual adhesives.

Durability of ECAs

One area of particular concern for ECAs is long-term reliability. Due to the need for touching contact between conductive particles and substrate/device surfaces, there is concern that oxidation of such surfaces will increase the resistance of joints, particularly in hot/wet environments. The extent and effect of this oxidation will depend on the surfaces in question. Tin-lead (Sn/Pb) surfaces exhibit oxidation, in the form of tin-oxide, producing an insulative barrier in the joint. Work has shown that passivated copper and gold surfaces display less oxidation, although it is not eliminated. ^[3] Recently, nickel-palladium surfaces have been successfully investigated, although devices using such surfaces are more expensive than the conventional Sn/Pb. The presence of an electric current can accelerate oxidation, although the rate of conductivity decrease is reduced if the current is passed continuously through the joint. ^[4] This is unlikely in practice, as most components experience regular 'on/off' cycles.

Silver migration has also been a recognised problem for ECAs. This phenomenon displays itself as dendritic growth of silver metal from anodic to cathodic polarised conductors ( Fig.6).

For silver migration to occur, conditions of humidity, heat and electric current are necessary. It is also often associated with ionic contamination in both the adhesive and service environment. Ensuring ECAs possess low ionic contamination, plus using a conformal coating for assembly protection (although this introduces an additional process step and cost) can reduce silver migration.

Electrically conductive adhesives are often claimed to possess superior fatigue properties to solder joints, and it is true that ECA joints retain their mechanical integrity. However, controversy exists regarding their ability to retain electrical properties. Work has shown that ECA joints display considerable scatter in resistance when thermally cycled, with individual joints apparently opening (losing conductivity) and closing (regaining conductivity) during exposure, as shown in Fig.7. ^[5]

Such behaviour can have various causes. Differences in the thermal expansion characteristics between the ECA, device and boards, combined with reduced adhesion and small joint areas are influential parameters, while build up of insulating layers between particles and substrates has also been suggested. The initial quality of joints is likely to be of greatest impact on performance.

Summary

Electrically conductive adhesives offer significant potential for the electronics industry, particularly due to the increasing legislation regarding the use of lead-containing materials. However, at present, the industry remains set-up for lead-solders (device and substrate finishes, processing methods etc), hence ECAs cannot be considered simply as 'drop-in' replacements.

Various factors need to be addressed before successful implementation is achieved - such as processing parameters including dispensing and placement, device and substrate finishes (Ni/Pd, Cu, Au), device geometry and durability. If such factors can be successfully addressed (currently being investigated), ECAs offer benefits to all areas of electronics applications.

References