Adhesives, electrically conductive nickel filler

CondUCtiVG FillGrS. Conductive fillers are used to provide the adhesive with electrical conductivity. The simplest fillers are metal particles such as gold, silver, nickel, indium, copper, chromium, and lead-free solders (SbBi) (6,7,9-11). The particles are usually spherical and range 3-15 /tm in size for ACA applications (12). Needles or whiskers are also quoted in some patents (6). 

Electromagnetic/radio-frequency interference shielding materials have to meet much lower demands in terms of overall electrical conductivity (typically 4-5 orders of magnitude lower than a silver-flake-filled adhesive). This means that cheaper conductive fillers can be employed, for example, silver-coated copper flake, nickel flake, and carbon black. Typically the adhesive has to form a compliant joint between two mating surfaces, and hence room temperature vulcanizing or heat-cure silicone is often a convenient choice of matrix material. 

A new area of concern for electrical stability arises because of the increasing use of conductive adhesives as replacements for solder. Some conductive adhesives show unstable electrical-contact resistance when used on non-noble metal surfaces such as copper or tin-lead solder. Although stable on gold, palladium, platinum, and silver surfaces, the same adhesives were found to be unstable on tin, tin-lead, copper, and nickel surfaces.The unstable resistance and increase in resistance in temperature-humidity exposures have been attributed to the growth of an oxide layer separating the filler particles from the substrate at the interface, a mechanism similar to that for the loss of backside contact in die-attach materials. 

Nickel is a good nominee for conductive filler in conducting adhesives because of the numerous benefits it possesses. Nickel shows chemical stability and oxidizes relatively slowly compared to copper. But, nickel has disadvantages compared to silver nickel has a higher electrical resistance than silver (about 25% of silver). However, it is less expensive than silver filler (Goh et al. 2006). 

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