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Regarding solderable finishes for high-temperature PCBs, electroless nickel immersion gold (ENIG) was the popular choice, although variants such as ENEG and ENEPIG were sometimes specified. Solder mask was an issue with high-temperature PCBs: Standard materials were epoxy based and would oxidise at prolonged temperatures above 150°C, resulting in a colour change from the usually green original to dark brown. There was a polyimide-based solder mask available but it was not photoimageable and had limited technical capability, with Tg quoted at only 165°C.
“Solving the Interconnection Challenge” was the presentation from Martin Wickham, from NPL’s Electronics Interconnection Group. Expanding upon the subtitle, “Sintered Silver for Interconnect,” he described how high-temperature electronics, for which there was an increasing demand, were pushing the boundaries for solder. Few solder alloys were available for operation above 200°C. Alloys with sufficient headroom were limited and of those, alloys not containing lead were very limited. Examples such as gold-tin were prohibitively expensive. RoHS recast proposals were due to go open-scope from 2019 onwards, although current RoHS exemptions might stay “until alternatives become available,” when those without alternatives would be left behind.
Sintered interconnect metals offered an alternative to high-lead solder alloys for high temperature applications and there was considerable interest both in silver-based materials for die bonding applications and in copper-based materials for forming conductors. It was known that nano-particle silver could be sintered at temperatures of 275°C or lower and that once sintered, the material would perform close to the melting point of metallic silver, 961°C. Wickham shared the results of current research at NPL on sintered silver interconnect technology with different component types and PCB finishes. Early trials with nano-powders had shown that it was possible to create basic sintered silver joints at 250°C with reasonable attachment strength using parameters consistent with a surface mount assembly process.
The aim of current work was to create circuit assemblies fabricated using sintered silver interconnects. Four different components had been investigated together with three different board finishes and two commercial silver pastes. The degree of densification of the sintered metal was dependent on factors such as uniformity of dispersion, particle size and particle size distribution, temperature, heating time, and the effect of the surrounding die material. A major issue with nano sized particles was their tendency to agglomerate, so dispersants had to be added, along with binders and solvents, to yield a printable paste, and all of these added components were detrimental to the sinter process. Principal ongoing challenges to the use sintered metals as high-lead solder replacements for PCB assembly were their compatibility with existing processes and components, particularly the finishes on substrate and component terminations, and the minimisation of sintering pressure. The mechanical performance and reliability of sintered assemblies had not yet been fully characterised.
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