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Immersion Silver as Universal Surface Finish for COB Technology
Keywords: immersion Ag, universal metallization, COB
Immersion Ag is currently being discussed as an innovative alternative to Ni/Au and Ni/Pd/Au metallization for PCBs in COB technology. Its advantages over conventional metallizations include its non-toxicity, due to the absence of Ni, and significantly lower costs, as Au and Pd are not required. In addition, immersion Ag is easily processed in die attach or SMD processes using adhesives or solder while at the same time being compatible with wire bonding. The first generations of immersion Ag in the 1990s were handicapped by a lack of storage capability. The Ag quickly reacted with oxygen and sulfur in the surrounding atmosphere and, after only a few hours of storage, high quality wire bonding was no longer viable. The current generation of immersion Ag was primarily developed to ensure long-term processability for solder technology. However, its suitability for wire bonding and other COB processes, such as glob topping, after periods of storage in non-inert atmospheres had not yet been verified. This paper addresses this issue, presenting the results of wire bonding tests in initial state and after periods of storage in inert and non-inert atmosphere, as well as after die attach processes using adhesives and solder. In addition, the suitability of immersion Ag for die attach with adhesives and solder, as well as processability for glob-top encapsulation, was investigated. Finally, the results from reliability tests on open and encapsulated samples are presented. The results show that wire bonding on immersion Ag yields bonds of high initial quality and superior reliability even after storage in non-inert atmosphere for several months prior to wire bonding. The same is true of samples that had been exposed to soldering or gluing processes. In addition, immersion Ag performed exceptionally well in other COB processes, confirming its suitability as a universal surface finish for COB technology.
Jens Goehre,
Fraunhofer IZM, Berlin, Germany
Berlin 12489,

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