EFFECTS OF ACCELERATED STORAGE ENVIRONMENTS ON THE SOLDERABILITY OF IMMERSION SILVER-COATED PRINTED CIRCUIT BOARDS

摘要

A study was performed to assess the physical metallurgy and solderability performance of an immersion Ag finish over Cu on printed circuit boards after exposure to the Battelle Class 2 environment that accelerates a simulated long-term storage conditions. Evaluations included visual inspection of tarnish build-up, Auger depth profile analysis, and a qualitative assessment of the solderability of pads using the 63Sn-37Pb (wt.%) alloy paste. Several storage methodologies were investigated, including the combination of vapor phase corrosion (VPC) paper and sealed moisture-barrier bags as well as unprotected exposure. The degree of surface tarnish was not an adequate indicator of Sn-Pb solderability for immersion Ag coatings. The immersion Ag coating was sufficiently robust that only prudent and reasonable protection measures (e.g., a polypropylene bag) were required for short-term storage or transport intervals. The primary driving force for Cu diffusion and Cu-S formation was the presence of the Class 2 environment above the immersion Ag coating, not solely thermal activation due to elevated temperatures. Copper diffusion and Cu-S formation during accelerated storage was not the sole mechanism responsible for solderability loss of immersion Ag coatings. Rather, degradation of an apparent Ag-C inhibitor layer caused by the long-term exposure to elevated temperature (30°C, 336 hours) was a second source of solderability loss. Void formation in the Sn-Pb solder joints was minimal or was not observed at all. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Dept. of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.