EFFECTS OF ACCELERATED STORAGE ENVIRONMENTS ON THE SOLDERABILITY OF IMMERSION SILVER-COATED PRINTED CIRCUIT BOARDS
Authors: Paul Vianco, Edwin Lopez, R. Wayne Buttry, Alice K Company: Sandia National Laboratories Date Published: 9/24/2006
Abstract: 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 moisturebarrier 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.