EFFECTS OF STORAGE PROCEDURES AND BAKE OUT ON THE SOLDERABILITY OF IMMERSION SILVER-COATED PRINTED CIRCUIT BOARDS
Authors: Paul Vianco, et al. Company: Sandia National Laboratories Date Published: 4/30/2009
Abstract: A study determined the solderability performance of an immersion Ag finish on Cu after accelerated storage aging that simulated 7 – 10 year shelf lives for life-of-program-buy (LOPB) applications. The aging conditions included exposure to the Battelle Class 2 environment for time periods of 33.6, 168, and 336 hours. Test assets were also exposed to a bake out treatment of 100°C, 8 hours, in air. Laboratory solderability test performance was based upon the contact angle, ?C, and wetting rate metrics. Solderability remained excellent for surfaces protected by vapor phase corrosion (VPC) inhibitor paper and a moisture barrier bag during the Class 2 exposures. When unprotected, the loss of solderability was attributed to Cu diffusion to the surface that was driven by the H2S and Cl2 components of the Class 2 environment. The resulting Cu-S and Cu-Cl layers could not be dissociated by the RMA flux. The immersion Ag coating on the PCB test vehicles reacted to the Class 2 environment in a similar manner. Visual inspection and cross section analysis of the PWAs that were built after exposure to the Class 2 environment (protected) followed by the bake out procedure (100°C, eight hours) exhibited excellent solderability as did those PCBs that were exposed only to the bake out step. The use of VPC inhibitor paper and moisture barrier bags provides an effective means to assure the solderability of immersion Ag-coated PCBs following long-term storage (7 – 10 years).