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: 2/12/2009
Pan Pacific Symposium
Abstract: A study was performed to determine the solderability performance of an immersion Ag finish on Cu after accelerated storage aging times 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 those coupons protected by the VPC inhibitor paper and 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 layers of Cu-S and Cu-Cl formation could not be removed by the RMA flux. The immersion Ag coating on the PCB test vehicles reacted to the Class 2 environment, whether protected or not, in a manner similar to that of the solderability test coupons. 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) that is often required of LOPB directives for high-reliability electronics.