COMPARISON OF THE SOLDERABILITY PERFORMANCES OF INHIBITOR CONTAINING AND INHIBITOR-FREE IMMERSION SILVER COATINGS
Authors: Edwin P. Lopez, Paul Vianco, Sam Lucero, and R. Wayne Buttry Company: Sandia National Laboratories Date Published: 1/31/2007
Pan Pacific Symposium
Abstract: Solderability tests were performed on Cu coupons coated with an immersion Ag finish. The immersion Ag coatings were either with or without a co-deposited, organic inhibitor. The effect of inhibitor presence was investigated with 63Sn-37Pb (wt.%) solder. Test temperatures were 245°C and 260°C. The flux was a rosin-based mildly activated (RMA) solution. Solderability tests were performed on coatings in the as-fabricated condition as well as after exposure for 8 hours and 16 hours in a Battelle Class 2 environment. The contact angle and wetting rate data demonstrated the requirement that the immersion Ag coating have an inhibitor in order to assure adequate shelf life performance. The loss of solderability by the non-inhibitor containing finishes was due to a combination of oxidation, chlorination, and sulfidation (tarnish) of the Ag surface. Weaker fluxes are sensitive to even modest changes in the solderability of immersion Ag finishes. The solderability of inhibitor-containing immersion Ag coatings was evaluated for a matrix of aging times (2 or 4 hours) and temperatures (72°C or 93°C) representing potential, pre-assembly circuit board pre-conditioning treatments. For both the 63Sn-37Pb and 95.5Sn-3.9Ag-0.6Cu Pb-free solders, the contact angles and wetting rates were not degraded by the pre-conditioning treatments. Similar trends were observed between RMA and low-solids fluxes. These results suggested that the driving force for Cu diffusion could be narrowed down to the Cl and S contents of the environment rather than simply oxidation. Key words: Solderability, immersion Ag, storage environments, inhibitors.