PWB solder wettability after simulated storage

A new solderability test method has been developed at Sandia National Laboratories that simulates the capillary flow physics of solders´ on circuit board surfaces. The solderability test geometry was incorporated on a circuit board prototype that was developed for a National Center for Manufacturing Sciences (NCMS) program. The work was conducted under a cooperative research and development agreement between Sandia National Laboratories, NCMS, and several PWB fabricators (AT&T, IBM, Texas Instruments, United Technologies/Hamilton Standard and Hughes Aircraft) to advance PWB interconnect technology. The test was used to investigate the effects of environmental prestressing on the solderability of printed wiring board (PWB) copper finishes. Aging was performed in a controlled chamber representing a typical indoor industrial environment. Solderability testing on as-fabricated and exposed copper samples was performed with the Sn-Pb eutectic solder at four different reflow temperatures (215, 230, 245 and 260°C). Rosin mildly activated (RMA), low solids (LS), and citric acid-based (CA) fluxes were included in the evaluation. Under baseline conditions, capillary flow was minimal at the lowest temperatures with all fluxes. Wetting increased with temperature at both baseline and prestressing conditions. Poor wetting, however, was observed at all temperatures with the LS flux. Capillary flow is effectively restored with the CA flux