Sensitivity of Copper Dissolution to the Flow Behavior of Molten Sn-Pb Solder
Authors: P. T. Vianco, J. A. Rejent, A. C. Kilgo, and S. E. Garrett Company: Sandia National Laboratories Date Published: 6/1/2015
Abstract: As printed circuit board (PCB) feature sizes continue to become smaller, it is critical to understand the details of the Cu dissolution process that accompanies the use of solder fountain and wave processes. In response to an application in which surface mount pads experienced unexpected dissolution by the outflow of molten tin-lead (Sn-Pb) alloy from a solder fountain, a study was initiated to examine the spatial characteristics of Cu dissolution caused by flowing Sn-Pb solder at 271°C. Initial trials with Cu coupons yielded a dissolution rate of 0.54 ± 0.33 microns/s. But, the data displayed too much scatter to fully capture important spatial effects. Therefore, PCB test vehicles having a Cu pad pattern were exposed to flowing solder for 20, 60, or 90 s. The data indicated that dissolution occurred on two length scales: (a) a large scale comparable to that of the solder fountain and (b) a small scale similar to the size of each Cu pad. The dissolution rate was separated into two values based on pad location relative to the stagnation point:
Inner pads (near to the stagnation point): 0.31 ± 0.04 microns/s
Outer pads (distant from the stagnation point: 0.53 ± 0.04 microns/s
There were no indications of pad lifting. The appearance of the remnant solder film did not provide an indication of the degree of Cu pad loss underneath it, including any dewetting caused by the local loss of entire pad thickness. This study provides important insight into the dissolution behavior of flowing molten solder as a critical consideration towards the development of a successful assembly process.