Control Of Spalling in SAC Pb-Free Solder Alloys When Used with a Ni Substrate
Authors: L. Snugovsky, et al. Company: Department of Materials Science and Engineering - University of Toronto and Celestica, Inc. Date Published: 5/22/2009
ICSR (Soldering and Reliability)
Abstract: A basic feature of the formation of a solder joint is the occurrence of a single layer of intermetallic compound between the solder and the substrate on which the joint is made. However, under some conditions, such as repeated reflows, when a solder of the Pb-free SAC type is used with a nickel (or Ni(P)) substrate, a second layer of intermetallic compound is formed in the solder, near to but separated from the layer that is adjacent to the substrate. The appearance of this second layer of intermetallic compound is called “spalling” because it gives the impression that it was initially formed on the substrate and later displaced from that location by the introduction of a second intermetallic layer on the substrate. By consideration of the possible reactions that can occur during cooling of an SAC solder to which is added a small amount of Ni it is shown in this paper that spalling is accounted for as a nonequilibrium effect associated with a quasiperitectic reaction of the Cu-Ni-Sn alloy system. It is shown that the occurrence of spalling can be delayed to longer liquid holding times by an increase in the Cu content of the solder to a value greater than 0.6wt%. Attempts to control spalling by increasing the cooling rate of the solder to values as high as 4.7oC per second were not successful.
Spalling; SAC solders; Ni substrate; quasiperitectic reaction