Authored By:
D.D. Perovic, L Snugovsky and J.W. Rutter
Department of Materials Science and Engineering
University of Toronto, Toronto, ON, Canada
P. Snugovsky
Celestica Inc., Toronto, ON, Canada
Summary
In view of potential damage to heat sensitive electronic components during soldering at the high temperatures required for some Pb-free solders, such as the SAC alloys, it appears likely that more use will be made of solders based on the much lower melting Sn-Zn eutectic alloy. Consequently, it is of interest to determine what reactions can occur in ternary systems derived from the Sn-Zn eutectic. One such system is Cu-Sn-Zn.
This paper reports a number of experimentally determined reactions that occur in Sn-rich alloys of this ternary system. In the expectation of a possible connection between the Sn-Cu6Sn5 and Sn-Zn binary eutectics, several alloy compositions were selected along a possible joining path. Samples of each were solidified unidirectionally at a very slow rate, approximating freezing with complete mixing in the liquid (the Solidification Path technique). This procedure allows detection of reactions by the change in the phases frozen out with change in composition of the liquid due to segregation.
It is shown theoretically and confirmed experimentally that each of these three quasiperitectic reactions can produce, on a copper substrate, the undesirable effect known as "spalling" in which multiple layers of different phases are formed at the solder-substrate interface during freezing.
Conclusions
1. The Sn corner of the Cu-Sn-Zn alloy system is characterized by the presence of three quasiperitectic reactions, lying on a path joining the two binary reactions.
2. Each of the quasiperitectic reactions is capable of producing the spalling phenomenon when used in making a solder joint on a copper substrate, provided that the solder composition lies within the appropriate quasiperitectic quadrilateral.
3. The fact that all quasiperitectic reactions of this alloy system are capable of producing spalling confirms the conclusion that this reaction is the cause of the spalling effect.
Initially Published in the SMTA Proceedings
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