ICSR (Soldering and Reliability) Conference Proceedings

Reactions In The Sn Corner Of The Cu-Sn-Zn Alloy System

Authors: D.D. Perovic, L Snugovsky, J.W. Rutter and P. Snugovsky
Company: University of Toronto and Celestica Inc.
Date Published: 5/17/2010   Conference: ICSR (Soldering and Reliability)

Abstract: 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. Three quasiperitectic reactions (U1-U3) and a ternary eutectic reaction were found:
U1: L+Cu6Sn5 <--> CuZn+Sn
U2: L+CuZn <--> Cu5Zn8+Sn
U3: L+Cu5Zn8 <--> CuZn4+Sn
E: L <--> (Sn)+(Zn)+CuZn4

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.

Key words: Lead-Free solder, Cu-Sn-Zn system, quasiperitectic reaction, microstructure

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