Eutectic orientation relationships in Sn-Bi solder

The Sn-Bi eutectic is used increasingly as a low temperature solder (LTS) in electronics manufacturing. Here, we measure eutectic orientation relationships (ORs) in Sn-58Bi and compare the ORs formed during solidification of undercooled solder balls with those from unidirectional solidification. In both solidification conditions, three ORs formed, each involving { 01 1 ̅ 2 } as the main interface planes in (Bi) with different interface planes in β-Sn. In each OR, the eutectic bismuth was commonly twinned, with multiple (Bi) orientations related to each β-Sn orientation which is explained in terms of the orientation variants of the ORs and the twin variants in (Bi). It is found that the dominant eutectic OR changes as the nucleation undercooling becomes deeper in solder balls which is discussed in terms of the effects of undercooling on the fraction of tin dendrites and the eutectic growth velocity. The ability to change the dominant eutectic OR may provide a pathway to control interfaces between the β-Sn and (Bi) phases which play an important role in the reliability of Sn-58Bi joints. • Two Bi twins and three orientation relationships (ORs) formed in Sn-Bi eutectic. • A single OR with different variants occurred by variants of Bi twins. • One Bi twin occurs in high frequency to maintain a small misorientation in OR. • The preferred OR is selected by the interface velocity. • The eutectic OR changes as a function of melt undercooling.