Improving Fatigue Life in Near-Eutectic Sn Pb Solders [Dissertation]
- Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
Due to technical advances in packaging technology, solder joint thermal shear fatigue failures pose reliability concerns and limit current packaging designs in the electronics industry. Past work has shown that thermal fatigue failures, as well as isothermal fatigue and creep failures, in the commonly used near-eutectic Sn-Pb solders are preceded by the formation of a band of coarsened material. Examination of crept and fatigued eutectic solder joints shows that shear deformation concentrates along relatively soft heterogeneities, the eutectic grain and colony boundaries, in the as-cast microstructure which happen to line up into long straight bands parallel to the applied shear strain. Improvement of the fatigue properties requires an elimination or, at least, a reduction in the length of these long straight boundaries. Several methods of doing that are proposed. These methods basically fall into two groups: one involving homogenization of the as-cast microstructure aimed at eliminating these heterogeneities and the other involved with breaking up the as-cast microstructure in such a way that long straight boundaries do not form parallel to the direction of shear. To study the effect of homogenization of the microstructure, room temperature aging of solder joints was studied. Softening was found to occur over a period of about 40-50 weeks levelling off at about 70-75% of the initial strength (31 MPa). Microstructural changes noticeable after about 40 weeks increase the elongation to failure during shear testing and improve resistance to fatigue. The effect of breaking up the as-cast microstructure was studied by comparing various off-eutectic alloys. The alloys studied consisted of Pb with 40 and 50 wt.% Sn in addition to the near-eutectic 63Sn-37Pb alloy. The proeutectic particles in these off-eutectic alloys were shown to spread out the deformation sufficiently to prevent the formation of long straight shear bands. As expected, the fatigue life of these off-eutectic alloys was significantly greater than the eutectic even though all three failed by formation of a coarsened band when tested at 75°C with strain ranges between about 5 and 10%. The 40Sn-60Pb alloy, which is further off eutectic and therefore has more of the beneficial proeutectic particles than the 50Sn-50Pb alloy, was less resistant to fatigue failure. It was concluded that the interdendritic, eutectic regions in the 40Sn-60Pb alloy had a structure, for reasons which are proposed, promoting heterogeneous deformation than those in the 50Sn-50Pb alloy. In addition, the Pb-rich 20Sn-80Pb and 5Sn-95Pb alloys were studied in fatigue at 75°C. Both alloys showed evidence of locallized recrystallization ahead of the fatigue crack and failed intergranularly at all strain ranges tested between about 5 and 10%.
- Research Organization:
- Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Office of Energy Research (ER); USDOE Office of Science (SC), Basic Energy Sciences (BES)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 5555245
- Report Number(s):
- LBL--30605; ON: DE91013844
- Country of Publication:
- United States
- Language:
- English
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