Microstructural observations of the tin-lead solder/copper system and thermal fatigue of the solder joint

When an electronic package encounters thermal fluctuations, cyclical shear strain is imposed on the solder joint interconnections. The thermal cycling leads to a condition of thermal fatigue and eventual solder joint failure. This study was performed in order to understand the microstructural mechanisms that lead to solder joint failures in thermal fatigue.; The solders studied were 5Sn-95Pb and 60Sn-40Pb joined to Cu. The precipitation process of {dollar}beta{dollar}-Sn in 5Sn-95Pb was fully characterized. On fast cooling from single phase {dollar}alpha{dollar}-Pb the Sn precipitates homogeneously. On slow cooling the precipitation process is cellular. Both precipitate structures overage and soften rapidly. The 60Sn-40Pb had a two phase microstructure. The Sn-rich phase was interconnected and polygranular. Both phases, and Sn grains, coarsened with time at 125{dollar}spcirc{dollar}C. Coarsened 60Sn-40Pb microstructure was deleterious to the mechanical properties. The 5Sn-95Pb solder on Cu formed a single phase columnar interfacial intermetallic. The 60Sn-40Pb on Cu formed columnar Cu{dollar}sb3{dollar}Sn adjacent to the Cu and rod-like Cu{dollar}sb6{dollar}Sn{dollar}sb5{dollar} adjacent to the solder. Failures initiated and propagated through interfacial intermetallics only at extremely fast strain rates. Interfacial intermetallics had no influence on thermal fatigue properties. Cu{dollar}sb6{dollar}Sn{dollar}sb5{dollar} {dollar}eta{dollar}-phase whiskers formed at the intermetallic solder interface. The whiskers break off into the molten solder, the dislocation core dissolves, resulting in hexagonal tubes of Cu{dollar}sb6{dollar}Sn{dollar}sb5{dollar} in the bulk solder.; Thermal cycling tests were performed on both 5Sn-95Pb and 60Sn-40Pb joints using a {dollar}-55spcirc{dollar}C to 125{dollar}spcirc{dollar}C cycle and 19% imposed shear strain. Failures in 5Sn-95Pb occurred at Pb grain boundaries. Isothermal fatigue tests on 5Sn-95Pb give similar results when compared to thermal fatigue. A heterogeneously coarsened region of both Pb and Sn-rich phases develops within the 60Sn-40Pb solder joints. Cracks initiate and propagate through the heterogeneously coarsened region. Cracks initiate at the Sn-Sn grain boundaries due to their inability to accommodate the strain. Heterogeneous coarsening and failure occurs in high (35{dollar}spcirc{dollar}C to 125{dollar}spcirc{dollar}C) and low ({dollar}-55spcirc{dollar}C to 35{dollar}spcirc{dollar}C) thermal cycles. Isothermal fatigue cannot be compared to thermal fatigue results of 60Sn-40Pb on Cu because heterogeneous coarsening in isothermal fatigue occurs to a lesser extent than in thermal fatigue.