Observations on the Mechanisms of Fatigue in Eutectic Pb-Sn Solder Joints

Abstract

Near-eutectic Pb-Sn solders are widely used for joints in electrical devices. These are liable to failure by thermal fatigue during operation of the device. Since the thermal fatigue load is often in shear, mechanisms of thermal fatigue in shear are of particular interest. Recent research has shown that the thermal fatigue of eutectic solders in shear is preceded by the formation of bands of coarsened material in the eutectic microstructure, which concentrate the deformation and cause the nucleation of fatigue cracks. Such coarsened bands are also observed in isothermal fatigue and unidirectional creep in shear. Since creep experiments are relatively simple to conduct and analyze, these have been used to study the formation and growth of coarsened bands. The mechanism includes three steps: the formation of inhomogeneous shear bands, the onset of recrystallization in the shear band to create a planar region of coarsened material, and the propagation of the coarsened band by progressive recrystallization at its tip. The results are applied to thermal fatigue and some of their implications are discussed.

Isothermal Fatigue Behavior of Sn-Pb Solder Joints

“…The eutectic-rich alloys: 63Sn-37Pb , 50Sn-50Pb and 40Sn-60Pb are shown together in Figure 2 while the Pb-rich 20Sn-80Pb and 5Sn-95Pb alloys are shown in Figure 3. In the eutectic solder joints, a colony structure described previously [14,16] for as-cast near-eutectic Sn-Pb alloys is observed as can be seen in Figure 2(a) and (b). The eutectic regions of the 50Sn-50Pb and the 40Sn-60Pb microstructures shown in Figure 2(c)-(f) is somewhat similar to the as-cast eutectic alloy except that it is coarser and has a less well-defined colony structure with smaller over-all colony size.…”

“…Figure 6(a) shows deformation in a eutectic sample. As has been shown to be the case for creep of eutectic solder joints [16], eutectic solders deform primarily in the colony boundaries under these loading conditions. The "failed" specimen of Figure 6(a) shows a band of concentrated deformation along the bottom of the joint.…”

“…coarsened band [16]. This mechanism involves the nonuniform deformation seen to occur The region ahead of the "crack tip" will the!l recfYst~llize thus propagating the coarsened band until it has traversed the joint.…”

Microstructural development and mechanical behavior of eutectic bismuth-tin and eutectic indium-tin in response to high temperature deformation

“…The coupled zone is symmetric about the eutectic composition, so that growth is within the coupled zone for any degree of undercooling. Also, pro-eutectic 3 phases will only form in alloys of off-eutectic composition. Often, specific " crystallographic relationships exist between adjacent lamellae of the two phases:· Several crystallographic relationships are usually possible between the phases, and any of these may occur in a given system.…”

Creep in Shear of Experimental Solder Joints

“…The mechanism that leads to the formation of such coarsened microstructural regions has recently been proposed (Tribula et al, 1989b). The development of an inhomogeneous shear deformation pattern within the solder microstructure results in regions of concentrated shear within the solder joint.…”