Microstructural evolution during the thermomechanical fatigue of solder joints.

摘要

Solder joints in electronic packages are electrical interconnections that also function as mechanical bonds. The solder often constrains materials of different coefficients of thermal expansion that, when thermal fluctuations are encountered, causes the solder joint to experience cyclical deformation. Due to the catastrophic consequences of electrical or mechanical failure of solder joints, a great deal of work has been performed to develop a better understanding of the metallurgical response of solder joints subjected to thermomechanical fatigue. This work reviews the microstructural and mechanical evolution that occurs in solder joints during thermomechanical fatigue. The eutectic Sn-Pb solder alloy is highlighted. Unlike most materials that experience thermomechanical fatigue, solder is commonly used at temperatures of up to nine-tenths of its melting point. Therefore extensive creep, solid state diffusion, recrystallization and grain growth occur in this alloy resulting in the evolution of a heterogeneous coarsened band through which failure eventually takes place. Two other solder alloys are compared with the Sn-Pb eutectic, a Pb-rich Sn-Pb alloy and a ternary near eutectic (40In-40Sn-20Pb, all alloys are given in wt. %). The Pb-rich alloy is a precipitated single phase matrix that does not evolve during thermomechanical fatigue and subsequently has a shorter lifetime. Conversely, the 40In-40Sn-20Pb solder is a two phase eutectic in which the microstructures refines during thermomechanical fatigue giving it a longer lifetime than the eutectic Sn-Pb solder. The microstructural processes that occur during thermomechanical fatigue and final fracture behavior are discussed for the three solder alloys. 47 refs., 14 figs.