Thermomechanical Reliability in Packages

Under field-use conditions or under accelerated thermal cycling qualification conditions, the solder joints experience cyclic thermomechanical loads due to GTE mismatch or thermal gradients among various parts of a packaging assembly and failure due to such thermomechanical loads. In addition to thermomechanical fatigue loads, the solder joints experience stresses due to mechanical loads such as vibration, shock, etc. However, such mechanically-induced failures and other chemically or electrically-induced failures are not the focus of this chapter. 

Before discussing the fatigue failure prediction models, a brief overview of the microstruc-tural effects is presented here. The microstructure of Sn-3.8Ag-0.7Cu is shown in Fig. 9(a) (Ref 37). The dark-colored regions represent Sn 

A study (Ref 38) compared fatigue failure modes of lead-free solders to lead-tin solder joints, for a BGA package mounted on an or-gaiuc board. In the study, the assemblies were subjected to thermal cycling until failure, and a comparison of the failure modes for lead-free solder and lead-tin solder was made. For the case of Sn-Pb solder, the crack path lies in the bulk of the solder material near the component body (Fig. 10). For lead-free solders, however, two distinct fracture paths are observed. The first fracture path is similar to that of lead-fin solder passing through the bulk of the solder near the component side. This is shown for SAC solder (Fig. 11) and is also observed for Sn-Ag solder. The second fracture path is characteristic only of the Sn-Ag and SAC solders. This fracture path consists of very fine cracks with multiple fronts, has a shattered appearance, and is seen near both the component and the board sides (Fig. 12). 

Life-Prediction Models for Lead-Free Solder 

Coffln-Manson type equations are commonly used to predict the low-cycle fatigue life of solder. The general form of the Coffin-Manson equation is as follows  

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