Solder fatigue failure studies

Continuous flexing of a PBA will fracture component leads and, more important, surface-mounted component solder joints, due to mechanical fatigue failure. (Mechanically induced flexing or vibration in assembled PBs is used under controlled conditions to induce failures in solder joints for quality and reliability studies.)... 

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). 

Frear, D. Grivas, D. Morris, J.W. A microstructural study of the thermal fatigue failures of 60Sn-40Pb solder joints. J. Electron. Mater. 1988, 17, 171-180. 

Once failure mechanisms were identified, solder joints were rated according to fatigue levels of solder joints (Figure 1). For this study, failure mechanisms were grouped as either component failure, design failure, or alloy failure. 

The alloy, Sn-3.4Ag. 8Bi, developed and patented by Sandia National Laboratories, was determined in the NCMS Lead-Free Solder Project to exhibit outstanding fatigue properties for surface mount applications under both accelerated thermal cycle test conditions used in the NCMS study. Sandia performed ATC testing of this alloy to 10,000 cycles for 0-100°C with 10°C/min ramp rates and 5-min dwell times [9,10]. There were no electrical failures at end of test (10,000 cycles) for 68 I/O PLCCs, 241/0 SOICs, and 1206 chip capacitors on FR4 boards, no cracks after 5000 cycles, and only minor surface cracks after 10,000 cycles. The Sn-3.4Ag-4.8Bi alloy has demonstrated considerable promise for use in surface mount applications, exhibiting greater fatigue resistance than eutectic Sn-Pb and most other lead-free alloys. It should be... 

---