Transitioning to Lead-free: the Effect on Low Cycle Fatigue of Contaminating Solder Alloys

As RoHS comes into force, the impact on industry is to force it to struggle with mixed inventory, conventional tin-lead and lead-free. There are many stories circulating in the industry where the label does not correspond with the components. Consequently there is real concern of mixing different alloy systems that will have unpredictable results in terms of low cycle fatigue performance. Some workers have shown that lead at the 1 to 10% level may not be evenly distributed throughout the solder joint (Oliver, et. al., 2002). Segregation of contaminates at the joint level may well result in structural weakness. What is the effect of this on thermal fatigue. The work reported here will describe experiments where the lead level in solder joints was controlled by altering the plating on component terminations and using controlled solder compositions. Microstructural examination verifies the segregation of lead. The built assemblies were then thermally cycled between -55 and 125degC for 2000 cycles to assess this effect on reliability. The work has indicated that there should be few solder joint reliability problems when mixing SnPb and LF components and solder alloys (with Pb contamination in the range 1 to 10%). Very few thermal cycle fatigue failures were experienced other than within two component groups. Ball grid array components did fail generally, in the rings of balls adjacent to the edge of the silicon die within the package. However, the failures in these devices were largely restricted to SnPb alloy dominated systems, i.e. SnPb terminated components soldered with SnPb or SAC alloy solder pastes. Uncontaminated SAC systems or those systems contaminated with low levels of Pb showed fewer failures and thus must be considered more reliable. Indeed, the system showing greatest thermal cycle fatigue in BGA components was the SnPb terminated EGAs with SnPb solder. All other systems were shown to perform better. The other component type to show significant failures, - were the QFP components where failures were confined to Sn-plated components with SAC solder. The process window for SAC alloy soldering is narrower than for equivalent SnPb processing and small additions of Pb may help widen the process window, improving the reliability for these soldering batches. It is therefore probable that these differences in QFP component reliability are batch related. Hot peel tests were also run to simulate problems that may occur is secondary wave operations where the fillet strength collapses and components can detach with little force at temperatures above 180degC This paper will discuss these results and the likely impact on the industry and the necessary precautions