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Reliability Testing and Failure Analysis: Lessons Learned

So how to predict reliability? The goal of thermal cycle testing was to accelerate failures with much shorter cycle times than would be seen in service, either to predict life in service or at least to achieve the same rankings as life in service. Thermal expansion mismatch between component and PCB caused stresses in solder joints and the amount of thermal fatigue per cycle was related to the time of the applied stress and the stress relaxation properties of the joint. The performance of lead-free alloys in thermal cycle testing was more significantly dependent on dwell time than that of tin-lead. SAC alloys appeared better at shorter dwell times, whereas when dwell times were longer, and hence more representative of life in service, tin-lead appeared better. Clearly, alloy selection was critical and Dr. Anselm stressed the importance of making intelligent choices of test conditions and careful interpretation of results in attempting to predict reliability. Moreover, because of crystal orientation effects, there tended to be more scatter on lead-free results, so that failure was less predictable. Drop-shock was a separate issue, and tin-lead had been observed to give better reliability than any of the lead-free solders tested, although there were significant variances between different alloys.

“Mixed alloys” was another issue, particularly in “backwards-compatibility” circumstances where lead-free components had been soldered with tin-lead paste. Reflow temperatures could be kept lower than for all-lead-free to minimize thermal damage to components, but if reflow temperature was too low or paste volume was too small, head-on-pillow defects could result and/or thermal cycling life could be reduced.

The overarching message was to choose the solder alloy carefully to fit the application, particularly to decide whether thermal cycling or drop shock resistance was the priority. Solder considerations aside, mechanical reliability could be greatly enhanced by the use of full underfill or even simple corner bonds.

The higher melting points of lead-free alloys, for example 217 °C for SAC305 compared with 183 °C for 63:37 tin-lead had driven the development of more thermally resistant PCB laminates. These materials already had a whole list of requirements to satisfy: The ability to withstand processing heats and chemicals, the mechanical strength to support components, manufacturability in drilling and machining operations, low thermal expansion, low dielectric constant, plate-ability of the dielectric material, and resistance to the absorption of excessive moisture. With lead-free they also had to withstand more thermally aggressive assembly conditions.

Dr. Anselm described experimental apparatus for monitoring PCB samples under a range of reflow profiles and temperatures, typically 240 to 245°C but maybe as high as 260°C, and it was realistic to expect that in the worst case a double-sided assembly that had been reworked could have experienced up to nine reflow cycles. He showed a whole catalogue of PCB failures after reflow: laminate colour changes indicating material degradation, examples of solder mask damage, surface blistering, many examples of delamination of core layers, prepreg layers and bond interfaces; also examples of inner layer damage, outer layer pad lifting, barrel cracking, and corner cracking. But it was not all bad news--other examples showed no deterioration after nine reflow cycles. Even if the PCB survived the thermal stresses of assembly, the robustness of the different available laminates varied greatly. Pad cratering had become an issue since the introduction of lead-free: Alloys were stiffer and stronger, out-of-plane stresses were higher, together with the tendency for high-temperature laminates to be more brittle. Another issue was copper dissolution, which was much higher in lead-free than tin-lead alloys, such that the thickness of surface-mount pads could be seriously reduced during soldering or rework operations.

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