Effects of Cycling Parameters on the Thermal Fatigue Life of Mixed SnAgCu/SnPb Solder Joints

Abstract

The mandated switch of the overwhelming part of microelectronics assembly to lead-free soldering has left the manufacturers of many high reliability products, which are still exempt from the requirement to change, with a choice between imperfect alternatives. One of these involves the inclusion of lead-free BGAs and CSPs in a eutectic SnPb solder paste based reflow process. In order to assess the consequences of this for reliability it is not enough to subject samples to common accelerated tests. Mixing of the alloys is almost certain to affect the acceleration factors so that comparisons of test results may not reflect relative performances under service conditions. This was illustrated with for a set of model BGA components with 20 mil (0.5 mm) diameter SAC305 solder balls, which were reflow soldered onto printed circuit boards using a eutectic SnPb solder paste and tested in thermal cycling together with controls soldered with a SAC305 paste. Mixed joints were found to outperform pure SAC305 in −40/125°C tests but not in 0/100°C, and general trends suggest that mixed joints may compare more poorly in service.