Interpreting accelerated test results for lead free solder joints

Abstract

Accelerated testing of microelectronics products and/or specially designed test vehicles may be useless or worse unless the results can be counted on to somehow reflect performance in service. This is obviously the case when it comes to the quantitative assessment of life in long term service. However, it is also important for so-called ‘engineering tests’ intended to compare alternative materials, designs or processes or to compare a product to previous ‘similar’ ones. As far as lead free solder joints are concerned this is more often an issue than commonly recognized. In fact current specs requiring assemblies to pass, for example, a certain number of cycles in a particular test are not necessarily meaningful in the many cases where acceleration factors to realistic service conditions cannot be assumed to be similar for the different solder joints. In general, reliability engineers need to know when ‘best in test’ is unlikely to mean ‘best in service’, how large deviations may be, and which comparisons can be counted on to be conservative. This includes cases where acceleration factors vary with solder volume, pad sizes, pad finishes or process parameters. A mechanistically justified model for the fatigue of lead free solder joints does allow for the generalization of observed trends and should be considered when formulating test protocols and guidelines. Another concern is the effects of the ongoing variations in cycling amplitude common under realistic service conditions. These are not appropriately accounted for in accelerated test protocols. A dependence of the effects on the solder alloy suggests that the sensitivity to these will vary with assembly process parameters and subsequent thermal history as well. The behavior is explained by the effects of cycling on the deformation properties of the solder joints, and recommendations can be made with respect to reliability assessment/optimization and Environmental Stress Screening (ESS). This includes guidelines for vibration testing and combinations of vibration and thermal cycling.