An Approach for Assessing the Long-Term Reliability of Lead-Free Electronics Under In-Field Conditions

For many microelectronic products, the long-term in-field life may be limited by thermal-mismatch-induced fatigue, a factor generally assessed by accelerated thermal cycling and extrapolation of the test results to in-field use conditions. For this purpose, thermal cycling tests are accelerated strongly by the use of larger temperature ranges, shorter dwell times at the maximum and minimum temperatures, and often, higher ramp rates than encountered in-field. The sensitivities of common lead-free solder joints to these factors and their interactions with a range of designs, materials, and process parameters are very different from those of the Sn-Pb joints that have been in use for decades. Systematic research efforts are currently aimed at the establishment of new test protocols and acceleration factors; such work has already shown lead-free solder joints to be much more sensitive to the dwell times in testing, but extrapolations of this dependence to in-field conditions remain complicated by interactions with design parameters, as well as with the more obvious effects of cycling parameters. Neural network analysis offers important advantages for the recognition of patterns, correlations, and trends among empirical accelerated test results, but it usually does not lead to the kind of mechanistic understanding required for extrapolations. Nevertheless, as part of a larger effort to develop tools for the ongoing analysis and generalization of major databases, we identified a systematic trend that should be useful in life assessment. This is particularly important because it contradicts predictions, based on widely used models, of a more problematic behavior. In short, we shall suggest a practical approach to the assessment of the effect of time at operating temperature on the life of lead-free solder joints in-field.