High speed ball pull - A predictor of brittle solder joints

With the increasing popularity of portable electronic devices and industry moving to lead-free soldering, the brittle fracture of solder joints has become a major concern for component as well as system manufacturers. Normally testing of a solder joint´s resistance to brittle fracture is done via drop shock. In addition to being an expensive and time consuming test, the drop shock measurement has multiple variables for multiple joints complicating data analysis and understanding of the failure mechanism. There are several high strain rate test methods such as high speed ball pull, high speed ball shear, impact bend test etc. under evaluation to replace or minimize the drop shock testing. At Cookson Electronics we have carried out extensive studies on high speed ball pull and have used it as a tool to predict the brittle fracture of solder joints. In this paper results of recent studies on lead free alloys soldered to different pad finishes under a number of reflow conditions are presented. Changes in alloy bulk properties as well as growth of the intermetallic layer under aging lead to changes in failure mechanisms. These changes take place at room temperature as well as at high temperature aging. Drop shock data on the same alloy-pad combinations will be presented that shows that high speed ball pull test can be used to predict the brittle fracture failure probability of the solder joints. Failure analysis of the failed samples in ball pull as well as in drop shock shows a clear transition from bulk failure to IMC-pad interface fracture after high temperature aging. The probability of a brittle failure in drop shock shows a clear correlation with fraction of interfacial failures (mode 4) after high temperature aging. Different alloy-pad combinations show different fractions of brittle fractures but a similar correlation between high speed ball pull and drop shock results is observed.