Assessing the risk of “Kirkendall voiding” in Cu3Sn

A common location of sporadic, usually unpredictable, premature failures of both SnPb and Pb-free solder joints is within the intermetallic bond to one of the contact pads. Cu pads tend to be less prone to such defects than Ni/Au coated pads, but one phenomenon is a cause of growing concern for high reliability applications. Sporadic intermetallic problems are notoriously difficult to screen for, but at least a number of them are detectable right after assembly. However, the occasional formation of a network of voids within the Cu3Sn layer forming when soldering to Cu pads is rarely noticeable until much later. To make matters worse, efforts to establish practical engineering tests for purposes of identification and screening have usually not been successful.The root cause of the so-called ‘Kirkendall voiding’ has been shown to be associated with the quality of the electroplated Cu, apparently as determined by the incorporation of minute amounts of specific organic impurities. The present work describes and discusses a series of potential tests that allow the sorting of electroplated Cu in terms of its propensity for Cu3Sn voiding during soldering and subsequent aging. The times and efforts involved in these tests differ greatly, as do their effectiveness and accuracy. Individual tests may therefore be useful at various stages from supplier qualification to monitoring of batch to batch variations.