Wheatstone bridge method for electromigration study of solder balls in flip-chip packages

In this paper, we describe a new approach and a new system developed for EM tests of solder balls in a packaging assembly. The approach was based on the Wheatstone bridge method, which provided significant improvement in the sensitivity for detecting EM damage in solder balls. In the bridge circuit, each of the arms can contain an ensemble of solder balls connected in series and/or in parallel to increase the number of solder balls being tested. Thus the method can be used for EM tests of a large ensemble of solder balls, extending the range of statistical detection of early failures and reducing EM test time. Using this method, EM tests were performed at 165° and 235°C on 97Pb-3Sn solder balls in ceramic flip-chip packages. The results yielded an activation energy of 0.85 eV. By extending the test time to 2000 hrs at 165°C, all of the test circuits except one showed resistance saturation, suggesting the existence of a threshold jL_c product. Subjected to a maximum resistance change of about 15 mΩ, the jL_c product was estimated to be 110 A-cm for 97Pb-3Sn solders at 165°C.