Evaluating the electromigration resistance of lead-free solders by utilising the sharp tip of a Cu trace

Abstract

Owing to the miniaturisation of solder bump connections, evaluations of the electromigration (EM) resistance of various lead-free solders have become increasingly important. In this paper, by utilising the sharp tip of a Cu trace, a simple method including easy sample preparation is proposed for EM resistance evaluation. The sample is prepared by surrounding the Cu traces with solder, and the EM resistance is evaluated by supplying a current between the traces. The current flows in the anode trace and flows from its sharp tip to the solder. The total volume of observed hillocks in the designated area near the tip of the Cu trace per unit time of current supply is used to determine the EM resistance. Rankings of the EM resistance in various solders under the same supplied current and sample temperature are theoretically proven to be invariable, regardless of their values. For testing materials, Sn-3.0Ag-0.5Cu and Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge solders (wt %) are used with 4N (99.99%) purity Sn as a reference. The usefulness of the present method is demonstrated by evaluating the EM resistance of these solders.