ELECTROMIGRATION STUDIES OF FLIP CHIP BUMP SOLDER JOINTS
Authors: Scott Brandenburg Company: Delphi Delco Electronics Sys. Date Published: 8/23/1998
Surface Mount International
Abstract: This paper describes investigations into the effects of electromigration and thermomigration on flip chip solder bumps. Among the variables studied were bump temperature, bump current, and UBM (under bump metallurgy) geometry. The test assemblies consisted of a standard Delphi Delco Electronics test flip chip fabricated with eutectic Sn-Pb bump metallurgy mounted to FR-4 substrates. To induce electromigration, the assemblies were tested at two steady state DC current levels of 1.25 and 0.625 amps per bump; and two steady state I.C. junction temperature levels of 150C, and 175C. Metallurgical analysis of the solder bump identified metal migration as the primary failure mechanism. An examination of the relationship between flip chip bump life to current density and bump temperature resulted in a mathematical model based upon Black’s equation. Activation energies determined from the data resulted in a model with excellent correlation to actual life data. The model was sufficient to develop design guidelines given inputs of bump operational parameters and environmental conditions.