Single current source electromigration system and its application to copper pillars with tin based solder

Abstract

The trend towards finer pitch interconnections in current generation microelectronic packaging, coupled with the introduction of 3D chipstacks and high power ICs operating at low voltage have all contributed to increasing current density in conductors to the extent that electromigration is a threat to long term reliability. Traditional electromigration (EM) test systems were designed for back end of line (BEOL) IC interconnect with dimensions of micrometres or less and so typically were limited to a total current of 25 mA per device under test (DUT). However for packaging applications, higher currents, 100mA to 1A, are typically needed to provide sufficient current density to generate electromigration failures. Because the traditional EM test systems use dedicated source-measure units for each individual DUT, they are expensive and cannot easily be modified to provide higher currents or to support different package types. In this paper we describe an alternative approach to package EM testing in which a single current source supports many DUTs while the resistance measurements are obtained using a separate multimeter accessing the DUTs by means of a software controlled switch matrix. This provides a flexible, easily reconfigurable system at a cost of approximately 10% that of a traditional system. Since many reliability test labs already have many of the components needed the cost can be reduced still further.