Electro- and thermomigration induced Cu3Sn and Cu6Sn5 formation in SnAg3.0Cu0.5 bumps

Abstract

Two important trends in the microelectronics business are the development of three dimensional packaging solutions which increase the number electronics components on the same area, and the application of VLSI electronics under harsh environment conditions. Both trends lead to a growing importance of intermetallic compound (IMC) formation in Sn based solder joints. Due to miniaturization a growing part of the solder joint volume is transformed into IMCs and finally the reflow process becomes a transient liquid phase soldering (TLPS) process. For harsh environment applications TLPS enables the transformation of low melting Sn contacts into high melting IMC joints. In both cases a model for the prediction of migration-induced IMC formation is required for the fabrication of IMC joints.For the general prediction of the migration induced IMC formation the related material parameters are needed. Against this background the Cu3Sn and Cu6Sn5 formation was observed during temperature storage tests on Amkor® Package-on-Package packages (12×12mm) with SnAg3.0Cu0.5 ball grid arrays. A mathematical model was developed to calculate the average mass flux of Sn and Cu during the stress tests. Based on the mass flux values the activation energies and diffusion constants for Cu and Sn in Cu3Sn and Cu6Sn5 were determined. Afterwards the temperature storage was combined with an AC and a DC current load to investigate thermo- and electromigration-related phenomena. Based on the IMC formation speed during the AC and the DC tests the heat of transport Q* and the effective charge of the moving ion Z* were calculated. An interpretation of the material parameters is given in consideration of the high defect density in Cu3Sn and Cu6Sn5.