Crack Growth Analysis of Lead Free Passive Component Assemblies

Abstract

This work is aimed at understanding thermo-mechanical failures in lead free solder joints of passive components. More specifically, this work has attempted to identify locations of crack initiation, nature of crack propagation and the rate of crack propagation. For this, R2512, R2010, R1206 and C1206 devices were assembled on 1.6 mm thick FR4 board with SnAg(3.8%)Cu(0.7%) solder paste on chemical Tin and electroless NiAu solder pad finishes and were subjected to a -40°C to 125°C cycle. Samples were removed at selected intervals and the assemblies were cross sectioned to study the crack behaviour, growth rate and crack shape. Assemblies on NiAu pad finish showed a high level of voiding (est. ~ 35% of volume underneath the component termination) in contrast to assemblies on Sn finish (~10-15% of the volume). Crack growth was invariably seen to occur within the bulk of the solder joint and no separation between solder and intermetallic layers was observed. Crack initiation was observed at the device corners for both capacitors and resistors, as well as at the toe of the joint for the resistors. This was in agreement with the results of finite element modeling simulations, which indicated maximum creep strain accumulation at these locations. In all the components, assemblies on NiAu pad finish showed a much higher crack growth rate than assemblies on Sn pad finish.