Effect of gold and copper on microstructural evolution and mechanical durability of SAC305 solder joints

Electroplated Ni/Au over Cu is a popular metallization for printed circuit board (PCB) finish as well as for component leads, especially for wire-bondable high frequency packages, where the gold thickness requirement is high (≥ 20 μinches) for wire bonding. This study examines the effect of redeposition of bulk AuSn₄ IMC as a function of varying gold content (2-5 nominal weight-%) in the solder joint and copper access (by presence or absence of nickel barrier layer on top of Cu plating) on the cyclic mechanical durability of isothermally aged SAC305 solder joints. The specimens are divided into two batches: i] in the first batch (Type I), both the copper platens to be soldered are electroplated with Au and Ni. Ni barrier layers are used to completely stop the solder from accessing the Cu in the substrate; ii] in the second batch (Type II), one Cu platen is electroplated with Au and Ni barrier layer but the other platen is electroplated only with copper (no nickel layer), to allow access to Cu from the substrate. The first phase to form at the interface of type I specimens after initial reflow is Ni₃Sn₄/ (Ni,Cu)₃Sn₄ and type II specimens is (Cu,Ni,Au)₆Sn₅/ (Cu,Au)₆Sn₅. During the subsequent solid-state annealing for 1 month at 120°C, AuSn4 IMCs redeposited on top of initial IMC formed after reflow in type I joints, whereas there is no redeposition observed in type II specimens up to solder joints containing nominal 4 wt-% of Au. Next, the effect of redeposition on cyclic mechanical durability of three different configurations (type I joints with 4 wt-% of Au and type II joints with 3 & 4 wt-% of Au) are characterized at room temperature at a strain rate of approx. 5.5E-2/s using a custom-built thermo-mechanical micro-scale (TMM) test system. Type II joints are found to be more durable than type I joints and within type II specimens, joints - ontaining 3 wt-% of Au are more durable.