(V010T11A057)Inhibiting the Re-deposition of AuSn_4 on Au/Ni Metallization Pads by Varying the Accessibility of Cu in Isothermally Aged SAC305 Solder Joints

摘要

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, w here the gold thickness (> 20 pinches) requirement is high for wire bonding. Redeposition of bulk AuSn_4 intermetallic compound (IMC) at Au/Ni contact pads of isothermally conditioned SnAgCu (SAC) solder joints is a critical reliability concern in these packages because the interfacial layer between rcdeposited AuSa_4 IMC and initially formed IMC during reflow at the contact pad after reflow is brittle in nature. Redeposition of bulk AuSat IMC in Pb-free SAC solder joints (most popularly SAC305) is also believed to be dependent on the degree of access to copper. This study examines the effect of varying gold content (2-5 nominal weight-%) in the solder joint and accessibility to copper (by presence or absence of nickel barrier layer on top of Cu plating) on redeposition of AuSn_4 IMCs at the interface of isothermally aged SAC305 solder joints for 720 hours at 121°C (0.8*T_(melt)). The modified lap shear Iosipescu specimens used for the study are divided into two batches: i] In the first batch, 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, 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 accessibility of Cu from the substrate. Representative solder joints from above two batches are then cross-sectioned and analyzed using environmental scanning electron microscopy (ESEM) and energy-dispersive x-ray spectroscopy (EDX) to investigate the composition, thickness and morphology of both bulk and inierfacial IMCs. The first phase to form at the interface of the first batch of specimens after initial reflow is (Ni,Cu)_3Sn_4. During the subsequent solid-state annealing, the redeposition of AuSn_4 occurred in systems plated with Au/Ni on both sides. Contrarily, in the second batch when the solder joint has copper access from one side of the joint, the first intermetallic after reflow to form is (Cu,Ni,Au)_6Sn_5/ (Cu,Au)_6Sn_5 and no redeposition of AuSa_4 is observed after solid state annealing except for the solder joint containing nominal 5wt-% of Au.