Abstract:

This study investigates the effect of varying gold content between three and eight percent by weight (wt%) in the solder joint and accessibility to copper (by presence or absence of nickel barrier layer on top of Cu plating) on the deposition of AuSn4 intermetallic compounds (IMCs) at the interface of Sn3.0Ag0.5Cu (SAC305) solder joints that are isothermally aged for 720 hours at 121°C (0.8 × Tmelt). Two types of modified lap shear Iosipescu specimens are used for this study: i] In Type I, two copper platens were electroplated with Au and a barrier underlayer of Ni, and then soldered together with SAC305. ii] In Type II, one Cu platen was electroplated with Au and a Ni barrier layer and then soldered with SAC305 to another platen that was electroplated only with copper (no Nickel barrier layer), to allow the solder access to Cu from the substrate. The first phase to form at the interface of the Type I specimens after initial reflow was Ni3Sn4/ (Ni,Cu)3Sn4. During subsequent solid-state annealing, AuSn4 IMC deposited at the interface from the bulk in Type I specimens. In contrast, in Type II specimens, where the solder material had copper access from one side of the joint, the first intermetallic after reflow to form was (Cu,Ni,Au)6Sn5/ (Cu,Au)6Sn5 and no deposition of AuSn4 was observed after solid state annealing at any Au content below 8 wt% of the solder content. Three different configurations (Type I joints with 6 wt% of Au, Type II joints with 6 wt% of Au, and Type II joints with 5 wt% of Au) were selected for characterization of mechanical durability and creep constitutive response. Cyclic isothermal mechanical durability tests were conducted at room temperature at a strain rate of approximately 5.5E-2/s. Type II joints were found to be more durable than Type I joints and within Type II joints, the durability was found to decrease as the Au wt% increased. Subsequent failure analysis on cycled specimens showed mostly ductile failure in Type II joints upto 5 wt% of Au, whereas a combination of ductile and brittle failure was observed in solder joints containing 6 wt% of Au, due to the presence of large brittle AuSn4 platelets in the bulk. Creep tests were also performed at constant stress levels between 5 MPa to 15 MPa at room temperature on the above three plating configurations. The difference in creep resistance between the above three configurations was not found to be statistically significant at the tested stress levels at room temperature. Main motivation behind the study is the usage of lap shear modified iosipescu specimens which were used to characterize the material properties (elastic, creep, hardness) and fatigue life characterization of microscale SAC305 solder specimens with varying wt% of Au dissolved in the solder joints and different IMC configuration at the interface due to presence/absence of Ni barrier layer at one side.

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