Experimental evaluation of SnAgCu solder joint reliability in 100-μm pitch flip-chip assemblies

The intermetallic compound (IMC) evolution and the thermal-mechanical reliability of Sn-3.0Ag-0.5Cu (SAC305) solder joints were studied using air-to-air thermal shock testing of 100-μm-pitch peripheral-row flip chip assemblies. Flip chips assembled on organic substrates were subjected to air-to-air thermal shock testing between −55 °C and 125 °C, and the samples were removed at regular intervals for cross-sectioning and failure analysis. It was seen that on the die side, after 2000 cycles, all of the (Cu,Ni)3Sn4 had transferred to (Cu,Ni)6Sn5 due to strong cross-pad interaction between the chip-side Ni pad and substrate-side Cu pad, and thus, there was no premature solder cracking possibly due to the absence of dual IMC structure. On the substrate-side Cu interface, the Cu3Sn growth was hindered, and thus there was very little increase in Kirkendall voids in the Cu3Sn after 2000 cycles. Therefore, there was no premature brittle failure in the intermetallic. Failure analysis shows that the cracks in the outermost corner solder joint started to form after 2000 cycles near the chip-side pad, and the cracks propagated in the solder matrix around the IMC like a ring to create solder open. From the experimental data, crack propagation rate equation parameters and characteristic mean life were determined.