Reliability Investigation of Mixed BGA Assemblies

The effect of different reflow profiles on the reliability of lead-free (LF) Sn-3.0 Ag-0.5 Cu (wt.%) (SAC 305) ball grid array (BGA) devices assembled with a SnPb eutectic paste was investigated. The memory modules in a back-to-back configuration were reflowed on standard graphic cards finished with immersion silver (IAg) or hot air solder leveling (HASL) coatings. The reflow peak temperatures ranged from 209 degC to 227 degC, while the time above liquidus (TAL) varied from 45 to 80 s. Depending on the reflow conditions, the solder interconnects displayed varied degrees of SnPb and LF solders intermixing. It was established that in order to receive a homogeneous solder alloy, the reflow peak temperature had to be in the 218 degC-222 degC range. The reliability of solder interconnects of memory modules was assessed by subjecting the cards to 1500 cycles of accelerated thermal-cycling with a profile from 0 degC to 100 degC. It was found that the control SnPb/SnPb assemblies displayed superior reliability to that of the mixed assemblies. Regardless of the degree of homogeneity of the BGA balls, the predominant failure mode of the mixed solder joints was interfacial cracking through a Pb-rich phase near the intermetallic layer. In contrast, only partial cracks propagating diagonally through the bulk solder were present on the control boards. It was concluded that a combination of state of stress and segregation of the Pb-rich phase at the interface was responsible for the shortened thermal-mechanical fatigue life of the mixed solder interconnects