Dependence of Solder Joint Reliability on Solder Volume, Composition and Printed Circuit Board Surface Finish

Thermal fatigue and room temperature isothermal mechanical performance of various Pb-free and SnPb solder joints were examined. Various solder alloys doped with Ni (SN100C) and Mn (SAC105-Mn and SACM) were evaluated and compared to SAC105, SAC 205 SAC 305 and SAC 405 and eutectic SnPb alloys. Solder spheres ranging from 10 to 20 mils were reflowed on various printed circuit board (PCB) surface finishes such as copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG). The mechanical behavior of these solder joints was evaluated in low speed and high speed shear tests, and also in shear fatigue test. The effect of isothermal aging was examined. Custom made ball grid array (BGA) packages with the same alloys and sizes were tested in accelerated thermal cycling (ATC) test. The results from room temperature mechanical tests were correlated to package level ATC results obtained for the thermal cycling profile of -40/125 degrees C. The effect of solder volume and composition on the solidification temperature of each solder joint was carefully measured by differential scanning calorimeter (DSC). Precipitate sizes and distributions were analyzed using backscattered scanning electron microscopy (SEM). Sn grain morphology was characterized by polarized light microscopy (PLM) and electron backscatter diffraction (EBSD). Investigation of the lifetimes of various solder joints in room temperature fatigue and accelerated thermal cycling tests showed distinct dependences of lifetime on solder composition. Distinct increases in lifetimes with increases in Ag content were observed. Results suggested the recrystallization and failure mechanism in Pb-free solder joints are strongly affected by Ag3Sn precipitates. Combination of Cu/ENIG surface finishes generally resulted in some improvement in thermal fatigue performances. Results also showed that solder volume can greatly affect the microstructure and performance of SAC solder joints in mechanical and ATC tests. Larger samples generally solidified at higher temperatures and revealed different Sn grain morphologies than smaller samples, which generally undercooled more. Addition of dopants generally reduced the undercooling, resulting in different solder joint microstructures. The effect of variation in solder composition and volume and PCB surface finish on solder joint microstructure and lifetime was carefully evaluated.