Aging Studies of PBGA Solder Joints Reflowed at Different Conveyor Speeds

PBGA packages are more susceptible to solder joint fatigue problem due to the significant mismatches of the coefficient of thermal expansion (CTE) between bismaleimide triazine substrate (∼15ppm/ºC) and silicon die (∼2.5ppm/ºC). In this paper, the shear cycle fatigue properties of PBGA (Topline DC169) solder joints reflowed with different profiles, and aged at 125ºC for 4, 9, 16, 25, and 36 days are studied. The profiles are devised to have the same “heating factor”, which is defined as the integral of the measured temperature above the liquidus (183ºC) with respect to dwell time in the reflow profile, but to have different conveyor speeds. The effects of conveyor speed on the solder joint (non-aged and aged) fatigue lifetimes are investigated. It is found that with increasing the conveyor speed the solder joint shear fatigue lifetime can be improved substantially. Moreover, the shear fatigue lifetimes of aged specimens decrease with extending aging time and the lifetime variation of faster conveyor speed specimens is larger. From our previous work, the “heating factor” can characterize the reflow profile in the melting section. Using the theory of heat transmission, it can show that solder joint cooling rate during solidification increases with increasing conveyor speed. SEM and optical micrographs show that faster cooling rate causes a rougher interface of the bulk solder/IMC and the refinement of the grain size in the solder joints. The thickness of IMC increases with increasing aging time, and the growth rate of samples with faster cooling rate is larger. SEM cross-section views reveal that cracks initiated at the point of the acute angle near the solder pad, then propagated along the interface of the bulk solder/IMC layer. The variation in the roughness of the bulk solder/IMC interface is likely to be responsible for the fatigue lifetime difference of non-aged solder joints. On the other hand, since the growth of IMC is a diffusion-controlled process, i.e., tin diffuses into copper, and the diffusion activation energy in the grain boundary region is far smaller than that in the lattice region, so diffusion is most likely to be occurred in the coarse grain boundary region. Hence, the IMC growth rate of samples with faster cooling rate is larger and the growth of IMC layer decreases the fatigue lifetimes of aged solder joints.