Investigation on lead-free solder joint reliability of edge-bonded CBGA under temperature cycling

The use of edge-bond adhesive is recently found to be a possible alternative to underfill for solder joint protection. This paper investigates the effectiveness of using it for the enhancement of solder joint thermal fatigue reliability. Both the accelerated temperature cycling (ATC) test and simulation are performed for the study. The test vehicle is a 51×51 mm ceramic ball grid array (CBGA) package. There are three batches of samples prepared. One of them utilizes no edge-bond and the other two adopt different edge-bond adhesives. Opposite results, surprisingly, are obtained from the two edge-bonded cases. It is observed that the use of edge-bond can either enhance or degrade the solder joint reliability. The simulation results agree with those from the experiments. This suggests that there are controlling factors for the phenomenon observed. Through an analytical consideration, it is identified that these factors are indeed tied with the mechanical properties of the edge-bond adhesives. The coefficient of thermal expansion (CTE) and the Young´s modulus are namely the properties in concern. In order to enhance the thermal fatigue reliability of solder joints, edge-bond with large modulus and CTE close to that of the solder joints should be employed.