Test Methodology And Equipment Investigations On Brittle Behavior Of Lead-free Solder Joints In Ball Grid Array Packaging

In modern electronic manufacturing industry, it is a prevalent trend for handhold electronic products to minimize in the dimension. As a result, the anti-shock ability of solder joints have become a big issue when subjected to shock conditions, say, dropping onto the ground. On the other hand, for environment protection concerning, lead-free solder alloys are widely adapted which behavior much more brittleness under impact force, owing to the fragile intermetallic components formed on the bonding interface. Board-level drop test is always employed to quantitatively evaluate the anti-shock ability of joints soldered on the board, which is costly and time consuming. Alternative methods might be package-level impact test, including shear, pull, peeling or bending test, in which high speed ball shear test is considered as an advisable choice.In this thesis, finite element method was firstly conducted to simulate the structural responses of solder joint in both drop test and ball impact test, followed by a conclusion revealing the relationship between both tests. And then, a drop hammer based impact tester was developed and solder ball shear test was carried out under a constant shear speed of 1.8m/s. Two types of solder alloy, Sn-3.0Ag-0.5Cu and Sn-2.5Ag-0.5Cu were investigated with respect to different reflow profiles and reflow times during soldering process, in order to study the impact of soldering parameters on the reliability of solder joints. Finally, failure modes and micro-structural analyses were preformed and evaluated.Simulation results demonstrated that failure of joints highly depended on the configuration of boards. The vibration period of impact force induced by drop shock was around 1ms, lower than that in ball impact test under shear speed of 2m/s. There were three kinds of failure modes in ball impact test, and failure time was proportional to the IMC strength.Shear test indicated that conventional quasi-static shear test could not measure bonding strength, as all failures occurred in the bulk. When subjected to a high speed condition, three types of failure modes were observed, including pad lift, interface fracture and bulk failure. The anti-shock ability of solder joints were significantly affected by heating factor and reflow times, while two sort of solder alloy behavior in a different manner.Microscopy analyses revealed that, the thickness and morphology of intermetallic layer was different when subjected to different reflow conditions. It was obvious that the variance in anti-shock reliability was a reflection of interfacial micro-structure.