Cohesive fracture mechanics based finite element analysis to the performance of lead free solders in BGA packaging under drop impact

Lead free solders are replacing lead rich solders in the electronic industry, the performance and safety of lead free solder joints in electric packaging under drop impact becomes a critical concern of semiconductor and electronic product manufacturers. Compared with the lead rich solder alloy, lead free solder alloy typically has higher rigidity and lower ductility. The presence of the Intermetallic Compound (IMC) layer can also affect the drop impact response of the solder interconnect, which may lead to quasi-brittle solder/IMC interfacial fracture. The traditional drop test is expensive and time consuming, and it is quite difficult to observe the full dynamic responses during the drop impact. In the present study, numerical analysis is performed to investigate drop impact effect on ball grid array (BGA) electronic packaging with the intention of predicting the performance of solders under drop impact and providing the fundamental understanding required to design a reliable electric packaging. A three dimensional finite element model is developed to simulate the solder interconnect and electronic packaging failure under board-level drop impact. An impact analysis procedure coupled with submodeling technique is established. The Cu6Sn5 and Cu3Sn IMC layers are incorporated in the solder interconnects model; cohesive fracture mechanics based method is applied to predict the crack initiation and propagation near the IMC/solder interface. A lead-free solder alloy constitutive relationship comprising elastic and rate dependent plastic effects is incorporated in the computational model. The mechanical shock is the main failure mechanism of solder joint during drop impact. The susceptible failure location is concluded at the interface between the solder and intermetallic compound based on the stress criteria. The developed model can be used to compare the drop impact performance of different components and solder alloys, which can guide the proper selection of component and optimize the layout of BGA electric packing. The behavior of electric packaging and different types of solder interconnects under drop impact can be predicted with corresponding material parameters determined from experiment.