Dynamic Response And Failure Analysis Of Lead-Free BGA Package Under Drop Impact

As growing demand for portable electronic products as well as the compulsory use of lead-free solder, for the industry which support the portable electronics, solder joint drop impact reliability has become one of the important challenge. Study on dynamic response of lead-free solder joints under drop impact load is very necessary.The damage of lead-free solder joints and tin-lead solder joint under drop impact was analyzed through drop test. The specimens and experiment scheme of drop test were designed, and strain signal on PCB center and dynamic strain of solder joint were monitored. Dyeing method was used to mark the damage of the solder joint morphology. The experimental results showed that the outermost corner solder joint near PCB side failed first both for lead-free package and tin-lead package; The damage of tin-lead solder joint was ductile fracture, and lead-free solder joint was brittle failure; As can be seen from the dyeing figure of tin-lead solder joint, the failure mode was welding pad fracture, but for lead-free solder joint, the failure mode was brittle failure of the IMC layer. The failure mechanism of solder joints is the bending rigidity difference between package and PCB, when PCB is bending downwards, the outmost solder joints are under tensile stress, and when PCB is bending upwards, the outmost solder joints are under compressive stress. As the repeated bending of PCB, tensile and compressive stress was generated alternately in the outmost solder joints which results in crack and then eventually complete failure.ABAQUS implicit dynamic analysis method was used to calculate dynamic response of lead-free BGA package under drop impact. BGA package three-dimensional finite element model was built. The load was according to JEDEC standard condition B and was applied with Input-G method. The boundary conditions were set as the horizontal displacement constraints on width direction. The modal analysis of BGA package was conducted to study vibration mode and frequency of each order modal. The simulation results showed that the simulation strain curve in length direction of PCB center was essentially coincident with the experimental strain curve. The maximum stress was at the outmost corner solder joint near PCB side which consisted with experimental results. The conclusion illustrated the correctness of the model. PCB deflection curve was a sine curve, which indicated that PCB bent up and down under drop impact. The first order modal is the main vibration mode of package under drop impact, and the high orders have little effect. Peeling stress of the outmost corner solder joint that obviously greater than other stress is the main cause of the solder joint crack.Based on the correctness of modal established with ABAQUS, three different kinds of solder joint distribution with the same peripheral size was analyzed. The results showed that biggest Mises stress were all at the outermost corner solder joints in three different kinds of distribution. Among the outmost solder joint, the stress of horizontal solder joints significantly less than the longitudinal ones; Different solder joint distribution has little impact on PCB deflection. For the full array distribution, the peeling stress of dangerous solder joint for lower density of solder joint distribution obviously bigger than higher density distribution under the same drop impact load. Removing some of central ones has extremely small influence on peeling stress of dangerous solder joint. It indicates that peripheral solder joint distribution density has significant effect on peeling stress of dangerous solder joint. When peripheral solder joint density is small, peeling stress of dangerous solder joint is big, and it will fail easily. The results provide theoretical references to the effective improvement of solder joint distribution and package reliability.