Thermal Failure Analysis And Reliability Researches Of Lead-free PBGA Package

As people's awareness of environmental protection, the traditional SnPb solder will be replaced by environment-friendly lead-free solder. Lead-free device thermal analysis and reliability have become the research hotspot of microelectronic packaging at home and abroad. Temperature cycling experiments is a regular method to study the thermal failure and reliability of the electronic devices under actual service conditions. Finite element analysis method can get the stress and strain solutions comprehensively and conveniently, which is also a good way to improve the efficiency of failure analysis and reduce the design cost.In this paper, the temperature cycling experiment is carried out for lead-free plastic ball grid array chip by using programmable high and low temperature test box. The morphology of solder balls and the substrate cracks growth have been observed by using a high power microscope, and the chip failure mechanisms have been analyzed. The resistance values between solder joints on the lead-free chip have been tested by using the equipment measurement of electrical properties before and after temperature cycling experiment. The chip performance has been analyzed with the changed resistance value after experiment. The results show that: the changes of the solder balls'morphology and the substrate cracks growth are more likely to happen on the non-uniform distribution of the chip. Temperature cycling test do not result in open circuit or short circuit of the chip, but it result in the resistance values between solder joints grown disproportionately, which reduce the electrical properties of the chip, mismatch the resistance and reduce the reliability.Based on ANSYS finite element software, the stress distribution and strain displacement of the 2D model of lead-free plastic ball grid array device has been simulated under temperature cycling loads. Simulation results indicate that the maximum points of the stress and strain are all in the non-uniform distribution part of the lead-free plastic ball grid array device. The thermal fatigue is prone happen in the connections of the device, which leading to the substrate cracks initiation and growth, eventually leading to device failure. The uniform distribution of the solder balls on the chip is helpful to improve the device reliability. The simulation results are consistent with the experimental results, which prove the correctness and feasibility, of the finite element simulation method.Time course of post-processing prove that with the time increase the stress-Y, plastic strain-Y, elastic strain-Y and total strain-Y of the dangerous point of the device are all periodicity change, and after a few temperature cycles the changes become stable. In the case of the same device structure, the stress and strain of lead-free device is bigger than the lead devices'. Big stress and strain can lead to solder joint fracture, crack initiation and growth of chip substrate, eventually leading to device failure. To improve the reliability of lead-free devices, people need to find materials which thermal match with the lead-free solder balls or find lead-free materials which physical characteristics and electrical properties are similar to lead solder balls material. This study has a certain reference value for improving the reliability of lead-free plastic ball grid array chip.