Modeling of Failure Mechanism and Life Prediction of Lead-Free BGA Solder Joint under Repetitive Mechanical Drop Loading.

摘要

With the recent development of mobile consumer electronics, a unique set of reliability issues those are associated with portable products and their using environment. In particular, accidental dropping is believed to cause substantial damage to the solder joints. The increasing occurrence of drop failure of portable electronics has been traced to the failure of the solder joints that interconnect the integrated circuit (IC) components with the printed circuit board (PCB). The increasing market pressure to replace lead based solder in electronic products has resulted in research being carried out in many areas related to lead-free solder. Thus, the reliability of lead-free solder joints under mechanical shock loading is a major concern.;The main objective of this thesis is to study the failure mechanism and life prediction for lead-free ball grid array (BGA) solder joint subject to repetitive mechanical drop loading. The emphasis will be placed on the strain hardening effect of lead-free solder joint under repetitive mechanical drop loading.;The mechanical properties of SAC lead-free solder alloy were first studied by the tensile test. The results showed that the SAC lead-free solder was strongly strain rate dependent. The constitutive model of SAC lead-free solder was built. Nanoindentation tests were employed to obtain the mechanical properties of lead-free solder and IMC. The nanoindentation hardness value for SAC lead-free solder and IMC was found to decrease with increasing aging time. The IMC layers grew in thickness during thermal aging tests, while there were no significant changes in the Young’s modulus of IMC.;High speed ball pull tests were used to find the brittle fracture strength of IMC. The simulation of the high speed ball pull tests were studied and correlated with the high speed ball pull tests. Then the IMC brittle failure criterion was established.;The strain hardening effects of SAC lead-free solder were found by solder wire drop test and board level drop test (BLDT). The structure stiffening effects were also found by the strain measurement during the BLDT experiment. The failure analysis after the PCBA BLDT experiment showed that all the failure modes were brittle failures. A 3D finite element model of the PCBA was built and studied under repetitive drop loading. The dominant strain rates of the SAC lead-free solder joints were found by correlating the material properties with tensile tests. After gradually correlating the drop loading condition, PCB strain, failure location and mode, and the IMC strain rates, the drop to failure number under repetitive drop loading was found. The predicted life from the simulation model matched well with the BLDT experiments. Finally this simulation model was validated on the industrial PBGA BLDT.