Study of thermal fatigue lifetime of fan-in package on package (FiPoP) by finite element analysis

Thermal fatigue lifetime of the Fan-in Package on Package (FiPoP) was analyzed based on the plastic strain model by finite element analysis (FEA). Both the stress and strain of FBGA and PBGA solder joints were studied under thermal cycling. It was found that the outmost solder joint on the PCB was the most dangerous position. The fatigue lifetime for the key solder joint on the PCB was estimated using Engelmaier modified Coffin-Manson model. Results showed that the fatigue lifetime of the solder joint of the FiPoP was up to 2073 temperature cycles. The effects of different Ag contents of the lead-free solder paste used for board level assembly on the fatigue lifetime of solder joints of the FiPoP were studied. Results revealed that Sn-4.0Ag-0.5Cu solder had a longest fatigue lifetime of 2441 cycles, the Sn-1.0Ag-0.5Cu had a shortest fatigue lifetime of 1830 cycles, and the Sn-3.0Ag-0.5Cu which was used in previous work showed a middle lifetime. The effects of thermal cycling temperature profile on the fatigue lifetime were also investigated. Results showed that the faster temperature ramp rate decreased fatigue lifetime significantly. When the ramp rate increased 16.5degC/min to 33degC/min, the fatigue lifetime dropped to 1686 cycles. On the other hand, the shorter the temperature dwell time, the longer the lifetime. When the dwell time reduced to 10mins, the fatigue lifetime was improved to 2572 cycles. It meant that the creep suffered by solder joints had a contribution to the lifetime of the solder joints as well.