Effects of Underfills on the Thermal-Cycling Tests of SnAgCu PBGA (Plastic Ball Grid Array) Packages on ImAg PCB (Printed Circuit Board)

The effects of underfills on the thermal cycling test of lead-free (SnAgCu) PBGA packages on ImAg (immersion silver) PCB are investigated. Three different underfills are studied. These underfills are dispensed on the PCB to fill the gaps between the BT (bismaleimide triazene) substrate of the PBGA, the Sn3wt%Ag0.5wt%Cu (SAC) solder balls, and the PCB. In addition, two more sets of samples (without underfull) are tested (one is with SAC solder balls and paste and the other is with Sn37wt%Pb solder balls and paste), which serve as controls. The sample size for each set of samples is 48. The life distribution, reliability function, failure rate, and mean time to failure (MTTF) of the PBGA solder joints under thermal cycling for these 5 different sets of samples have been determined. The true characteristic life, true MTTF, and true Weibull slope of the PBGA solder joints under certain confidence requirements have also been determined. Failure analysis has been performed and examined on the failed samples. The results presented in this study should be useful for understanding the effects of underfills on the lead-free PBGA solder joint reliability.