Effect of Convection and Conduction Oven to the Intermetallic Formation and Solder Joint Reliability

This paper presents the effect of the reflow oven type to the intermetallic (IMC) formation and solders joint reliability for the wafer level packages (WLPs). Two common used reflow oven in electronic manufacturing industry which are conduction and convection oven have been studied. Four reflow profiles with same peak temperature and wetting time for both ovens were developed. In this study, two low (240 degrees C) and high (255 degrees C) peak temperature were investigated. SAC107 solder ball with Cu UBM pads of WLP daisy chain were reflowed by using each ovens. The formation of IMC was observed at the interface of the solder and Cu UBM pad and found that oven type is significantly influence the IMC formation. The reflow profile for convection oven produced longer IMC needles and visible over the whole UBM pad. On the other hand, IMC needles are only visible at the perimeter of the pad for the conduction oven. Drop Test (DT) and Temperature Cycle Test (TCT) were used to evaluate the solder joint reliability. From the board level test results, solder balls reflowed by convection oven have achieved better thermal performance than conduction oven. Failure analysis was performed on the TCT units and found that crack line propagated in the solder bump but not in the IMC layer as the IMC needles acts as a barrier to crack propagation. With the 25% improvement in TCT performance by convection oven, Infineon Technologies is now exceeding customer expectation and sustaining competitive advantage in WLP market.