Comparison of substrate finishes for flip chip packages

In order to achieve substrate routing densities required for high pin count flip chip package, electroless nickel immersion gold (ENIG) substrates have been the technology of choice. However, ENIG substrates are found to be more susceptible to brittle fracture at the nickel to intermetallic interface during high strain rate events. This mode has been sporadically found in PCB assembly and handling operations involving excessive and uncontrolled flexural loads on the PC boards. The unpredictable nature of this failure mode has driven suppliers to look for alternatives to ENIG. In this paper, electrolytic nickel-gold and solder on pad (SOP) are studied and contrasted against ENIG. A significant outcome of this work is the identification of a viable surface finish with improved toughness to withstand mechanical shocks in early assembly and with continued good long term reliability.