Moisture blocking planes and their effect on reflow performance in achieving reliable Pb-free assembly capability for PBGAs

Moisture/reflow testing on plastic ball grid array (PBGA) packages for Pb-free assembly applications has revealed a potential performance issue associated with moisture diffusion within the organic layers of PBGA substrates. Diffusion of moisture within the composite organic layers of the substrate is found to be effectively blocked by the presence of large metal power and ground planes. For short ambient exposure times, these planes impede the ingress of moisture and limit initial moisture uptake. However, when dry baking becomes necessary after extended periods of exposure to ambient conditions, the metal planes are then found to hinder the egress of moisture, thereby forcing significantly longer bake times. This paper addresses the critical nature of moisture interaction within the inner-organic layers of PBGA substrates and demonstrates the potential concerns for long-term handling effects. Residual moisture contained within the organic layers is found to play an important role for moisture/reflow performance at the higher Pb-free assembly temperatures. Finite element analysis (FEA) is used to detail the moisture diffusion process within the internal package features for PBGAs. Key material property data are measured and presented. Important aspects of fast diffusion paths associated with glass fiber reinforced substrates are also accounted for. Finally, experimental moisture/reflow data collected on 2 and 4-layer PBGAs are presented.