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

摘要

Moisture/reflow testing on plastic ball grid array (PBGA) packagesfor Pb-free assembly applications has revealed a potential performanceissue associated with moisture diffusion within the organic layers ofPBGA substrates. Diffusion of moisture within the composite organiclayers of the substrate is found to be effectively blocked by thepresence of large metal power and ground planes. For short ambientexposure times, these planes impede the ingress of moisture and limitinitial moisture uptake. However, when dry baking becomes necessaryafter extended periods of exposure to ambient conditions, the metalplanes are then found to hinder the egress of moisture, thereby forcingsignificantly longer bake times. This paper addresses the criticalnature of moisture interaction within the inner-organic layers of PBGAsubstrates and demonstrates the potential concerns for long-termhandling effects. Residual moisture contained within the organic layersis found to play an important role for moisture/reflow performance atthe higher Pb-free assembly temperatures. Finite element analysis (FEA)is used to detail the moisture diffusion process within the internalpackage features for PBGAs. Key material property data are measured andpresented. Important aspects of fast diffusion paths associated withglass fiber reinforced substrates are also accounted for. Finally,experimental moisture/reflow data collected on 2 and 4-layer PBGAs arepresented