Assembly, reliability, and rework of 0.4 mm pitch wafer level chip scale packages

Among the different Chip Scale Packages (CSPs) available, Wafer Level Chip Scale Packages (WLCSPs) offer an immense potential as a packaging format for low to medium I/O components primarily due to its potential for low cost and small form factor (i.e. die to package ratio). Until now, WLCSP offerings have mainly been between 0.5 to 0.8 mm pitch. However, increased demands for smaller footprints have forced the electronics industry to consider moving from a 0.5 mm to a 0.4 mm pitch.; The global objective of this dissertation was to provide the electronics industry with a drop-in high yield paste printing assembly and a rework process for the 0.4 mm pitch WLCSPs. The paste printing assembly process was aimed to be Surface Mount Technology (SMT) compatible requiring use of 4 to 6 mil thick stencils. Solder bridging defects commonly observed during the assembly of fine pitch components were extensively studied and indications of possible mechanisms for the occurrence of solder bridging defects were presented. Flux bridging, a printing defect that is commonly observed during enclosed printhead printing process, was researched and methods to solve flux bridging problems were delineated. The effect of board support, board bending, and board width variations on printing using an enclosed printhead was studied. Typical mask shift tolerances on substrates were incorporated in the assembly process by intentionally printing offsets. The effect of assembly process variables such as pad finish, reflow atmosphere, and standoff on thermal cycling reliability was investigated.; A rework process was developed for underfilled and non-underfilled 0.4 mm pitch WLCSPs. Non-underfilled assemblies showed no difference between reworked and non-reworked (non-underfilled) assemblies. Rework of underfilled assemblies showed considerable challenge in removing underfill residues from mask openings. Additionally, edge underfilling techniques, (i) four corner attach and (ii) perimeter attach, showed considerable promise in reworkability and reliability.