WLCSP Assembly

Abstract

Assembly of WLCSP components involves surface mounting technology (SMT), which includes pickup WLCSP components from tape and place them onto a printed circuit board (PCB), solder reflow, and optional underfill. Figure 9.1 provides a schematic diagram of a typical assembly line setup involving WLCSP, in which solder paste or flux is first printed or dispensed on the PCB respectively before WLCSP pick and placement. Reflow is followed to finish the solder joint formation between the WLCSP component(s) and PCB. Optical or X-ray inspections are arranged at various stages of assembly line to ensure correct solder paste printing (height, area, and volume of the solder paste bricks deposited on the PCB soldering pads), accurate component placement (XY offset and skewness), and proper solder joint formation. In-circuit test (ICT) is an electrical probe test on the assembled PCBs, checking for shorts, opens, resistance, capacitance, and other basic quantities which will show whether the assembly was correctly fabricated. Post solder reflow, underfill could be applied after flux clean to provide the needed protections to WLCSPs and solder joints going through subsequent assembly processing steps and to ensure robust reliability of the WLCSP devices in the everyday usage. Underfill is especially desired when die size is big or when low-K dielectrics are present in the WLCSP.