Study of SMT Assembly Processes for Fine Pitch CSP Packages

摘要

The SMT (surface mount technology) assembly process for 0.4 mm pitch CSP (chip scale package) components was studiedrnin this work. For the screen printing process, the printing performance of different solder pastes, aperture shapes and sizesrnwas investigated. Square apertures and a fine particle size in the solder paste provided a better paste release. Besidesrnoptimizing the printing process capability and minimizing the printing defects such as bridging and missing paste, the totalrnvolume of solder consisting of the paste and the solder ball has to be considered in order to maximize the final process yield.rnFor the pick & place process, the accuracy required for the placement equipment was determined by studying the selfalignmentrnof the lead-free CSPs (with Sn/4.0Ag/0.5Cu balls) during the reflow process using lead-free Sn/3.9Ag/0.6Cu paste.rnThe components were intentionally misplaced up to ～50% off-pad. After reflow, the x-ray inspection showed that therncomponents had aligned to the pad. By considering the stack-up of the PCB (printed circuit board) pad location and sizerntolerances, the solder paste printing tolerances and the placement tolerances, the required alignment accuracy for the pick &rnplace equipment was established to meet the total process capability requirement.