Comprehensive evaluation of solder paste for assembly of PQFN packages

Solder paste has been extensively used to attach power dies in assembling Power Quad Flat No-lead (PQFN) packages. An appropriate solder paste ensures good die bonding quality and good package thermal and reliability performances. A comprehensive procedure is proposed to evaluate new solder pastes for PQFN packages. First, the solder void performance of a given paste is evaluated, as solder void is the main defect related to solder paste in die attach application. The Design Of Experiment (DOE) of reflow profile is conducted on the solder paste. A reflow profile yielding the best solder void performance is accepted as the optimal reflow profile for that solder paste. Then, the impacts that flux residues remaining on the lead frame surface have on the wire bonding and molding qualities are evaluated, as they may deteriorate the qualities significantly. The wire pull test and Temperature Cycling (TC) test are conducted on bonded wires and molded packages. The minimum wire pull force must meet the wire bonding requirement, and no delamination and function failure may be observed throughout the TC test. By evaluating four solder pastes using the above procedure, it is shown that each solder paste possesses its own characteristics and that the proposed comprehensive evaluation is necessary. Only a solder paste that yields acceptable solder void performance and wire bonding quality and that causes no delamination is qualified for assembling PQFN packages.