Ultra-fine pitch Pb-free & eutectic solder bumping with fine particle size solder paste for nano packaging

With the demand for electronic component miniaturization with high functionality, high density assemblies requires finer interconnect features and pitches, which cannot be met using conventional solder paste screen printing. Ultra-fine pitch solder deposition can be carried out by electroplating but with related cost and environmental issues. This paper discusses a feasible deposition process for ultra-fine pitch using fine particle size solder pastes. Two types of pastes of differing particle sizes, each with lead-free SnAgCu and eutectic SnPb compositions are compared. 50 pin bumps at 100 μm pitch are deposited via paste printing through a photo-sensitive polymer mask which is strippable after solder reflow. Printing was carried out on a whole wafer with depopulated and full array die with around 4000 I/Os per die as test vehicle. The test vehicle consisted of BCB passivation and under bump metallization (UBM) on aluminum pad. The processability of the pastes is compared. Good bump height uniformity is found where difference observed between the paste types is briefed. Due to the fine particle size of these solder pastes, the possible oxidation of the bumps after reflow is examined. The implications of reduced solder bump volumes and cross-sectional area of the UBM-to-solder joint on bump shear strengths and failure mechanisms are analyzed. Also discussed are the micro structural evolutions of the bumps after reflow, where differences are observed between the types of solder pastes and compositions.