Fine-pitch, low-volume SoP(Solder-on-Pad) process

Low-volume, low-cost and lead-free Solder-on-Pad (SoP) process using solder bump maker (SBM) is proposed for the fine-pitch flip-chip bonding process. The SBM is composed of a lead-free solder powder and a resin. The resin in the SBM consists of a polymer matrix, a deoxidizing agent and additives. The deoxidizing agent and additives remove the oxide layer on the surface of the solder powder. The bumping process features no-mask process so that a fine pitch bump array can be easily achievable. It mainly consists of two thermal steps; one is for the aggregation of the solder powder on the metal pads on a substrate and the other for the reflow process to make the round solder bump array. The thermo-rheological behavior of the SBM was characterized using a differential scanning calorimetry (DSC) and a dynamic mechanical analyzer (DMA). With this material and process, the solder bump array was successfully formed with pitch of 130μm.