Levels for Lead-Free and Mixed Assembly with Various Surface Finishes

Void Levels for Lead-Free and Mixed Assembly with Various Surface Finishes 

Recent publications cite the reflow profile as the primary factor in mitigating voids for lead-free solder. The theory is that by using a high temperature soak in the reflow profile, the volatiles that create voids have an opportunity to burn off. If the volatiles do not escape before the 

Mixed assembly, i.e., tin-lead solder ball with tin-silver-copper (SAC305) solder paste, also showed a higher level of voids when compared to lead-free assemblies (Fig. 22). The tin-lead and SAC solder reach liquidus at 183 and 221 °C (361 and 429 T), respectively. Therefore, the tin-lead solder balls become molten well before the lead-free solder pastes, possibly blocking the paste outgassing. In spite of the voids, it was shown that all lead-free assemblies and all surface finishes produced class III, 0-9% area voids, specified by IFC 7095. 

Immersion-tin and OSP appeared to produce comparable voiding, with ENIG (Ni-Au) being slightly better. 

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