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28 SMT Magazine • March 2014 profile temperature impact on the component's process temperature limitation. conclusions There are many alternative low/no sil- ver solder pastes in the market today. In gen- eral, alternative low/no silver solder pastes have higher liquidus temperature and require a higher reflow profile than SAC305 solder pastes. Thus, the process window of low/no silver solder paste is much narrower than the process window of SAC305 solder. Many al- ternative lead-free solder paste materials have good printing and wetting performance. How- ever, low/no silver alloy solder pastes may result in more defects such as solder balling, voiding (especially on QFN components) and solder bridging. Flux chemistry and supplier play a critical role in the performance of alter- native lead-free solder paste materials. A good selected low/no silver solder can have similar performance as SAC305 solder pastes in many aspects when the process conditions are op- timized. The higher reflow profile associated with alternative low/no silver alloy solder paste creates some concerns about the component's process temperature and its limitation. When alternative low/no silver solder paste is used, it is recommended to review the component's temperature limitation to make sure that they can survive the reflow profile. Further work on the reliability of lead-free solder joints assem - feaTure FEaSIBILITY OF LOW/NO SILvEr aLLOY SOLDEr PaSTE maTErIaLS continues Figure 13: x-ray images of bga solder joints reflowed with Sn100C solder pastes. good Sn100C solder paste had similar voiding level as SaC305 solder paste (Figure 13a). Many evaluated Sn100C solder pastes had slightly more voids than the control SaC305 solder paste (Figure 13b 13c). figure 14: X-ray images of Qfn88 components assembled with different lead-free alloy solder pastes. low/no silver alloy solder pastes resulted in more voids and larger voids than SaC305 solder paste.