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March 2014 • SMT Magazine 27 FEaSIBILITY OF LOW/NO SILvEr aLLOY SOLDEr PaSTE maTErIaLS continues 9). However, many alternative low/no silver al- loy solder pastes had excessive solder balls (Fig- ure 10). During the previous lead-free (SAC305) solder pate evaluations, almost all materials performed well in this solder balling test, and excessive solder balls weren't seen. Voiding The X-ray image of BGA solder joints re- flowed with SAC305 solder paste is shown in Figure 11. The X-ray images of BGA solder joints reflowed with SAC0307 and SN100C solder paste are shown in Figure 12 and Figure 14, respectively. In general, many alternative low/no silver alloy solder pastes resulted in more voiding than SAC305 solder paste. Some alternative alloy solder pastes had similar voiding level as SAC305 solder paste for the BGA solder joint (Figure 13a and 14a). For the QFN components, the voiding level was sig- nificantly increased for the samples reflowed with low/no silver solder pastes, as shown in Figure 15. Process Window Alternative low/no silver alloy solder pastes had a narrower process window than SAC305 solder paste. Many alternative alloys hadn't completely reflowed at the low lead-free profile (Figure 15). Medium or hot profile may be need- ed when alternative low/no silver alloy solder paste is used. This raises the concern about the feaTure figure 11: X-ray image of bga solder joint reflowed with SaC305 solder paste. little voids were seen. Figure 12: x-ray images of bga solder joints reflowed with SaC0307 solder pastes. good SaC0307 solder paste (12a) had similar voiding level as SaC305 solder paste. Many evaluated SaC0307 solder pastes (12b,c) had slightly more voids than the control SaC305 solder paste. Again, flux chemistry has a greater impact on the material performance than the alloy com- position. Solder Balling Test Some alternative alloy solder pastes per- formed good in the solder balling test (Figure