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22 SMT Magazine • March 2014 slump test, the solder bridges were counted at the room temperature. For the hot slump test, the test vehicles were baked at 125°C–150°C for about 20 minutes. The solder bridges were then recorded and compared. Solder Ball Test Solder paste was printed on solder mask and reflowed (Figure 4). The solder balls, appearance and flux residues were analyzed. A quantified test for solder balling can be done by counting the number of solder balls at a designed loca- tion. Wetting Tests Solder wetting test was done by reflowing the solder paste printed at time zero and at 4-hours of stencil waiting time. Two wetting tests were used in the initial evaluation. In Wet- ting Test 1, the solder spreading (diameter) was measured and compared (Figure 5). In Wetting Test 2, the solder paste was printed at different aperture openings and reflowed. Lead-free sol- der paste usually does not wet as well as tin lead and thus requires an over pad print to achieve full pad coverage. For Wetting Test 2, the mini- mum print area to achieve 100% solder coverage of the pad was observed and analyzed (Figure 6). Reflow and Process Robustness Tests To verify the process robustness of the solder paste materials, further tests were per- formed on the top performing solder pastes from the screening tests. Real components were used to simulate the production environ- ment. Three different reflow profiles (low, me- dium, hot) were used in the low/no silver sol- der paste evaluation. Table 2 summarizes the difference between the profiles. All the boards were reflowed in air environment. Print quality (volume and standard deviation), solder balls, wetting, voiding, flux residues and appearance were then evaluated. A good solder paste should have good performance across all the tests, and its quality should be consistent within a wide process window. FEaSIBILITY OF LOW/NO SILvEr aLLOY SOLDEr PaSTE maTErIaLS continues feaTure Figure 4: Solder ball test. figure 5: Wetting test 1. figure 6: Wetting test 2.