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JULY 2019 I SMT007 MAGAZINE 63 – We observed a considerable amount of flux residue after reflow. In time, these solder pastes may evolve to improve this • More slump and less wetting – Our tests indicate a bit more slump (hot and cold) than our controlled material, but again, this is more likely to do with the maturity of the paste. However, the alloy itself does not wet as aggressively as SAC-305, which can be an issue in certain circumstances, although we still found it to be acceptable. In some cases, less wetting can be an advantage as well, such as for paste in hole Experimental and Results Materials Tested Figure 1 shows the materials tested. Material A is the control material. Materials B and D are the "doped" alloys, and Material C has no dopants added. Assembly Conditions All test boards were baked, inspected, and labeled before solder screen print. Solder paste inspection (SPI) was performed as well as SMT assembly and reflow. Reflow Profile for LTS Although peak temperatures as low as 165°C can be used (and should be used when appropriate), a higher temperature of 190°C was used to drive mixing on the SAC-305 BGAs (Figure 2). The solder paste and BGA ball interface experiences a sufficient phase change at these temperatures to drive diffu - sion and hence mixing of the BGA ball with the solder paste. Mixing of the BGA with the SnBi paste shown in Figure 3 was approximately 50%. This is comparable to Time 0 as well as a characteris- tic of all the solder joints we observed, which fell into the 40–60% mixing range. Figure 1: Materials tested. Figure 2: Reflow profile for the LTS.

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