Circuits Assembly - December 2008 - (Page 31) Solder Joint Reliability F igure 6. A HOP defect formed using a lowtemperature activation system. Figure 5b. Void-reducing profile for Pb-free pastes with BGAs and CSPs. Figure 5a. Ramp-soak-spike recommended profile. Figure 7. A joint formed with a high-temper ature activation system; no evidence of HOP. appeared similar, the flux interaction was different. This difference was determined by a viscosity test conducted while the paste medium was in contact with the solder alloy doped with the aforementioned elements. Other factors that appear to influence HIP include types of reflow, reflow profiles and solder paste chemistry. 1,2 Some data obtained suggest vapor phase reflow may result in more HIP defects than convection reflow. It is not clear whether this is truly related, however, as it has only been seen as a trend. An experiment was performed to measure the impact of reflow profile on HIP solder joint formation. The experiment utilized two different reflow profiles. The first profile was a standard ramp-soak-spike (Figure 5a). The second profile (Figure 5b) included a hotter soak zone and longer dwell time at liquidus. There was no perceived difference in the defect rate, depending on the profile; each resulted in circuitsassembly.com random cases of HIP, depending on the component tested. Solder paste chemistry was the next factor tested to determine impact on HIP. During this experiment, it was found solder paste chemistry appears to have the single greatest effect on the HIP defect. When changing from an older Pb-free solder paste to a new higher-temperature activation paste, the defect, in many cases, was eliminated. In other cases, it was more difficult to remove. However, in the experiment run, the solder paste chemistry appears to have the largest impact on HIP. An experiment was conducted using various solder paste chemistries to measure their effect on HIP incidents. It was determined that, regardless of the reflow profile used, the novel solder paste eliminated HIP. Although this solder paste is halidefree, a solder paste containing >0.5% halide also was used in this experiment, and the defect was once again eliminated. Continued on pg. 46 Circuits Assembly DECEMBER 2008 31 http://www.circuitsassembly.com
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