![]() ![]() It demonstrated full compatibility with the cool range of the SAC305 window, establishing itself as a viable drop-in replacement for almost any SAC-based SMT process. Despite SnCuNi’s higher melting point, it formed good joints, passing every standard test to which it was subjected. A reflow process with a peak temperature of only 234☌ and 60 seconds above 217☌ is considered the low end of the window for SAC305. ![]() The results of this test were eye-opening. Therefore, thermal cycling was eventually eliminated from the test plan. A similar study that compared the two alloys on a more complex PCB completed 3000 cycles (0-100☌) with no remarkable results. However, given the good appearance, microstructure and solder joint strength, it is highly unlikely that any solder joints would fail during cycling, and no new knowledge would be gained from the effort. Thermal Cycling: Original experimental plans included thermal cycling 5 full assemblies from each alloy set. The thermal aging had no significant impact on shear strength values, as seen in figure 5. Shear strengths averaged 4-6kg, which is typical for this component size and comparable to previous tests run with on assemblies with peak temperatures of 245☌. Joint Strength: 0805 components were shear tested at 15° angles before and after thermal aging. Both alloys demonstrated similar IMC growth during the second aging, as shown in figure 4. The initial cycle had no significant effects on the IMC or the solder joint shear strength. They were then subjected to two thermal aging cycles an initial one at 125☌ for 96 hours and a subsequent cycle at 150☌ for 240 hrs. In the as-reflowed state, both alloys formed continuous IMCs approximately 3μm thick. Microstructural Analysis: Optical microscopy showed good wetting to both leaded and leadless terminations, and good IMC formation. Although the voiding levels are acceptable, they could likely be mitigated by adding a soak to the reflow profile. The X-ray analysis showed more voiding in the SnCuNi solder joints than the SAC305, as shown in figure 3. ![]() They all passed, and the assemblies were earmarked for thermal cycling. Photos of the typical solder joints produced in this process are shown in figure 2.ĥ PCBs from each alloy group were then fully assembled with PTH and wave soldered, and installed into chassis for functional test. The SnCuNi solder joints formed under the cooler profile did not exhibit the typical luster that is characteristic of the alloy, but were not as dull as the SAC305. No defects were found by either inspection process. Inspection & Electrical Test: AOI took place on the assembly line, and visual inspection was performed by the line’s IPC-certified inspectors. Following assembly the boards were subjected to numerous tests and analyses. Both pastes used the same flux, and the two products ran down the assembly line sequentially with no changes to any portion of the SMT process. The experiment consisted of two runs – one with SAC305 solder paste and one with SnCuNi solder paste. was dictated by thermally sensitive components on the PCB, which was a mixed-technology industrial controller of low-medium complexity. It dropped the SN100C alloy into a reflow process considered cool for SAC305 the temperture peaked at 234☌ with only 60 seconds above 217. But a newly published study 1 explored exactly that situation. Until recently, we had no real-world, production-based data on the soldering behavior of the SnCuNi alloy near the low margin of the reflow window. Laboratory tests have demonstrated good wetting at lower temperatures, but they’re just that – lab tests. It’s that 7-degree difference in minimum reflow temperatures that generates the fear – of cold joints, incomplete wetting, inconsistent IMC formation or other reliability issues - if the peak temperature of a solder joint falls into that questionable space between 233 and 240☌. Applying the 13-degree guideline to the SnCuNi alloy results in a minimum peak temperature recommendation of 240☌. Recommended reflow temperatures are typically at least 13☌ higher than melting temperatures hence the SAC305 peak temperature window of 233 – 255. SAC305 begins to melt at 217☌, reaching its fully liquid state at approximately 220.6☌. Fear of full compatibility with SAC reflow processes. Why has it not gained major acceptance as an SMT alloy? In large part, fear. Because it contains no silver, it is much more economical than SAC alloys containing 1, 3 or even 4 percent of the precious metal, and it produces smooth, shiny, easy-to-inspect solder joints. Nickel-modified tin-copper solder, known commercially as SN100C®, is a leading lead-free alloy for PTH soldering, rework and hot air leveled PCB final finishes. Sn/Cu/Ni soldering performance at low temperatures.
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