1/6/2023 0 Comments Reflow solder paste![]() ![]() 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☌. REFLOW SOLDER PASTE FULLFear 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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