Currently we are using a 2 step vapor soldering process to manufacture our std. lead soldered parts. The first step was done with a ls240 vapor fluid and the second was done with a ls200 fluid. Using a 221 deg C sn95/Ag5 solder for the first step and the a std 60/40 solder on the second. this worked very well. Now, with the change over to a Pb free method I am being faced with the issues of higher reflow temps. Trying to stay with a 2 step method I have went to a Sn95/5Sb solder (240 deg C) for the first step and the Sn/Ag/Cu solder (217 deg C) solder for the second. My issues are coming from the availability of fluids. The options are ls215, ls230, and ls240. The ls 215 is out as it is not hot enough to reflow the Sn/Ag/Cu paste but the ls230 and ls240 do not offer enough temperature seperation. When using the ls230 for a few days the vapor temps stabilizes @ 232 degrees. The lower plastic range for the Sn95/5Sb is about 233 degrees. So what I end up with is dull joints on the primary soldered parts after seeing the secondary soldering process. Does anyone have any input on this?
In general two step temperature reflow process requires minimum 40 deg C delta to be sucessful. We are not using vapor phase soldering but we have similar situation when we change to SAC lead free solder for PCB component population and secondary sweatdown operation. We chose 42Sn/58Bi eutectic alloy which is melting 138 deg C. If you can go to the lower temperature rather than going to high temperature you might try to use this alloy. It is Indalloy #281 and solders very well. Hope this may help. Regards,