Anyone tried the 183C tin/bismuth solder paste? (not 138)

I spotted some on aliexpress and ordered it, of course, very curious....

My initial impressions (haven't yet used with reflow, and obviously using paste with an iron isn't a great test, but it didn't seem to flow or behave much worse than leaded, and miles better than the usual higher temperature lead free garbage. The stuff seems to be virtually unknown in the west. You look on reputable distributors, they've got leaded, and they've got tin or tin silver, and they've got the tin bismuth eutectic (58% bismuth, 42% tin), but at138C the melting point is way too low and it's said to be brittle as well. Hell, it's easier to find solder containing gold from western supply houses...
This stuff is 64% tin, 35% bismuth, 1% silver. Flows like solder should, was able to recklessly create bridges and clear them, responds well to my go-to flux. I hand-soldered a 1206 4-element resistor network with more ease than I expected....

Certainly the thought went through my head "hmm, this is china, I've gotten stuff that was flagrantly mismarked from these aliexpress sellers, before, and I don't think a single tube of flux I bought from them wasn't counterfeit (rthe stuff inside ranging from "might as well have been the real thing" to "this doesn't do anything but make smoke"), maybe it's just leaded solder paste with a new label! Like the 30 AWG wire with "22awg" printed on it (yes, seriously. Didn't realize because it had connectors at both ends, until it had been running a load approriate for 22AWG for quite a while, and had of course overheated, turned the red insulation black, and then broke when Itried to move it! That was really nasty....). So I tacked a spare part down onto some leaded HASL board, soldered it up with this paste - it did seem to flow a bit different and was doing the same thing, bridging and clearing the pins on one side.... and then the chip fell off. Huh, it was clearly soldered before... and I was only working on one side. google brought me some tin-lead-bismuth phase diagrams that pointed to the problem - just a couple percent lead gets into it, and a fraction of it melts at a WAY lower temperature. Certainly proves that it's not just relabled tin-lead solder....

Okay, got it, separate tips for this and leaded solder when reworking.... and it's for boards with enig finish only, I was going that way anyway, so nbd there. I'm going to try using it with reflow soon. Very excited to see how that goes - though I don't think I'll try it on my most advanced designs first :wink:

for the hell of it, I ordered some other weird solder paste, 158C that claimed to be lead free, plus another brand of the 183C unleaded stuff (I do make boards in quantities sufficient that that isn't totally insane amount of solder paste), and even a thing of the 138C stuff (don't expect to use it for anything serious, but it was a small amount and cheap so why not).

One other notable thing, beyond it's absolute invisibility in the global west - there's no wire form of it that I can find. Only paste! That's gotta mean something >.>

Interested to hear of anyone else's experience with this stuff. What's the catch? I'm certain there is one - It seems too good to be true, from initial impression. It'll probably do a crap job when I reflow with it or something, or two months from now the chips will fall off, or who knows what...

Haaa! Found one western manufacturer - Kester. A single product, sold in 500g tubs for $125 on digikey. And they specifically warn against lead right at the top of the datasheet! Okay, now things seem more consistent. my impression is that it's not the typical choice for normal PCBs anywhere, but is the only option for some BGA parts. If you can do lead free, why sacrifice anything to use this other solder. Since I'm having process issues with high temp lead free, this is much more interesting.

Never used it but the phase diagram indicates that increasing the proportion of silver moves the eutectic towards higher tin content and higher temperature:
https://www.metallurgy.nist.gov/phase/solder/agbisn-l.jpg

Which agrees with your observation that it flows well as a eutectic is a requirement for that.