Recently I was running out of solder paste and I’ve bought some chinese paste to try out from a (very) popular online electronics outlet. I think somewhere in the reviews or in one of the tutorials they say they actually use that paste internally so I got confident and the price was right (I guess… even though I later found the same paste on dx.com for under $5), about $14 for a jar of 50g which would last a long time. Great I thought, I’d get it in time to assemble more boards, without the insane shipping delays from china, so I ordered and got peace of mind.
A few days later I got it and I assembled a batch of boards with it and peeked through the microscope to inspect how it reflowed. Here’s what I saw:
Holy cow! What happened? Tons of tiny balls of solder stuck in tons of solder paste residue. Not one of my best days, but certainly memorable.
It was going to be a lot of work to fix these boards. I was certain the chinese solder paste was at fault and instantly decided I won’t trust chinese paste again for such a critical assembly step. First of all the massive amounts of flux residue was shocking, that alone speaks of how cheap this paste is. Then massive amounts of solder balls, that indicates very poor flux properties that prevented the solder to reflow properly. I haven’t done anything different as far as reflowing, but the results were horrendous. I’ve even reflowed by hand with the hot air gun, and results are very good usually. Not with this paste. The solder melted but I suspect the flux was so bad quality that it stopped the solder balls from reflowing together, instead causing small balls to form everywhere. It could also be that the flux is very old, I don’t know for sure. Here’s a mugshot of the offending paste:
I got busy fixing that batch and realized that simply touching up with the soldering iron wasn’t enough. That flux would simply not evaporate nor help the small solder balls to reflow into the larger solder joints nearby. I had to scrape away the residue and then rework every joint with the soldering iron. You cannot see this with the naked eye, so if you’re using this brand of paste you might think it’s okay but it’s not. Another lesson learned…
I got some US branded paste that is much higher quality, and is not much more expensive. Kester paste is really nice, I used their leaded paste and it works very well, almost no residue to clean up and no orphaned tiny balls of solder mess. Someone recommended ChipQuik paste so I got some from techni-tool to try out. I really like this one, it’s no-lead no-clean and reflows at about 190-200 degrees celsius. Here’s some yield from this paste:
Big difference eh?
The US branded paste usually comes in small syringe like tubes (10cc) which are in the $20-30 range, or very large 500g jars or tubes that cost up to $100 or more per jar. I only need a small tube every few months, so to make the best of this deal I just threw away the bad paste and emptied the good paste from its tube into the left over jar, this way I can easily apply the paste with a credit card and recycle it (even though they don’t recommend mixing used and unused paste, I don’t really see any quality degradation).
Did I say I threw away the chinese paste? Well sort of. Destroying it first and having some fun while doing it sounds better. Here’s a paste burning experiment, I think it’s obvious which paste reflows better, with less residue, and without tiny solder balls left behind: