What causes this on SMD soldering?

Community PCB Design
1

I’ve done probably 20 or so SMD boards on a hot plate since I brought the hot plate instead of hand soldering, and all have had really nice fillets and soldering so far like this:

http://img525.imageshack.us/img525/1082/cap3m.png

But today I’ve just done 2 boards and both have come out like this:

http://img32.imageshack.us/img32/7581/cap2l.png

I presume that is either because I have put too little paste on the pads so it has bubbled and seperated, or because the temperature of the hotplate has warmed up too quickly this time or the solder paste is getting old or something similar? Anyway, all connections are sound and there are no problems with the board its fully functional, but it just looks nasty and I wondered what I perhaps did different this time to cause it?

2

It’s probably caused by an incorrect heating profile. It’s impossible to achieve the correct profile with a hotplate.

3

Looks like burnt flux, can you clean it off with IPA/toothbrush?

4

There are a few things that can cause that.

  1. slow heat profile. A slow heat profile can dry flux fully before the solder has a chance to flow.

2.open air time. If you have solder paste open to the air (or not refrigerated), the flux can absorb too much humidity, and not function. Also remember that solder will oxidize in the open air.

The last is what looks to have happen. Flux will be come useless if left in the open air at high humidity for a long time. All solder pastes will have a recommended open air time for a certain temperature and humidity.

Also…almost every paste I have ever bought required refrigeration. Solder paste is an active compound…so the flux is always working. Refrigeration slows the process to where the paste is viable for much longer.

James L

5

I’d like to refrigerated mine at home, but unfortunately putting it in the food refrigerator is just a risk I cannot take. My cheap stuff has lasted surprisingly long for just leaving it in the plastic jar. Longer than I expected.

6

Thanks for the comments. I will try to clean it off tonight with IPA.

Also, two things I can see I probably did wrong this time, I didn’t turn the hot plate quite as high (I have a marker of the levels and times and noticed this morning it wasn’t turned up full on the last heat :shock: ). And secondly, I was doing a new boards and fumbling about with getting components out after applying the paste to the boards, so the paste probably sat in open air for a good 10-15 minutes this time compared to probably 2.

I will keep all the comments in mind and hopefully this wont happen again as its only happened once out of 20 odd boards so thats not too bad for a £15 hot plate and manually applying the paste. Can’t expect brilliant results every time at that.

Luke

7

Most paste is rated for at least 6-12 hours post application. 10 minutes shouldn’t do it.

I’d wager its a poor heating profile, likely from lack of heat (flux moved, solder started moving but stopped before it could flow all the way).