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Blow holes in solder on boards

I am a small sub-contractor that assemble and solder. I am having a real hard time with my solder joints staying smooth. After heating part and adding solder, the solder bubbles and leaves a gap or splatters. I use Kester resin core solder "44".
Help its driving me nuts?
Peg Wiggin
electronics - Brownfield, Maine, USA
2006

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Hi Peg,
Blowholes or craters in solder joints can be caused by outgassing of the base material through a crack in the barrel of the plated through hole, and into the solder joint. The copper plating thickness in the hole could be minimal, or the tensile strength and elongation of the copper deposit is out of spec. The sure way to tell is to have cross sectioning done on the defective boards to make sure the copper plating is thick enough, and to identify any cracks in the barrel of the hole. Good Luck!
Mark Baker
Process Engineer - Syracuse, New York
2006

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Mark,
Just wanted to say thank you for the response.
Now I at least have some kind of answer to my problem.
It was making me crazy,trying to figure out if was something I was doing wrong ,etc.
again,thank you,
Peg Wiggin
- Brownfield, Maine, USA
2006