Hi Ken and TechNetters!

Ken sent me some pictures of some solder problems he's seeing on some assemblies and wanted me to post them on my page. I got the pictures up. Go look at:

http://www.stevezeva.homestead.com/files/solderability_1.jpg

http://www.stevezeva.homestead.com/files/solderability_2.jpg

http://www.stevezeva.homestead.com/files/solderability_3.jpg

I have a few questions; are you seeing this quite a bit, or just very random? What kind of solder paste is it?

I had some strange looking joints one time in the past when I was out in California building memory modules. We were using water soluble paste at the time. One of the inspectors called me said that she was seeing cold solder. I went and looked at the assembly, and saw what really did look like cold solder. But the strange thing was that it was occuring on a couple of leads on a part, and all the rest of the leads were fine...including the ones right next to the leads that looked cold.

I was thinking;"Something doesn't add up, convection heat doesn't work like this...how can one lead have cold solder, then the one right next to it is fine?"

I started looking a little more, and took a pair of tweezers and gently probed the cold solder joint. To my suprize, there was a solid fillet of solder that was wetted to the lead and pad beneath a layer of what looked like cold solder. Things were getting really strange now.

To make a long story short, what it turned out to be was that the screen print operator was using a wet rag to clean the underside of the stencil periodically. It was an old MPM SP2400 that didn't have an automatic cleaning system. The operators heart was in the right place, but the rag he was using was overly wet. When he wiped the bottom of the stencil, some of the water was trapped in the apertures. So on the next print, the was enough water to alter the chemistry of the past that gave us these random cold looking solder joints.

After I took his wet rag away, gave him some decent wipes and alcohol, the problem went away.

The point of this story is that maybe whatever they're using to clean the stencil with might becoming trapped in some of the apertures, and the screwing the paste up on the next couple of prints.

Just a WAG...

-Steve Gregory-

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