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Unfilled PTHs during wave soldering

I have a problem with wave soldering on some boards. We have some holes on the boards which just cannot fill the via PTHs. Affected holes are random and the holes are completely unfilled (see thru'). Wetting is observed on the annular rings (top and bottom). Need to know what are the potential root causes. All variables and parameters are unchanged(flux, type of solder,etc) , and this problem only appeared in the last several days while we have run this for a very long time.

Yip Chee Keen
electronics mfgr - Singapore
2004

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Sounds like contamination in the holes, perhaps from the board plating or hole drilling. Or possibly your flux bath has changed viscosity due to evaporation.

If the problem only shows up on holes that have component leads in them, it's usually because the leads are too big for the hole. Or possibly dirty component leads.

If it's not component leads, you need to look in the unfilled holes. You can try viewing with a microscope using bottom illumination slightly to the side of the hole. Or use a borescope if you can get one. As a last resort, cut the board in two through one of the holes using a fine diamond saw. Then you can use a microscope to see what is going on.

If you see board material where the hole plating should be, that's either a drilling or plating problem. A drill that is not sharp or too fast a drill feed rate can leave surfaces that won't plate.

If you see contamination (usually gray or black) in the hole, the boards may have been in storage long enough to collect dirt. Clean with BOTH an ionic cleaner (detergent & water), AND then with a degreaser solvent.

The fourth possibility is flux not getting in the hole. Visual inspection immediately after wave soldering, before washing or lead trimming, may spot this.

Tom Gallant
- Long Beach, California
2004