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September 19, 2012

Solder Popping Out of Holes

Solder Popping Out of Holes
Have you seen this effect before? Solder appears to be popping out of through holes. What could be the cause?

We see this problem with a specific batch of PCBs. Older and newer batches have no problem. The problem does not seem to be related to the time that the PCBs has been on shelf.


Experts Comments

It's hard to say for sure, not knowing more specifics of the PWB construction and the thermal history of the board. What I think might be happening is that there was a solder plug in the via, and subsequent heating (e.g. reflow) caused a void within the solder plug to expand, pushing the surface of the solder up. Because no flux was present, it just domed up, and did not wet the hole wall. That's my story, and I'm sticking to it!
Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Solder "popping" out of holes during the assembly process can be caused by outgassing of moisture from the PTH. This is typically facilitated by substandard plating in the holes that allows moisture trapped in the substrate to outgass through the plating into the barrel when the solder is trying to wet and take hold.  It would likely be in your best interest to have the holes in question Microsectioned to see if they meet IPC-6012 requirements for plating thickness and consistency. A stopgap measure could be to thoroughly bake the moisture out of the boards, but this is a band-aid and not a fix and it may reduce your solderability due to oxide formation and intermetallic growth during the bake.
Bob Neves
Microtek Laboratories
Bob Neves has spent the last 27 years working for Microtek Labs involved in qualification, conformance and failure analysis of PCB's, PCBA's and the materials that go into making them up. Neves has served as the IPC's Rigid Board General Committee Chairman, HDI General Committee Chairman, TAEC Chairman and have served on more than 50 IPC Committees. He was awarded both the IPC's President's Award and Hall of Fame award for service to the electronics industry.
I'm not sure I've seen this, but I guess I can gamble and offer my two cents.  

If the solder is coming out of the holes the implication is there is no intermetallic layer being formed between the solder joint and the plating in the hole? Check the surface of the barrel or plated through hole for a coating of some kind, be it either solder mask or some other type of coating even perhaps oxidation. Many times when solder mask is improperly applied it is removed and replaced and the removal process could be a chemical wash which would dissolve the initial material, but also coat the entire board, which would include the plated through holes. If the conditions is caused by oxidation this will prevent the solder from wetting to the base material and this could also allow the solder not to be metallurgically connected to the barrel.  

When solder solidifies it shrinks, so if the solder is not metallurgically bonded to the barrel it can now be loose in the pth and if all conditions were correct, could in fact be pushed out of the plated through holes.
Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
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