|Ask the Experts|
September 19, 2012
Solder Popping Out of Holes
|Expert Panel Responses|
It'shard to say for sure, not knowing more specifics of the PWB construction andthe thermal history of the board. What I think might be happening isthat 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 thesolder up. Because no flux was present, it just domed up, and did not wet thehole wall. That's my story, and I'm sticking to it!
Solder "popping"out of holes during the assembly process can be caused by outgassing ofmoisture from the PTH. This is typicallyfacilitated by substandard plating in the holes that allows moisture trapped inthe substrate to outgass through the plating into the barrel when the solder istrying to wet and take hold. It wouldlikely be in your best interest to have the holes in question Microsectioned tosee if they meet IPC-6012 requirements for plating thickness andconsistency. A stopgap measure could beto thoroughly bake the moisture out of the boards, but this is a band-aid andnot a fix and it may reduce your solderability due to oxide formation andintermetallic growth during the bake.
I'm not sure I've seen this, but I guess I can gamble and offermy two cents. If the solder is coming out of the holes the implication isthere is no intermetallic layer being formed between the solder joint and theplating in the hole? Check the surface of the barrel or plated through hole fora coating of some kind, be it either solder mask or some other type of coatingeven perhaps oxidation. Many times when solder mask is improperly applied it isremoved and replaced and the removal process could be a chemical wash whichwould dissolve the initial material, but also coat the entire board, whichwould include the plated through holes. If the conditions is caused byoxidation this will prevent the solder from wetting to the base material andthis could also allow the solder not to be metallurgically connected to thebarrel. When soldersolidifies it shrinks, so if the solder is not metallurgically bonded to thebarrel it can now be loose in the pth and if all conditions were correct, couldin fact be pushed out of the plated through holes.
Vice President, Technical Director
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