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Issues With Fillets on Via Holes?
Board Talk
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TranscriptPhil Welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting, the assembly brothers today coming to you from high atop Mount Rialto. We are here to talk about electronic assembly, materials, equipment, components, practices and procedures among other things and who knows where we might go with that. Jim, what is today's question? Jim The question is about concave via fillets causing problems. When we wave solder our circuit boards, some of the vias have concave fillets giving them a dimple effect. The boards function fine but we are having problems when we test using our flying probe system. What can we do to create flatter fillets on via holes? And this is sent in by G.F. The first thing is there are no flat soldered surfaces. Solder is formed by surface tension and wetting forces which are always going to give you a curved surface. Now you can try to minimize that. First let's make sure we agree upon what our questioner is asking. He's talking about filling via holes. These I'm assuming are plated through holes which do not have leads in them. So they go over the wave and they just fill up with solder. He's talking about a concave fillet on the top surface which means that the surfaces of solder in the middle of the via hole is lower than the surface of the board. That's my assumption. Phil Jim I'm kind of surprised, a little puzzled as to the problem he's actually having with this flying probe in that to my knowledge flying probe generally can compensate in the Z axis for varying topography of the board and of course the test points it's going after. So I'm not sure why it's a problem but he might want to confer with the manufacturer of the flying probe system or reexamine the programs. Let's address the other issue, how our friend might be able to attain somewhat flatter vias, or shall we say better filled vias. Jim If you think about the situation we have this concave fillet. That means we haven't completely filled the through-hole. So we're up against a classic question of how do I get better hole fill in a wave soldering process? In most cases we're looking for the Holy Grail which is positive top side fillets when I have a lead in the hole. Even though IPC tells us that it is not necessary even on class 3, seventy-five percent hole fill is okay. Many of us in our heart of hearts want to see that top side fillet. But in general getting better hole fill on plated through-holes during wave soldering is a function of optimizing the process. Adequate fluxing, proper topside preheat, proper wave parameters, depth of immersion, immersion time and so forth. The idea of controlling the process to get flat vias would be a really fussy process. If you increased the hole wetting you might get a convex fillet where you have a bump sticking up that might also cause problems with the flying probe. It's going to be really fussy to try to fine tune the wave soldering to get exactly the level hole fill in via holes across a circuit board. I see it as being probably impossible to achieve. Phil Check with an expert at the flying probe manufacturer. They can probably make some recommendations on compensating for the varying topography that you're dealing with. Beyond that we wish you the best and when you are playing around with your soldering machine and trying to make those adjustments, try not to inhale too deeply when you're over the flux pot. And when you hit the wave, whatever you do, please don't solder like my brother. Jim And don't solder like my brother. |
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