Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Chris Sargent

#15254

Solder Joint Problem | 25 June, 1998

I have got a strange problem with a customers board. The problem being that I have had a number of boards returned to myself for Dry Joints. When you examine the joints, the solder fillet is perfect, the only problem being that it is attached to the leg, not to the pad. The problem suggests that the leg is more solderable than the pad, and so I guess this leads to intermettalics or contamination. Has anyone any other ideas, or has come accross this problem and can have a good stab at the root cause? Help much appreciated. Chris

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Steve Gregory

#15255

Re: Solder Joint Problem | 25 June, 1998

Hi there Chris! Are the leads you talking about fine pitch? Is the complete fillet attached to the foot, with it being cleanly separated from the pad? I bet if you look at the board there will be via's really close to the pads at the locations where you're having problems. If that is the case, what is happening is you're getting re-reflow (no, I wasn't stuttering...hehehe) when you're wave soldering the board. You're getting heat transfer coming thru the via's when it hits the wave and because they're so close to the pads, bingo- bango...you got the opens you're having. If the via's don't hafta be filled because of ICT or something, use some liquid mask and mask the area off before wave. If you do need the via's filled, mask it off anyway...just means you're gonna hafta fill em' buy hand now... -Steve Gregory-

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Chry

#15256

Re: Solder Joint Problem | 25 June, 1998

| Hi there Chris! | Are the leads you talking about fine pitch? Is the complete fillet attached to the foot, with it being cleanly separated from the pad? I bet if you look at the board there will be via's really close to the pads at the locations where you're having problems. If that is the case, what is happening is you're getting re-reflow (no, I wasn't stuttering...hehehe) when you're wave soldering the board. You're getting heat transfer coming thru the via's when it hits the wave and because they're so close to the pads, bingo- bango...you got the opens you're having. If the via's don't hafta be filled because of ICT or something, use some liquid mask and mask the area off before wave. If you do need the via's filled, mask it off anyway...just means you're gonna hafta fill em' buy hand now... | -Steve Gregory- Hey there - Sounds like re-reflow to me, too. If you can't mask the vias and can't change the design to get some heat sinking in, try modifying the wave solder profile. Getting it over the wave as quickly as possible is an option - increase your conveyor speed and up the preheats accordingly. With good flux and the right preheat, you should get nice joints with no more than 1.5 seconds of wave contact time. Or run without the chip wave if possible. In terms of heat transfer, the chip wave pumps a lot more heat into the board than the smooth wave, primarily because of the higher volume of solder that passes by. If you're using a hot knife, you may be able to lower the temperature, up the pressure a little bit, and still get good debridging. Whatever you do, don't make the mistake of sacrificing preheat. It's not the preheat that's reflowing the fine pitch leads. Its the solder contact. If you don't have enough preheat, you'll need more contact for good wetting and hole fill. You should be able to eek out a decent profile that fills your holes but doesn't nuke the topside. Good luck.

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