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SMD Spring Pins Reflow Crooked / Tilted

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SMD Spring Pins Reflow Crooked / Tilted | 4 January, 2017

We recently ran a board with Mill Max surface mount spring-pins which placed perfectly straight from tape/reel but came out tilted in various directions after reflow. We're right within the recommended paste thickness of 0.005" and a reflow temperature of 255C. Does anyone have any experience with this type of pin and this issue? Mill Max wasn't able to offer much advice other than paste thickness and reflow temperature.


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SMD Spring Pins Reflow Crooked / Tilted | 5 January, 2017

I'd recommend not using them, personally :) The long term reliability of the SMT joint is suspect, IMHO. I had them for an in-house manufactured bed-of-nails test fixture for testing somewhere north of 5000 units....after the first 100, we were repairing fractured solder joints all over the place. We didn't have any guide-pins, though (that's what happens when your engineers don't listen to or consult manufacturing), so, YMMV.

If you're stuck with them, here are a couple of things you can try: 1. Watch the size/shape of the paste aperture. Make sure that it matches the manufacturers spec. 2. Watch the size of the pad. Either meet the manufacturers spec, or reduce it somewhat (not below the od of the pin, but, right at the od would be pretty good). This makes reworking the pins a bit harder, but, will help. 3. Watch the thermal imbalance. This is likely a large contributor to your root cause. In the picture, I can see some larger traces coming in to one side of the pin, etc. If you can, balance out the thermal load in the copper that the pins are attached to. If you can't balance it for whatever reason, consider wagon-wheeling the pads to the copper, to help with thermal imbalance. 4. Reduce the fan-speed in your reflow oven, if possible. While I consider this a minor influence, it definitely could be an influence. 5. Increase placement force...but, only a make sure that the pins are seating down in the paste. This is one of those shot-in-the-dark solutions, though, and could cause more issues if the placement force gets too high, the pin could be placed askew. 6. Double check the reflow profile to verify TAL. The part *should* have enough mass to self-center/self-level, but, the TAL is going to need to be pretty tight. If it's too short, it might not have enough time to shimmy into place, if it's too long, it might have enough time to shimmy too much. Also, check the cooling ramp. 7. Is the board run on the mesh in the oven, or on a pin-rail-chain? Is the board handled between placement and reflow? Is the travel through the oven mostly smooth, or are there vibrations (especially during liquidous)?

Those are the points that I'd start at. I want to think, though, that the last time I used them, I ended up reworking a lot. Thankfully, it was a low volume runner (like 1 board/year), so, the rework didn't impact things as much as it could.

Cheers, ..rob

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SMD Spring Pins Reflow Crooked / Tilted | 5 January, 2017

You have too much paste; I have used a lot of test points/spring contacts, etc. A single pin placement, especially one like this one that has a high lead to pad ratio, meaning 80% or more of the pad is covered by that single flat “lead” on the bottom of the part. When that solder reflows there will be no where for it to go, it will try and make a round solder bump and will settle unevenly. Easiest way to fix this would be a 40%-50% reduction on the aperture to pad; even dropping to a 4mil may not take off enough volume. You’ll be getting into reliability issues, but at least there are straight.

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SMD Spring Pins Reflow Crooked / Tilted | 9 January, 2017

Thanks for your replies. Customer designed this so we can offer them feedback on the design. Pin manufacturer said most likely too much paste as well. Reducing apeture size would be the easiest solution there. We are running a VIP70 oven with chain rails; it runs smooth with no vibration. We don't have control over fan speed. We can try adding a little extra force, we're using 150g right now. The pad is currently undersize from the spec. but even on tilted pins we don't see solder under the pin it appears to be close to the board under a microscope.

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