How to PCBA a surface mount component with flush hole throughs?

Hi,

I have a USB-C footprint which has small legs which fit flush with the PCB height. By default these holes do not have solder paste.

My query is, if they should be pasted on the rear (as per a double sided pcba), it’ll be hanging on only by the many small pins at the top. The front paste would help the connector seat properly but there’s a chance most of it will fall through the hole.

Thoughts?

Cheers, Andrew

The solder paste / reflow process is only suitable for SMT.
THT is usually done with wave soldering, and connectors like this are usualy soldered in a machine that does both at the same time.

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Ah ok. The pins are 0.6mm and the PCB 0.8mm, so the tin will flow into the pin hole a bit (and then a capacitor has to be placed on top).

It looks like this is not really manufactureable.

It may be possible to have the board assembled py Pick and Place machines and then solder the 4 THT holes of the USB connector manually from the other side if there is enough room to access it.

Putting those big Elco’s so close together may also be a problem for soldering.
I am definately NOT an expert on this but I do know that just being able to draw something does not mean it can be manufactured reliably.

The pick and place of the reverse layer would have to occur after the USB pins are soldered. Originally the PCBA company was intending to do by hand but I pointed out any sort of bump would cause the capacitor to not seat correctly.

The idea now is to apply solder paste on those pins and let it sink into the hole. I’ve done this on a reflow station and the gap is as such that it doesn’t just run through.

My understanding is the hot air comes top down. If the capacitor was completely over the pins then it would require two runs in the oven for the top layer. It looks like there’s enough exposure though just.

I forgot to mention, the legs are a little less than flush with the board. They don’t come out the other side as in the picture.

There are several issues which result in this being a poor design. Mainly:

  1. Mixing THT and SMD is painful and costly. Consider a USB conn that is completely SMD.

  2. Having too many parts jammed together results in a product that is not repairable. Unless there is a VERY strong requirement for a small size, make the PCB bigger. Students do this all the time and they can’t debug or repair boards.

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On the other hand, consider the cost of having a torn out connector. SMD vs. THT connector depends on the use case.

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Yes, the USB-C pins are very small, liable for solder failures and the THT helps for positioning. Mainly though it better resists force.

The product is non-repairable, it’s to be epoxied inside the case.

If epoxy reaches the connector it helps keeping it in place, so SMD component would be OK.

The USB could be soldered by selective soldering. That is a bit like wave soldering, but just the solder does come out of a needle. But you need enough distance to other parts on the soldering side. Say something like 2 mm. Ask your manufacturer! Selective soldering is kinda like hand soldering, but the PCB isn’t flipped over during the process.

To me it looks like the four tabs of the USB have to be hand soldered. But who knows what tricks your manufacturer is willing to play with selective soldering. Ask him!

Nick

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