2021-04-23

Solder paste, and reflow oven

I got a cheap reflow oven off Amazon. Seems to work, but I am no expert, and there is some advice on the internet about some tweaks and improvements to that model too. However, baby steps.

I also got some solder paste. I do not (yet) have a stencil. So advice on how one uses a solder paste stencil, and what paste you get, and so on, may be useful.

This does, however, pose a problem. Whilst it is very easy to put a spot of paste on the pads for the passives, there is no way in hell I could possibly put paste on each pad on the FT231XQ or USB-C connector. The pins are too fine.

So how to do it. Well, I did wonder if just putting paste over the whole row of pads and relying on surface tension may work. But the other snag is these short turnaround boards from PCB Train have no solder resist. Still, it is worth a try...



Well, not quite. That is the USB-C connector, and you can maybe just see that the end power pads have bridged. As it happens a touch with soldering iron sorted that and all the fine pads were OK. But it is clearly not reliable. Attempts to do this on a simple QFN-16 were less good. I suspect if I got exactly the right amount of paste it might work, but generally, no. And I doubt any chance with a QFN-40.

So what now?

Well, what I was doing with the hot air gun was tinning the pads manually (easy), and putting liquid flux on the chip, and then blasting it. It worked pretty reliably, but the heat was not very controlled and you end up charing the PCB.

So will that work in the over? Well, the short answer is yes! (well sort of)

I tin the find pads, flux the chip and place on tinned pads, solder paste the passives and put them on board, and then reflow. It seems to work most of the time, but occasionally I have to heat gun and push the chip down on to the board to ensure all pins connected.

Just as well, as I have just got the new NFC boards to play with. Fingers crossed.

7 comments:

  1. Might it be possible to make yourself a miniature "welder's third hand" (https://images.app.goo.gl/ai8mJNbM8rZzT2QP6) to hold the chip down to the board when in the oven?

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  2. Might it be possible to make yourself a miniature "welder's third hand" (https://images.app.goo.gl/ai8mJNbM8rZzT2QP6) to hold the chip down to the board when in the oven?

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  3. I use my vinyl cutter to make the masks using some old thick OHP acetate. For some boards to help the solder flow I stick two masks together to double the amount of paste applied.

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  4. Or get your boards made by one of the cheap chinese manufacturers for peanuts and get a stainless stencil for an extra fiver.

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  5. I get my boards done at Sitopway in China and they deliver a mask for extra £. I use a plastic credit card as a squeegee :) I could do with a reflow oven, so interested to see how you get on. Solder paste is pretty date code sensitive, so old paste doesn't work very well. I've tried re-fluxing old paste, but results were mixed. For now, I heat the boards from the underside and apply liquid flux to the top to aid the soldering process. The component count on my latest design is minimal, so I'm getting away with it.

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