Hand soldering a VQFN24 part to a PCB

RE: Hand soldering a VQFN24 part to a PCB

DAB — Sun, 17 Jul 2022 18:26:17 GMT

You are braver than I.

My eyes are no longer good enough for this small stuff.

RE: Hand soldering a VQFN24 part to a PCB

michaelkellett — Sun, 17 Jul 2022 12:42:49 GMT

I've often done it the other way round.

Big hole in the middle for the centre pad. (1mm diameter or even more).

Solder paste to the small pads ( applied with a needle if nothing better available.)

Place the part, cente it on the pads by eye.

Flow the solder paste, hot plate is best, frying pan is possible.

With hotplate set it to reflow tempeerature, let it get stable and place the board on it. Rebove the board when the solder flows.

With a frying pan you have no temperature control, so you need to warm slowly until the solder flows. Remove the board, the pan will go heating up.

I used this method on quite a few parts until stencils from China got nearly free and bought a proper reflow oven.

I found (cheap) hotplates more relaible than (cheap) hot air.

Once the pads are soldered you can solder the centre pad through the hole (heating up with a soldering iron). Make sure there is heat relief on the cntre pad if it's connected to a power plane.

If you put solder paste on the centre pad that can work too.

If you apply the solder paste by needle or similar you'll usually end up with far too much and some pads will be shorted. Use lots of really runny flux and solder wick (braid) to remove excess solder.

BIg holes in the centre pad work worse thermally but are useful for kitchen table processing because they allow you to check (with a very fine meter probe) that the underside of the chip is actually connected. I've had switching regulator chips not work because the centre pads were not soldering !

(This can also happen with TQFPs with a big centre pad, if the pins are a bit bent the pad doesn't touch the board. I've resuced an expensive FPGA on a very expensive board by re-soldering the centre pad through the better-safe-than-sorry large centre via.

RE: Hand soldering a VQFN24 part to a PCB

maxpowerr — Sun, 17 Jul 2022 10:08:04 GMT

A good idea. An interesting way to mount the chip.

RE: Hand soldering a VQFN24 part to a PCB

dang74 — Sun, 17 Jul 2022 04:44:31 GMT

You developed a very novel approach to deal with those pesky little QFNs. I am glad it was successful.

RE: Hand soldering a VQFN24 part to a PCB

shabaz — Sat, 16 Jul 2022 22:24:22 GMT

Hi Gene,

Great work! Very interesting jig. It gave me an idea.. since PCBs are cheap, maybe it could be worth getting a board made with a shape cut out, and a few holes that you can use for alignment with removable pins.

Then it could be possible to rapidly assemble using a hot plate or whatever.

Whenever I have tried QFNs, the slowest part of the procedure for me is the careful placement of the part, because even though it is supposed to self-centre, I still don't find it that easy to initially place it roughly correctly.

I might give this a shot if I do a board with QFN at any point in the future. I don't have a 3D printer anyway, so the board with shaped hole sounds like a suitable alternative.

RE: Hand soldering a VQFN24 part to a PCB

misaz — Sat, 16 Jul 2022 21:41:35 GMT

For testing shorts I use continutity (beep) mode of multimeter. I touch two sibling pins and connectin should be non-conductive (no beep shoul occur). With good slim test probes I am able to do this directly on pins of TQFP/TQFN/... chips. For testing that pin is "correctly" soldered you can use diode tester because most chips have ESD protection two diodes: one between pin and GND and second between pin and VCC.