Electronics > FPGA

FPGA/CPLDs that are hand solderable

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asmi:

--- Quote from: NorthGuy on January 26, 2022, 04:00:16 pm ---With QFNs, putting the correct amount of solder on the center pad is key. If you put too much, the chip will tilt one side and the pins on the other side will be left in the air. If you put too little, the solder on the center pad will suck the chip down and it won't self-align. It is easier to do with biggish QFNs (i.e. 20+ pins), but I often struggle with very small 0.4 mm parts.

If you use solder paste, you get better control of the solder quantity. In this case, QFNs are one of the easiest, way better than TQFPs. As stencils gets cheaper and better, solder paste becomes more advantageous than hand soldering.

--- End quote ---
You can't use stencil for rework, and it's just a matter of time until you will come across a dud of a QFN (or kill it due to schematic/layout/handling/testing screw-up) and will need to replace it. This is why I strongly recommend mastering hand-soldering without any solder paste and stencil, this way it just won't be a problem at all. For me it took about 5 or 6 attempts to get a hand of it, larger QFNs are actually harder to solder/desolder because it's harder to heat it up uniformly such that the whole thing reflows at the same time. Preheat can help with it, but I don't use it for practical reasons (it's hard to use it with microscope and not get burns), so for me smaller QFNs are easier.

woofy:
For soldering small QFNs on prototype boards I place a largish (iron tip size) PTH hole in the center under the pad. Then after soldering the outside pads, just pop the iron in the center hole and solder that down.
I use that trick for LAN chips where the center pad is the only ground pin on the chip.

nctnico:

--- Quote from: NorthGuy on January 26, 2022, 04:00:16 pm ---
--- Quote from: asmi on January 19, 2022, 08:54:28 pm ---Just get some cheap QFNs to practice if you don't have any already, and get cracking. It will only take a few tries until you get a hand of it.

--- End quote ---

With QFNs, putting the correct amount of solder on the center pad is key. If you put too much, the chip will tilt one side and the pins on the other side will be left in the air. If you put too little, the solder on the center pad will suck the chip down and it won't self-align. It is easier to do with biggish QFNs (i.e. 20+ pins), but I often struggle with very small 0.4 mm parts.

--- End quote ---
What works really well is to simply put non-tented vias in the exposed / thermal pad to wick away the excess solder. You'll need those anyway and they allow for hand soldering a QFN as well. Add flux and use hot air to heat the board. Solder will wick through the vias to the exposed pad. Works like a charm.

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