Many of the newest and coolest chips come in annoyingly small and difficult to (manually) solder packages, like DFN. For production runs this is no problem, but I typically prefer to assemble first boards manually - this allows me to catch stupid mistakes (like incorrectly set up regulator) without blowing up too many expensive components in the process. Manual assembly of course takes time, but at least you aren't paying 200€ each for five boards you immediately find out don't work at all due some stupid mistake that should've been caught at layout.
But back to DFN assembly. The following pictures are really crappy - I have to take them through my microscope's eyepiece, making it very difficult to focus them properly. I'll really need to get a microscope with camera one of these days...
So here is the situation. Chip in question is simple USB/serial converter. This one chip I salvaged from older board - finding problems is much easier when you have known-good (and properly programmed) chip already, allowing you to focus on problems on board itself. And there were a few such problems, as expected. But those are not today's topic either.
Like I said in previous post, I prefer to use leaded solder for difficult jobs. This counts as one. So I got a small syringe of solder paste from local electronics shop (note; you absolutely must keep solder paste in cold, like fridge - it will be quickly ruined in room temperature!) and apply some on the board. The amount seen below is actually more than enough, about half of that would be sufficient. Adding to much paste will only result in shorts under the chip.
Then place the chip on top of the solder paste. Positioning doesn't need to be exact; surface tension will move it to correct place during soldering. I have cheap Chinese hot air soldering station with adjustable temperature and air flow. I typically set temperature to slightly below 300C and air flow to low value (about 20 on scale 0-100). Even then the air flow will easily blow chip away, so start from a longer distance and gradually get closer with the nozzle. Oh, and you absolutely want to have a microscope (or at least some magnification) for this - seeing what is going on makes the entire process so much easier.
After solder starts to melt, the chip will stay put and reflow properly. Don't try to hold the chip with a tool - this prevents it from settling properly during the reflow process.
No picture of the process, sorry (too busy soldering). Just the end result. Not the prettiest solder job, and needs some cleaning, but it works - which is all I need from a prototype board.
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