QFN Reflow Soldering
I decided to try my hand at QFN soldering yesterday. I was really nervous about it... it just seemed like the kind of thing only a robot or an expert can really pull-off. I've seen it done many places online and everyone said that it was doable. Well, here's another voice in the chorus: It is possible with hobby-grade equipment. I've written about reflow soldering before, so I'm not going to re-hash those discussion, but that is the technique that I used. As you can see in the image above, I used a professionally manufactured PCB. This design is a breakout/prototype adapter for a Skyworks RF switch. The circuit is remarkably simple; there are two control lines that select which inputs are routed to the common (RFC on the silkscreen). There are a few DC blocking caps on each of the RF lines, and that's it!
In this image, I'm showing how much solder paste I used. As you can probably see, I made no attempt to keep it contained to any particular location. I assumed that surface tension and the solder mask would help out in this regard. This is the first board I did, and I used somewhat less solder on the second one.
Above is the result of the first board. You can see some serious solder bridges on the chip there. They were easily removed with solder wick.
This is the second board I tried. This time I used a little less solder. You can see that there aren't any bridges (you'll have to take me at my word that there aren't any on the other side). I did use some solder wick to clean up the solder anyway, as I don't like that they're bulging out the sides. As a final note about this image, you should be able to see a very small solder ball on the left side of the chip. This is very common with reflow. The extra solder paste under the IC was squeezed out by surface tension. I usually remove these with a razor blade.
I had another board and IC that I'm working on, also a QFN, that I thought I'd try. In this case, it's a 3x3 package with huge spacing between pads. This one was much easier to handle. You can see that I still have some solder overflow.The solder ball on the right was removed with the razor blade, and the near one was removed with solder wick.
Now that I've tried it a few times, I can say that soldering QFN isn't nearly as scary as I thought it would be. A steady hand and some method of soldering other than an iron is all that's really necessary. I imagine this would work with the skillet or any hot air method.
The last image is a skyworks 1.2W RF power amplifier breakout/prototype board.This will be the subject of a future post. Stay tuned!