Brooks, QFN's aren't that bad (you ought to try 0201 SMDs..... Like soldering dust. QFN's are tight, but with a little technique they solder pretty easily. I am using a little QFN28 for the CP2102 USB-to-serial converter on my new FC Oncore Eval Board design. There are a couple of tricks I have found that work well.
1. When you make your pad pattern in your layout program you want to make the SMD pads stick out beyond the edges of the package enough to allow you to get an iron tip in there and create a little solder fillet. If you use the normal package patterns suggested by the manufacturers it is almost impossible to manually install the part as there is not enough pad exposed. I have done this with almost every component layout I use when designing boards. The pads are always a little bit bigger so that I can manually build or rework if needed. This step is really important. 2. First glue the QFN to the board with a little TINY dab of slow curing epoxy. You want to use the slow stuff because it gives you enough time to push the IC around with a couple of toothpicks until it is aligned. 3. Liberally flux the part to make sure you get good wetting. 4. With a small tip (I usually just use my good old Weller) and small diameter solder (.015" if you can get it), carefully form bridges between the pads on the board and the pads on the QFN. DO NOT drag the iron. You want it to glide on a thin layer of solder. If you drag the tip you risk pulling the pads off the board and/or the QFN itself. Don't worry about bridges at this point. Add more flux if you need to. The idea is to keep the surface tension high so that the solder wants to form bumps rather than smearing. 5. Once you are happy that you have made connections to all pads, work back around the part with the tiny (.015" I think) solder wick and the iron. Again, you only want to slide around without dragging. The solder wick will pull up any excess solder but leave a perfect little bond underneath the QFN and a nice fillet on the side. Again, be liberal with the flux. If the solder gets too dry and/or overheated it will tend to smear between pads instead of balling up and separating. See mom, my 2 week DoD soldering class really did come in handy! ;-) Randy P.S. I hope I never have to do this with lead-free solder. -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Brooks Shera Sent: Friday, June 30, 2006 2:54 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Linear Interpolator Inspired by Ulrich's lead regarding the high resolution TDC, I asked the Acam US rep for a quote on the TDC GP2. As Ulrich indicated they are not too expensive: $28 quantity 1, $26 quantity 10. Sounds great. Temperature coefficient isn't quoted but it will be considerable - presumably can be corrected in software. For me the catch seems to be that they are packaged in a QFN-32. The pin spacing is 500 microns and the open space between the solder pads is only 200 microns. Perhaps I am too timid but this sounds like trouble for manual soldering, that's assuming the low cost PCB suppliers could make the appropriate solder mask. Any comments from the experienced? Acam makes a dandy evaluation board, the ATMD-GPX, but for this they want $2298. Oh well. Brooks _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts