[time-nuts] linear interpolators and itty bitty soldering

Randy Warner Randy at synergy-gps.com
Fri Jun 30 18:47:58 EDT 2006


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! ;-)


P.S. I hope I never have to do this with lead-free solder.

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 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
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
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
$2298.   Oh well.


More information about the time-nuts mailing list