Most points have been covered pretty well. Just a few other things: 1) I think the typical guidance for metal stencils is about 75% coverage of center pads, broken up into a window pane pattern. However, for mylar or other similar stencils which are more flexible and fragile, I think less coverage with thicker strips between the panes is a good idea. Generally the plastic stencils end up putting down more paste and so smaller openings are better and also less fragile. 2) For rework (and for hand soldering leaded packages like QFP) I much prefer a paste flux over the liquid flux that is in, for example, the flux pens. The thicker flux doesn’t evaporate away as soon as you hit it with an iron. Usually I can clear any bridges by using the paste flux and a quick swipe with a clean iron. MG Chemicals and ChipQuick both make a good flux available in a syringe. Frys sells them both in the $10 to $15 dollar range and one syringe will last a long, long time. Cleanup is more trouble than with the thinner flux but it works so much better IMHO. 3) Even if you are using solder paste and reflow for soldering a QFN package, you should consider extending the edge pads in your footprint outside the chip package outline. This makes it easier to add some solder with an iron if you find that a pad isn’t making good contact because of a lack of solder. 4) If you are putting solder paste on by hand from a syringe, note there are different types of solder paste which are meant for syringe application. They are a thinner consistency and,have smaller solder balls. Some other solder pastes are meant for squeegee application and are thicker. I think in general the paste which is supplied in a syringe is meant for hand application. So don’t take some solder paste from a tub and stick it in a syringe unless you like to have a sore thumb from pressing too hard on a plunger. I speak from experience on this one - before I realized that there are different types of solder paste ;) Scott
> On Mar 12, 2015, at 7:01 PM, Philip Odom <[email protected]> wrote: > > For hand soldering QFN packages, I've had success with putting a very large > via (to GND or whatever your pad is attached to) right in the middle of the > pad that I can stick a soldering iron tip through. > > After drag soldering the perimeter pads, the via lets me solder down the > center pad. I'm sure it's not 100% reliable but for a small number of hand > soldered parts, it's worked quite well. > > For reflow, I typically aim for 50% paste coverage of the center pad, broken > up into at least 4 squares/rectangles (more for larger parts). This prevents > the part from floating, lets gases from flux escape, and results in more > uniform paste application. Very large apertures in the stencil can result in > uneven paste. > > Philip > > > On Thu, Mar 12, 2015 at 6:19 PM, David Madden <[email protected]> wrote: > > On 3/12/15 5:50 PM, Jason Barnett wrote: > > dispensing the paste in a grid of 4 to 8 squares > > I've seen those on stencils, but I didn't know why they were done that > way. Makes sense. But I think you're talking about problems that are > light years ahead of where I'm operating, at least. I count myself > lucky when I can boil off the flux, suck out the excess solder, and only > have to fix a couple of pins! > > -- > Mersenne Law LLP · www.mersenne.com · +1-503-679-1671 > - Small Business, Startup and Intellectual Property Law - > 9600 S.W. Oak Street · Suite 500 · Tigard, Oregon 97223 > > _______________________________________________ > dorkbotpdx-blabber mailing list > [email protected] > http://music.columbia.edu/mailman/listinfo/dorkbotpdx-blabber _______________________________________________ dorkbotpdx-blabber mailing list [email protected] http://music.columbia.edu/mailman/listinfo/dorkbotpdx-blabber
