I have to disagree with Brad on this one. It is vital to get the proper amount of solder on the center ground pad. Too much paste will cause solder balls (and shorts under the pins) and too little paste will cause your component to fail due to thermal stress. We have been dealing with this issue lately at my company. Boardhouses deal with needing a fair amount of solder on a center ground pad, by dispensing the paste in a grid of 4 to 8 squares, to provide a channel for the flux to exit. Without these channels, the flux boils and forces its way through the molten solder causing it to splatter and throwing solder balls around and creating voids under the device where you cannot inspect them. Since the center ground pads are usually needed for thermal conduction, a lack of solder will cause the chip to get much hotter and end up failing early.
Here is a link to a nice document talking about these issues... http://www.smta.org/penang/papers/S7_P2_Wong_Hon_Sern.pdf On Thu, Mar 12, 2015 at 5:21 PM Brad Burleson <[email protected]> wrote: > On Thu, Mar 12, 2015 at 11:44 AM, David Madden <[email protected]> wrote: > > I've had pretty good luck with solder paste in a syringe, smear across > > the pads (and center), then hot-plate. > > While I've never done a QFN32, I have done more than a few QFN10's and > I wanted to add... the less paste on the center ground pad the better. > I had some stencils made with a large ground pad for the QFN10 and it > was a disaster. When I re-sized the ground pad to 20% the original > size my success rate (w/ a little hand re-work) went up to nearly > 100%. > > I've also made a few just like David says and all you have to do is > make sure a little bit of paste is on EVERY pad (and less is more). > > Good luck, > > Brad. > _______________________________________________ > dorkbotpdx-blabber mailing list > [email protected] > http://music.columbia.edu/mailman/listinfo/dorkbotpdx-blabber >
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