Soldering DFN and QFN devices - Page 1
Forum section: electronics / projects

author: Chasm
author_rank: Regular Contributor
timestamp: February 22, 2011, 01:36:47 am
content: Hi folks,how feasible is it to solder DFN and QFN devices on a PCB yourself?Dead bug method is not possible and the devices have 0.5 mm pitch and an exposed pad.To tell the truth the exposed pad is my biggest worry at the moment, the distance between the exposed and the other pads seems to be only ~0.6mm. Any chance to get this done without a stencil and junking a dozen boards?

author: allanw
author_rank: Frequent Contributor
timestamp: February 22, 2011, 01:47:02 am
content: Definitely doable and probably easier than you'd think with an iron.Use lots of good flux and it practically solders itself...I find QFN and DFN easier than QFP packages because there's much less change of bridging. The exposed pad can be difficult without using a heat gun if you do not use a large pad in the center for soldering iron access.What I like to do is just to wet the iron tip and wipe off most of the solder. Then I touch the tip to a few leads at a time and re-wet it often. This prevents bridging but still provides enough solder.

author: Lance
author_rank: Frequent Contributor
timestamp: February 22, 2011, 02:10:33 am
content: I found this video quite informative:c_Qt5CtUlqY

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 22, 2011, 10:30:40 am
content: Here's a vid I did on thisAnother tip is to make sure the pads extend beyond the package, to allow inspection and reflow of individual pins.

author: Lance
author_rank: Frequent Contributor
timestamp: February 22, 2011, 04:30:40 pm
content: Quote from: mikeselectricstuff on February 22, 2011, 10:30:40 amWould that not end up damaging the underside of the board?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 22, 2011, 04:49:51 pm
content: Quote from: Lance on February 22, 2011, 04:30:40 pmNo.

author: Psi
author_rank: Super Contributor
timestamp: February 22, 2011, 10:48:15 pm
content: Quote from: Lance on February 22, 2011, 04:30:40 pmremember that the center pad is on the bottom too, so all you're doing is heating up the metal pad underneath

author: Lance
author_rank: Frequent Contributor
timestamp: February 23, 2011, 04:45:04 am
content: Quote from: Psi on February 22, 2011, 10:48:15 pmAh, I see. Doesn't having the pad go all the way through the board introduce design issues? Or does it not matter because those pads usually go through anyways? Doesn't having the metal exposed increase the risk of corrosion?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 23, 2011, 09:21:43 am
content: Quote from: Lance on February 23, 2011, 04:45:04 amIt is normal to have vias through the centre pad of QFNs - often for thermal and/or ground impedance reasons, but doesn't hurt even if not needed for that, and makes rework easier. Exposed pad isn't an issue as you'd normally used gold finish on a PCB with QFNs as you want a very flat finish, but even if it was silver, it would only tarnish the surface and wouldn't affect the solder joint.

author: Chasm
author_rank: Regular Contributor
timestamp: February 23, 2011, 03:31:59 pm
content: Thank you.Especially both videos look promising, it should be possible to get the part soldered even if it is QFN32.Which is of course the moment you realize that you can't buy the part without throwing 20 Euro into Farnells throat or buying it directly and paying about the same for postage at Freescale.Annoying since the MC34700 seems to be a nice SMPS regulator that should fit into all current and most future projects at a price point at a ok price...I'll put the flux question in a new thread.