PCB Assembly Services - Screaming Circuits: QFN Solder Paste Layer


QFN Solder Paste Layer

LBDCminiI've got the fab order placed with Sunstone.com for my next demo project. The little board is represented here at pretty close to actual size on screen - provided you have a 22" monitor set at 1680 pixel horizontal resolution. Give that, you might want to click on it to pop up a bigger representation of it. That makes it about 4 X life size.

When you do that, take note of the QFN / DFN parts: The processor in the middle, the LiPoly battery charger right between the upper two mounting holes and the RS232 driver in the lower left. I've followed my paste layer advice and segmented the paste stencil layer to reduce the chance for float or major voids.

I found a footprint in the library for the big processor in the middle. I just had to modify the paste layer, as shown here. I made the footprint for the charger and RS232 chips from scratch. Neither had anything close enough in the library.

The DFN has a slightly different approach to segmenting the stencil layer. Little squares like I used on the other two chips work just as well, but this is effective as well.

Another thing to take note of is the markation on the LEDs. The original footprint for the 0402 LEDs does have a polarity mark, but it's one of the types that can easily be misinterpreted or can be difficult to see. The diode symbol put down in silk screen removes any possibility of ambiguity.

Duane Benson
I'm happy I live in a split level head.



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Ah, okay. Thanks!

My only experience is with TI's L293D, which has a very helpful datasheet, complete with example drawings of heat sink copper. A nice example that I mangled by forgetting to remove the thermals from the pins into it. Lots of quality time gently scraping off soldermask and globbing on solder...

Josh - It's an odd chip. It doesn't have a heat slug underneath or on top. The datasheet pretty much doesn't give any help so I just designed that part as well as I could.

If there were a thermal pad under the driver chip, I would have designed the stencil similarly to one of the QFNs and would not have open vias.

Are the vias under the motor controller (h-bridge?) big enough for the heat dissipation needed? It seems like a slightly odd setup to me, but I'm still learning about laying out higher power stuff (I don't do particularly high-power-dissipation components).

The three vias underneath the microUSB connector are in a non-contact area. There should be a tiny air gap to prevent any issues.

I will need to verify that once I get everything in hand. Often there are differences between one manufacturer and another such that one might have the necessary air gap and another might not.

Looks nice!

One question: are the vias below H4 (Micro-USB?) allowed? Or will they be covered with solder resist?

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