QFN Center Pads on PCBs


Reading time ( words)

The QFN (quad-flat no-leads) package can no longer be considered exotic. It was about a decade ago, but not anymore. In fact, with the wafer scale BGA, it's one of the more common packages for new chip designs.

Not all QFNs come with an exposed metal pad underneath, but most do, and that can still cause problems with reflow solder. The pad itself isn't the problem, but improper solder paste stencil layer design can be.

The default stencil layer in the CAD library footprint might have an opening the full size of the metal pad. If that's the case, you'll need to modify the footprint so that you get 50-75 percent coverage with solder paste. If you don't, you may very well have yield problems. With a 100 percent open area, you'll most likely end up with too much solder in the middle. The part will ride up or float, and may not connect with all of the pads on the sides of the part.

Figure 1_Benson_ScreamingCircuits.jpg

The figure below shows a stencil with too large an opening in the center, a segmented paste layer in the CAD footprint, and the resultant segmented stencil. You may note that I said to shoot for 50-75 percent coverage and ask: "Well, is it 50 percent or 75 percent? What gives?"

Figure 2_Benson_ScreamingCircuits.jpg

True, that is a bit of ambiguity. However, anything in that range should be fine for prototype boards. If you're going for volume production, you'll want to work with your manufacturer to tweak the design for best high-volume yield.

Good news on this front is that many QFN manufacturers and parts library creators have taken notice of this. It's far more likely now than it was 10 years ago to find a datasheet correctly illustrating this, and footprints created correctly. But, always check your footprints to make sure.

 

Duane Benson is the Chief Technology Champion at Screaming Circuits, a prototype PCB assembly electronic manufacturing company in Canby, Oregon.

Share


Suggested Items

Solder Printing Process Inputs Impacting Distribution of Paste Volume

12/14/2017 | Marco Lajoie and Alain Breton, C-MAC Microelectronics
The volume of solder deposition, like any process, has variations that may be characterized by a statistical distribution curve, whether normal or non-normal. As complexity, density, cost and reliability requirements increase, there may be value in narrowing the distribution curve. It is common sense that less variation serves the interest of quality of the more complex and dense circuit boards.

Tips & Tricks: Generating Stencil Tooling

12/13/2017 | Ken Horky, Peterson Manufacturing
Many engineers are leaving the editing up to the stencil fabricator these days. From the outside, this may appear as a time saver for process engineers, but considering how many stencil redos have been required and how many processes that have run 'sort of OK,' there's a tremendous amount of scrap and rework that could be saved from just a little more attention paid to stencil tooling.

Improving Solder Paste Printing

12/12/2017 | Stephen Las Marias, I-Connect007
Of the three elements involved in the solder paste printing process—stencil, solder paste, and printer—the stencil is considered one of the major factors affecting the transfer efficiency, accuracy, and consistency, of solder pastes into the pads, especially with the continuing trend towards miniaturization.



Copyright © 2017 I-Connect007. All rights reserved.