Electronic Swarms — Overhangs

SCFig1 As I’ve stated many times before, we see many, many different jobs go through our shop. In those jobs, we see some of the absolute newest components and packages; some not yet available to the public; some are so R&D that they never will be available outside of a lab. We see the best of the best in terms of design practices and complexity, and we see many that aren’t so much in that arena.

Given that, it would seem logical that the design problems we see would be pretty much scattered all over the map. By some measures they are, but on a day to day basis, they tend to cluster. For a few months we’ll see a lot of QFN footprint issues. In a different few months, we’ll see a lot of via in pad issues, etc. I don’t know why. It just works that way — problems come in swarms, or storms.

The latest swarm relates to panelized boards and components that stick over the edge of the board. We build things like that all the time. The problem comes in when the panel tabs come out right where the component overhangs. If the component overhangs in the cut out area, it’s usually not a problem. However, if the component is on the connection tabs, we can’t place that part without first depaneling.

SCFig2Probably the most common example is the surface mount USB Micro-B receptacle. It over hangs the board by a small amount, and that overhanging part is actually bent down. If it’s at the tab, it won’t even mount flush. Take a close look at the images along the right. That connector won’t mount as it’s sitting on a tab.

So, what do you do about it?

SCFig3 You can have your boards made as individuals. Although if you want short-run production, or if the boards are really small, that might not be possible or practical. You can also talk to your fab house about it. They may be able to place the tabs in a spot that won’t get in the way of the overhanging part, of they might be able to tell you where the tabs will be, allowing you to keep clear in your layout.

Duane Benson
Anyone ever drink Tab Clear?

http://blog.screamingcircuits.com