On My Via-In-Pad Soapbox (Again)

There’s never enough time. There’s never enough money. There’s never enough room.

I certainly say those things often enough, and sometimes it’s actually true. But other times, I’m just not looking in the right places. Here’s a board that is pretty much plumb out of room. Everything is so tight that many of the vias have to be put in the pads. Well, maybe.

Take the IC footprint (above). It needs a via to take a couple of connected pads to the other side of the PCB, but there isn’t enough room between the IC and the part just below it. Naturally, the logical thing seemed to be to put the via in the pads. Unfortunately, doing so will make it difficult to get a good solder joint. The big open hole will wick solder down to the other side of the board.

At first glance there doesn’t seem to be anything to do. But upon closer examination, there is some unused space here. I’d just slide the part up a little, as in the illustration below.

Then move the via south a bit and connect it to the pads with a trace just long enough to accept some solder mask. The solder mask will stop the solder from chasing the via off the pad and getting sucked down.

Duane Benson
Some solder suckers sit South of Sunday

http://blog.screamingcircuits.com

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About Mike

Mike Buetow is editor-in-chief of Circuits Assembly magazine, the leading publication for electronics manufacturing, and PCD&F, the leading publication for printed circuit design and fabrication. He is also vice president and editorial director of UP Media Group, for which he oversees all editorial and production aspects. He has more than 20 years' experience in the electronics industry, including six years at IPC, an electronics trade association, at which he was a technical projects manager and communications director. He has also held editorial positions at SMT Magazine, community newspapers and in book publishing. He is a graduate of the University of Illinois. Follow Mike on Twitter: @mikebuetow

1 thought on “On My Via-In-Pad Soapbox (Again)

  1. It is very , very rare… that one cannot find the room … it just takes time… as you indicated… often we are not given enough..
    time vs quality… often the root of short sighted gains.. (pay now or later).

    Your example points to another axiom…
    just like jello- there is always room for more (improvements in this case).

    The trace across the pads (common connections next to each other).
    Generally considered “bad form” to connect between them in the middle of the pads… (yet, I see it all the time).

    some of the reasons..
    a) the solder mask may lift between pads (hard to guarantee on fine pitch), causing solder to migrate (un-even solder distribution between the pads).
    b) the trace connections into the side of the pads can cause irregular shape on solder joints….
    c) thermal load during soldering will be different for those pads closest to via

    all good reasons for not doing this….

    still one more reason…

    There remains the possibility an engineer will want to disconnect one of these leads from the common signal (likely ground is this case)….
    disconnect from ground ? (example: allowing to float “high” by internal pull up)
    On many circuits.. software or some other external source may cause the designer the need to change a ground pin used for boot up configuration of the circuit..

    if the common connection is from the external edge… this is easily done (cut a trace).

    if the common connection is done from centers of pads..
    or
    if the common connection is done from the internal edge of the pads (body of part covers the traces) …
    it will be much harder to accomplish.

    basically… trying to accommodate the un-expected/un-specified…
    impossible to do completely…
    but often a few simple rules can accomplish much toward this goal.

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