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significance of small dots of solder on PCBs?

Discussion in 'Electronic Design' started by Michael J. Noone, Sep 25, 2005.

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  1. Hi - I've often noticed small round solder dots on traces on PCBs. They
    aren't a part of a component's footprint, and they don't appear to be
    vias as they don't have holes going through them. For example, in this
    picture of an XBox's motherboard:
    http://www.teamxecuter.com/x3/tutorials/pinheader/v10v15/x3_bottom.jpg
    they're very visible (for example the circled "D0" points). Can
    somebody tell me what these are for? My guess has always been that
    they're a sort of test pad - but why would test pads be needed on a
    mass produced PCB? I mean my understanding is that these days when a
    board like this goes bad it is replaced, not repaired. My only guess is
    that they're for testing and/or programming in the factory. Can anybody
    shed some light on this for me? Thanks,

    -M Noone
     
  2. Pooh Bear

    Pooh Bear Guest

    Why not ?

    It's common to test assembled boards that way !
    Yes, factory testing on a 'bed of nails'.

    Graham
     
  3. Kryten

    Kryten Guest

    yes, I expect they are test pads too.
    Yes, but you still need the test pads there to detect the fault.

    They may even help find if the fault is something easily fixed or not.

    Even if not, it can help highlight where faults are happening, to detect
    problem areas.
     
  4. Are they in sets of 3 widely placed across the board? If so, they are
    fiducials. These are alignment references that the auto placement
    machines use to get the solder mask and component placement precisely
    aligned with both the position and rotation of the board. The ones
    labeled D0 appear to be vias that have filled with solder from wave
    soldering. They are not fiducials, because traces run through them.

    http://www.tkb-4u.com/articles/other/fiducial/fiducial.php
     
  5. Mac

    Mac Guest

    I'm pretty sure those are testpads. Testpads are not only for
    troubleshooting, but also can be used to flag boards as bad prior to
    function or final acceptance tests. They can also be used to look for
    shorts between nets on bare boards.

    A few years ago I worked for a small design group within a large company
    we are all familiar with. Our group designed Intel architecture SBC's for
    CompactPCI chassis. The test guys always wanted us to have test points on
    every single net (even both sides of a series 10-Ohm resistor). The layout
    guy we had hated testpoints and thought they were stupid (this is because
    bring-up and debug and testing were not part of his job). Anything that
    made routing a board take longer was stupid, in his view. We had to watch
    him carefully. ;-)

    Usually the test guys would let us put the test pads on only one side of
    nets with small series resistors. And then there would always be some
    nets in densely-routed areas where there just wasn't any way to place all
    the test pads without creating long stubs. For high-speed nets, I would
    usually veto long stubs.

    --Mac
     
  6. Because at one point in time that very board was a prototype --
    with the appropriate test pads. No reason to redesign the board
    just to get rid of them.

    robert
     
  7. Rich Grise

    Rich Grise Guest

    They're vias. You can't see the holes, because they're filled with solder.

    Cheers!
    Rich
     
  8. Access to points on the board which are not otherwise brought out can
    improve testability. For example, on an analog circuit, you might want
    to probe an internally generated voltage to make sure it is within
    specs rather than trying to infer that it is from the outside. You
    might want access to an internal node on a digital circuit because it
    otherwise would take too long to test. The board can be put in a test
    jig with probes (eg. pogo pins) and tested without the cost and board
    area of connectors or the routing required to get different nodes to
    the connector.

    This kind of thing is probably more the rule than the exception in
    complex mass-produced boards.

    Some things don't lend themselves to easy testing- DIPswitches, for
    example. If the board is 100% electrically tested and you use a good
    quality DIP switch, and inspect you might be willing to assume that
    things are okay if the board tests okay with the switches in one
    position. However, there are many, many faults that this will not
    catch, and will only show up by switching a particular switch from one
    position to another. There may be faults that could show up only if
    combinations of switches are put in a particular set of positions.
    It's clearly impractical to test all combinations of, say, 2
    8-position DIPswitches, so brute force is out. Last design I did with
    those d*mn things (customer spec'd BTW), I tested each switch in both
    positions by forcing the operator to go through a programmed test
    sequence flipping one at a time. More than good enough for the
    application and it caught an almost invisible QFP-80 pin solder short
    in my prototype.


    Best regards,
    Spehro Pefhany
     
  9. Rich Grise

    Rich Grise Guest

    You could verify or refute this by looking at the other side of the
    board.

    Good Luck!
    Rich
     
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