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SMT R-chain layout choices

Discussion in 'Electronic Design' started by D Yuniskis, Jan 12, 2010.

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  1. D Yuniskis

    D Yuniskis Guest

    Hi,

    I have an SMT design that must support three R choices
    at final assembly. R1 < R2 < R3

    Differential stuffing is not an option. Boards need
    to be identical up to final assembly/test.

    One option is to install three R's in a series chain:
    R1, R2 - R1, R3 - (R2 + R1) and short none, the last, or
    the last *two* R's depending on the configuration desired.

    Another approach is to arrange three paralleled resistors such
    that R1 is formed by R3 || Ra || Rb and R2 formed by R3 || Ra.
    Then, removing Rb and possibly Ra, as required.

    The first approach could be accomplished with solder switches
    on the board -- but that eats up real estate (this is a *tiny*
    board!). Bridging R's with shunts seems like it could lead
    to components becoming "unsoldered" in the process.

    The second approach could possibly be accomplished by *cutting*
    the R's in question (gasp!).

    Are there other approaches that I should evaluate?

    Thx,
    --don
     
  2. D Yuniskis

    D Yuniskis Guest

    Grrrr... s/+ R1//
     
  3. You can get zero ohm resistors, but that would require removing and then
    reinstalling parts. So the 2nd option of removing parallel resistors sounds
    the least effort.

    Depending upon what your requirements, you might be able to use an EEPOT
    (digital programmable resistor) and have a small header on the board (or
    simply probed as part of the bead of nails tester) so you program the
    resistor value after it's assembled.

    Dave.
     
  4. Nemo

    Nemo Guest

    D Yuniskis mulls
    Is a trimpot too unstable for the application?

    I have seen select-on-test resistors (not SMT) fitted into a pair of
    single-pin sockets, but that may take up too much room. Of course the
    resistor can then fit above SMT components, so you win some real estate
    back.

    Snap-off sections of the board.
     
  5. krw

    krw Guest

    Cut lands to the Rs. Route the lands on the surface, between pads big
    enough to jumper, and forget the solder mask. A quick cut (one at
    each end of the trace is better) and it's customized.
     
  6. D Yuniskis

    D Yuniskis Guest

    Hi Dave,
    Correct. Or, adding spare sites in which to install those (as shunts).
    Two of them wouldn't take up *much* space, but...
    Yes. Unsoldering one or two components has to be no more
    effort than *soldering* one or two shunts! The downside
    of the parallel approach is it makes testing harder -- i.e.,
    you can only test the R1 condition (all R's installed).
    Yeah, I thought of that but I'm not sure how well that would work
    in this circuit topology. And, these are three *specific* R
    values so I would have to evaluate the consequences of the
    pot being set "wrong".
     
  7. D Yuniskis

    D Yuniskis Guest

    Hi,
    Yes. I was only offering it up as an "inspiration" for unusual
    approaches to this problem. :>
    Yes. Getting them *small* enough is the issue and, as I mention
    in another reply, it makes testing harder as you can only test
    the one "none cut" condition (i.e., if the two shunted R's
    are incorrect values, you will never know at final test)
    Two much real estate and two much money for the value that
    it provides (you can cut/solder components cheaper than that
    dollar spent)
    Understood. I am hoping to keep costs low enough and reliability
    high enough that this is not a factor.
     
  8. D Yuniskis

    D Yuniskis Guest

    Hi Nemo,
    Yeah, I think so. As I mention in another reply, there are
    a set of three or four *fixed* values that are acceptable
    here. I don't want to risk dealing with a pot that was
    set to some *other* value midway between two "legitimate"
    values.
    Now *that* may be an idea! I.e., use a regular 1/8W resistor
    with pigtails and install it above the other components.
    This "costs" roughly (labor) the same as soldering/unsoldering
    components that are already populated on the board. Only needs
    two pads -- which can be made "far enough apart" just by straddling
    some larger component, etc.

    It also lets the circuit be tested with all of the possible R values
    (the other approaches have limitations on this) easily.

    I will have to stew on that...
    <frown> Board is already *tiny*. I think it would be hard
    making a *tinier* portion that you could nick off.
     
  9. D Yuniskis

    D Yuniskis Guest

    I think that would be hard to do at this scale (the board is
    tiny intentionally!). Putting in big pads would use up more
    space than the SMT R's in the first place.
     
  10. krw

    krw Guest

    The lands only have to be as large as you want them to be. A wire
    between 0603 pads would be plenty big enough to cut without special
    tools. The pads would allow a 0-ohm resistor to be attached in case
    you change your mind. Just be careful not to run any lands underneath
    the cut area.
     
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