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Measuring PC Board Copper Thickness

Discussion in 'Electronic Design' started by EdV, Feb 22, 2007.

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  1. EdV

    EdV Guest

    We have been literally burned by boards that are supposed to have 6
    ounce copper and in fact do not. Actual fires at the customer.
    Eeeeow!

    Needless to say we would like to not do this anymore.

    I have googled and found a couple of micro resistance type systems.
    Seems reasonable enough but it will require adding test points and
    then doing some kind of conformal coat touch up.

    Anyone know a niftier approach? Ultasound imaging maybe?

    Thanks,
    Ed V.
     
  2. mpm

    mpm Guest

    Couldn't you just inspect these visually???

    With a suitable inspection microscope, you should be able to determine
    the copper thickness.
    To speed things up, maybe you could put a special trace on the PCB in
    a convenient location for initial accept / reject purposes...

    I've used 1 and 2 oz copper before, never 6. (?)
    So that said, I really don't have experience here, but the approach
    seems reasonable.
    I'll be curious to see what others have to say...

    -mpm
     
  3. TT_Man

    TT_Man Guest

    Add a waste strip to the
    Couldn't you just inspect these visually???

    With a suitable inspection microscope, you should be able to determine
    the copper thickness.
    To speed things up, maybe you could put a special trace on the PCB in
    a convenient location for initial accept / reject purposes...

    I've used 1 and 2 oz copper before, never 6. (?)
    So that said, I really don't have experience here, but the approach
    seems reasonable.
    I'll be curious to see what others have to say...

    -mpm

    Add a waste strip to the pcb that can be removed and micosectioned /cut it
    in half and measure the thickness.This is a common test for plating
    thickness in vias/through holes.
     
  4. John  Larkin

    John Larkin Guest


    Micrometer? Measure a copper-free area and then a nearby trace. Or
    just measure the resistance of a trace, with a power supply and any
    decent DVM.

    Right, when you ask for X ounce copper, you ususlly get less, often
    much less. I have one fab note that says

    START WITH 2 OZ COPPERCLAD.

    I have another that specifies maximum sheet resistance as 600
    uohms/square, with test traces. The best we've actually measured on
    these boards is around 700.

    Get used to it, I suppose.

    John
     

  5. Do you really need to do more than just spot check during incoming
    inspection?

    Fairly low resistances can be checked pretty easily with a Kelvin
    connection-- four test points (pogo pins, a little header or whatever)
    capable of a few A, set a lab supply to limit at 1A or 5A or whatever,
    and measure the voltage on the other two test points. Typical 100uV
    resolution on a cheap DVM gives you 20-100uOhm resolution. A 250 mil
    wide 6oz copper track an inch long has something like 300uOhm
    resistance, so that ought to be plenty. Or buy the fancy micro-
    ohmmeter if it sounds like more fun.


    Best regards,
    Spehro Pefhany
     
  6. EdV

    EdV Guest


    In this instance the traces are more like fill areas which may read
    nice in bulk across small areas but still have scant clad in areas. I
    would like something that could show that the copper is 6 oz and
    uniform across a 4"x6" area.

    In the end it probably will come down to the vendor providing
    breakaways with each board so we can crunch a bunch of numbers. Then
    verifying with a small sample of actual boards measured in several
    spots.

    Thanks to all,
    Ed V.
     
  7. Eeyore

    Eeyore Guest

    How would it be non-uniform ? Are you plating it to 6oz ?

    Graham
     
  8. EdV

    EdV Guest

    I am not certain exactly how it happens because I am not doing the
    plating but I have heard about these two:

    1. Chemical gradient in the bath because somebody forgot to turn on
    the agitator.
    2. Stuff dropped in the tank; pieces of pipe, tools,etc. that
    interfere with electrical characteristics of the bath.

    Nothing some good statistical methods shouldn't be able to wring out.
     
  9. Presumably you want it to carry current, not merely shield something. If so,
    drive a current through it and measure the V drop.


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  10. Jim Yanik

    Jim Yanik Guest

    ISTR NASA Tech Notes having some method for measuring coating thicknesses.
    Ultrasound,IIRC,but I'm not 100% on that.
     
  11. What if it's the right thickness, but porous or some damn thing? Might
    be better to measure the parameters that matter in the application.


    Best regards,
    Spehro Pefhany
     
  12. Rich Grise

    Rich Grise Guest

    Are you making these in-house, or is there really a vendor that's that
    bad?

    If they're in-house, clearly you need better process control - just
    turning on the agitator and not dropping crap into the tank might
    fix it. I've seen an ultrasonic thickness gauge, but used on a
    piece with nothing behind it - I don't know if it could catch just
    the copper and no FR4.

    But, if you keep your anode/cathode in good shape and keep the
    electrolyte fresh, you probably could keep track of thickness by the no.
    of amp-hours the plating process took. This would have to be calibrated,
    of course.

    Good Luck!
    Rich
     
  13. Fred Bartoli

    Fred Bartoli Guest

    Spehro Pefhany a écrit :
    Then after the ultrasound scan you can go for an osteoporosis diagnosis.
     
  14. whit3rd

    whit3rd Guest

    The sheet resistance technique preferred by NIST is
    based on four points in a square, probing a sheet larger than
    the point spacing. AC current is applied to NE and SE corners,
    and AC voltage drop is measured across NW and SW corners.
    It requires a substantial unetched area, and that means waste,
    usually.

    Since the specification is oz/sq. in, another obvious way to proceed
    is to
    punch out a known area, of 100% coverage with copper, weigh, and
    etch the copper off and weigh again. A hole punch and access
    to a good balance would suffice (and unlike the sheet-resistance
    meter, you can find lots of other uses for the balance).

    In either case, you can calibrate against a known sample
    and look at ratios, so absolute accuracy is of minor importance.
     
  15. That's why you need to drive a current through it. Same reason you need to
    load a battery to test it.

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  16. John Larkin

    John Larkin Guest

    You can also just mill or route an edge and optically scope the cross
    section. Edmund sells a decent little 50x handheld microscope with a
    measurement graticule.

    John
     
  17. Just my opinion: The test point resistance method would seem to be
    easier to implement for automated testing.
    Microscopes, micrometers and other such techniques will work fine for
    sampling incoming parts, but its going to be more time consuming for
    higher volume testing.

    Damn! I though this thread was about scantily clad broads, not boards.
     
  18. Robert Baer

    Robert Baer Guest

    Well, tell them to keep Frankenstein from drinking it and then peeing
    in it!
     
  19. Robert Baer

    Robert Baer Guest

    Fosamax anyone?
     
  20. Robert Baer

    Robert Baer Guest

    Well, if it is any consolation, thin boards can be *curved*!
     
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