I don't need a Christmas miracle . . .the 0.4% I mentioned was 0.4% per
degree C temperature change.  For my expected 50 degree C change from
ambient and self heating, I will get a 20% change in resistance.

I can probably cope with an additional 20% ambiguity caused by board
geometry and thickness variations.  I have found a raw board specification
of 10% on the starting copper thickness.

Today's questions:
1.  What variations can I expect to see (on finished boards) with a
    standard production specification of 2-oz. Cu ?
2.  Are any of "you guys" willing to share success (or failure) stories
    of your attempts at "board etch Cu for current sense resistors"?


Thanks,
Carl



-----Original Message-----
From: Edward Szpruch [mailto:[log in to unmask]]

Hi,
As far as I remember IPC allow 10% variations in copper thickness.Take into
account that 2 oz is unit based on weight while you are describing
thickness.Add to this topography of the foil from lamination side, thickness
reduction as result of different surface preparation processes - it seems
you need X-mass miracle to get to 4% while you are expecting 0.4%. Good luck
and Happy New Year Edward


> -----Original Message-----
>
> I have a project that seems to be a good candidate for a current sense
> resistor made as a part of the PCB.  I am using the free calculator,
> PCBTEMP, from the UltraCAD website (http://www.ultracad.com/calc.htm)
> for geometry calculations. I need to know the expected variability in
> thickness, batch to batch, for the copper foil on 2-oz stock.  I can
> live with the 0.4%
> TC of the Cu, but I need to evaluate all the other variables in my error
> budget before I present my proposal.  I would appreciate any success (or
> horror) stories from people who have (not) used the board copper as a
> current sense method.
>

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