At 01:15 PM 6/14/01 -0500, David Cary wrote:
>In my case, I
>finally got enough documentation to prove that PWB traces varied in resistance
>far more than the 1% we wanted in our current-sense resistor. (Mostly due to
>variations in the tin plating over copper, I hear).
Actually I'd be surprised if that was the major contributor. This is an old
figure, but Lund shows a thickness on 1 ounce copper thickness of 35 +10/-5
um. That is a *huge* variation for a sense resistor. There is also the
problem of etch control which would affect the width of a trace, but the
thickness variation would be a much larger error source. Add to that the
plating variation: It appears that foil plus plating on a 1 ounce plated up
1 ounce board is specified as not less than 35 um. The maximum does not
seem to be a matter of concern, as long as the board meets other requirements.
But half ounce copper was specified as 17.5 um +/- 5 um. That's a little
better, and plating could be avoided in various ways. Still, the resistance
is going to vary by +/- 35% or so.
That might be usable as a sense resistor, even for accurate measurements,
if it could be trimmed or calibrated. The variation of resistance with
temperature would need to be considered. There might be applications where
this would make sense.
Using thin track as fuse is a well-known bad idea, precisely because of the
problems noted above plus a few, though, again, even bad ideas can have
their day.
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Abdulrahman Lomax
P.O. Box 690
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