At 08:55 AM 7/23/98 -0400, JoAnn Amerson wrote:
>Now that I have EVERYONE'S attention.........I have some questions about
>necking down conductor widths.

And I thought this was a serious mailing list!

[...]
>If we have determined that we need a 15 mil wide trace (by using the
conductor
>width nomograph in the IPC spec), is it safe to neck it down to 10 mils for a
>short distance?  If so, define short.  Or do we assume that 15 is the
absolute
>narrowest a trace can be for ANY distance?

I am assuming that the question is about trace width determined by maximum
current. Sometimes one sees charts which give width vs. current for a 20
degree C. rise. I would answer that the standard, if I am correct, gives
trace width vs. maximum temperature rise in the trace. If one has a short
length of trace (but long with respect to its width, and 100 mils is long
with respect to, say, 10 mils), that length will experience the temperature
rise appropriate for its width and current.

Basically, it is not safe to neck down below those widths if there is only
a single path. However, note that if this is a trace distributing power,
some segments may not be carrying the full power, and those segments could
be narrower. But generally, with power traces, if I need to neck them down,
I will attempt to place redundant segments. Two 10 mil traces in parallel
equals an effective trace width of 20 mils. Another trick, if possible, is
to increase the copper thickness; doubling the thickness would allow half
the trace width.

If we are talking RF impedance, it's another story....

[log in to unmask]
Abdulrahman Lomax
P.O. Box 423
Sonoma, CA 95476

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