JoAnn Amerson wrote:
>
> Once again I turn to you pleading for help.  The "powers that be" are
> requiring us to use a 12 mil wide trace as a minimum unless we
> absolutely cannot because of lack of space.  It seems our board
> vendors have difficulty meeting a width and trace at 8/8.  My pleas
> to look into other vendors fall upon deaf ears.  "They're too
> expensive!", they cry.  So we pay the price over-designing our
> boards.  Our boards are predominantly low current, low voltage.  8
> mil usually is more than enough but......I'm sure you've all run into
> this argument at one time or another.
>
> What I'd like to hear from everyone are reasons why we *shouldn't*
> use a wider trace than necessary - if there are any.  Other than it
> takes longer to route a board with wider traces.  They don't care
> about that.  What documentation can I offer to them to make our lives
> just a tad bit easier?  Or am I lost in wishful thinking?
>

It would depend on your normal production run and other factors.  When I was
designing for automotive we used single-sided, thru-hole, punched boards with
13/12 trace/space.  The cost difference to go to a "higher tech" board was
_quite_ substantial, considering the usual production run was 1 to 2 million.
All these boards were low voltage, most were low current, but were manufactured
using silkscreened etch resist rather than photo to keep costs at a minimum and
number of boards/hour at a maximum.  This meant that if you went to 8 mil traces
the board fab would have to scrap a minimum of 25% more of the boards and they
charged you for that scrap.  Old technology is just a fact of life in some
industries; I guess it might be the case for your company, but you would need to
find out.

>
> And also, today's dumb question:
> I know that spacing between traces is governed by voltage and the
> width is governed by current load and temperature rise above ambient.
>  If I use a 12 mil wide trace, can I still maintain an 8 mil wide
> space to the next trace?  Assume it meets the spacing for that
> voltage.  Or is the spacing always at least track width?
>

"The only dumb question is the one that you don't ask!"

If the manufacturing technology permits you could decrease the spacing.  If you
are silkscreening the etch resist you have to allow for strech and movement
while the resist is going on, which may be the reason for the huge gaps between
traces.

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