At 01:41 PM 10/8/98 -0400, JoAnn Amerson wrote:
>Last night I locked myself out of my apartment and while waiting for my
>boyfriend to come over and let me back in, I spent the time pacing around the
>backyard and alternately cursed myself at my stupidity and thought about
>some specs the boss wants me to help write. Well, my brain works in strange
>(and wondrous) ways and I began to ponder something......
It may be weird, but consider the alternative!
>Would applying a gold plating to your component pads affect the electrical
>spacing required? IPC-2221, Para 6.3 and Table 6-1 do not mention board
>materials or any plating notes. Does this mean that it doesn't matter?
What is wondrous here is that JoAnn would consider that it would make a
difference. Perhaps one does not do one's best thinking sitting outside
cursing oneself.... Yes, it is a fairly silly question, but one who asks
silly questions, ten years down the road, is likely to be the resident
expert. Though it might help to take a basic course in electronics. One can
never tell when it will be helpful to actually know what is going on.
Others have already answered the question, more or less, but one person
thoughtfully tossed in some weird science. Steel balls in a pipe are nice
for explaining how it is that electrons move quite slowly compared to the
current they transmit, but the question is more about insulation and not
current transmission per se.
If I am correct (hey, I'm just a PCB designer!), there might be minor
variations in the ability of different metals to ionize the surrounding
medium (air, usually), especially when the effect of humidity is tossed in,
since copper, for example, or tin-lead, will oxidize, forming somewhat of
an insulating layer, unless it gets wet. I doubt that the differences are
significant enough to affect normal spacing requirements. Whether a surface
is smooth or has a roughness to it probably has more effect. I know for
sure that sharp angles make ionization easier. I once had some test pcbs
made to determine the breakdown voltage of various trace configurations at
a constant isolation distance. A sharp angle pointed toward the other trace
made a considerable difference, I don't recall how much, though 1500V for a
90 degree bend vs. 2500V for a smooth curve comes to mind.
I bet the specs don't mention the shape of the pads, either!
But, as far as actual breakdown is concerned, the specs are conservative.
Again, I don't recall the exact figures, but we began to get breakdown at
many times the voltage specified for the gap being tested; for the worst
trace, it was perhaps a factor of five or ten.
This is one spec one does not want to push too far, though! I follow the
IPC standards and the only exception was a board for a power laser which
was designed to be oil-immersed while operating. I don't have the spec in
front of me, but it does mention, as I recall, conformal coating.
Water-free oil does wonders for insulation distance. I would guess that is
why the power company uses oil-filled transformers, though perhaps the oil
also helps with cooling.
[log in to unmask]
Abdulrahman Lomax
P.O. Box 423
Sonoma, CA 95476
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