piclist-bounces@mit.edu wrote:
>   The calculators tell me that the trace should be 30 mils
> wide.  (I use 40 to be safe.) 

Bear in mind the underlying assumptions of the calculator you are using.
Some examples of assumptions are temperature rise, copper thickness,
external layer, etc. Generally the assumptions err towards the
conservative side.

> Now when I need to go from top
> to bottom metal, what size via (also called feed-thru holes)
> is required?

Good question, and one that I am sure many other people have wondered
about. My first attempt would be to equate the cross-sectional area of
your trace to the cross-sectional area of the via. Your board
manufacturer should be able to tell you plating thickness. As far as I
know, vias will generally have thinner copper than traces. this plus
your drill size should be enough to determine the cross-sectional area.

My second reaction is that since you're talking about a lightbulb at one
ampere, its unlikely you will have problems. Make the via as large as
you can for your existing 40-mil trace and be done with it.

> The only reference that I was able to find indicated that
> the via drill size should be greater than the metal width/3.
> So for a 40mil wide metal the drill size should be greater 
> than 14mils.

got a reference? I'd be interested in this.

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist