piclist-bounces@mit.edu wrote:
>   The reference I was using for via size is:
> http://www.ultracad.com/articles/viacurrents.pdf

I noticed they made the assumption that via wall thickness was equal to
trace thickness, and called it T. Then they assumed T is small wrt to
width and ignored it in the final equation, and rounded PI tp 3. Ok, ill
buy that .040 >> 0.0008, but I'd want to do the calculations myself
before using the Dia = width/3 rule. I'm not sure about their formula
for cross-sectional area of the via, either. Maybe they did some
simplifying, but I'd take a second look at that.

>   The fab house I use has a trace thickness of 1.3mil-2mil  (standard
> 1oz/sqft) and  a via wall thickness of 0.8mil-1.2mil.

always assume the worst case, so use 1.3 for the thickness of the trace
and 0.8 for the via thickness. Think of the via as two circles, the area
of one subtracted from the other to get the cross-sectional area of the
barrel. Set the two areas equal and solve for drill size as they did in
the article.

comments anyone?

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