Yes I did notice the 'simplifying' assumption.  Taking the fact that 
the via is 1/2 as thick as trace metal this means that the via should be 
greater than 2/3 of the metal width.  Also from the same article (I 
believe.  I have read so many and they are beginning to blur.)  and the 
suggestion that many is better than one,  the SUM of the via diameters 
should be greater than 2/3 of the metal width.

So in this case a 40mil trace could have
- one 27mil via,
- two 14mil vias,
- three 10 mil vias

  Thanks to all for the suggestions and the discussion.  I now have a 
new "rule-of-thumb" to use.

Phil

-- 
--------------------------------
Long ago when men cursed and beat the 
ground with sticks,  it was called witchcraft..

Today, it's called golf.


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