One solution is to calculate the copper area of your "standard" via and 
compare this to the recommended trace area.  Then use multiple vias 
instead of just one.  If one via calcs out to being close, then use ten 
vias and sleep at night.

Something else to do is make some measurements of voltage drop across 
vias that were made by the board shop you expect to use (if possible).

Take a bare board, locate an accessible trace with a via and apply a 
current limited supply to it, preferably at say 1 amp.  Then take a 
voltmeter and measure the drop along the trace.   This will give you at 
least a ballpark idea of what the resistance is across the via and thus 
what the I2R losses might be approximately.

Phil Keller wrote:
>   I have a design that is controlling an incandescent lamp with a 
> current of about 1 Amp.  I am not as concerned with the voltage drop as 
> I am with overall reliability and ensuring that the via does not become 
> a fuse.  There are many places that help in calculating the suggested 
> PCB trace width for a given current but what about the via?
> 
>   The calculators tell me that the trace should be 30 mils wide.  (I use 
> 40 to be safe.)  Now when I need to go from top to bottom metal, what 
> size via (also called feed-thru holes) is required?  Drill size and 
> overall diameter of the 'cover' metal?  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.
> 
>   Thanks for any suggestions.
> Phil
> 


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