Dave, 

In a word, 'No'. Not 'alright'. I can see that you were making a joke, of
course.

The personal risk in designing a board that is 'difficult to manufacture' is
that the general effect will be that it causes someone out there to curse
your name EVERY TIME they have to build your board... Day in and Day out...
How many boards with they make? That could potentially be a lot of negative
energy floating around there in the world... who knows what might happen
from that? Risky... very risky... Do you want to be responsible for that? I
prefer to play it safe if possible.

Better to make the design more manufacturable, and easier to solder and have
people curse 'someone else' for the thing 'they' designed that is hard to
build... and not your board.  Life is difficult enough as it is... let's
keep it simple and the negative energy always pointing away from you.

Here's a physical principle you can exploit... electrical current can be
divided or 'shared'... If one wire can handle an amp... two wires of the
same size together can handle 2 amps... and 4 of the same size can handle 4
amps...etc. 

So... figure out what equivalent amount of copper in cross sectional mils or
width of trace will handle the 30 AMPS...  Make sure you keep the total
copper quantity consistent all along the path where it must go. When you get
to the point where you absolutely cannot avoid using vias... make them do
'current sharing' as well. 

Calculate how much current a single via can handle with a 10 degree rise
over ambient and then add enough extra vias to give yourself some margin...
The vias do not need thermal spokes... nothing gets soldered to them... only
leads in holes and surface mount parts are really a concern. 

You must look at the design and figure out if the parts can be soldered
using a common manufacturing process... and not hand soldering if you can
avoid it... that is the most expensive way to solder if you are making
boards in quantity and a last resort when there is no alternative. 

I would consider that approach to be the 'safer way' to go with the
application you are describing. Trust me... Your employer will love you for
it, maybe even at bonus time. :)

Best regards,

Bill Brooks 
PCB Design Engineer, C.I.D.+
Tel: (760)597-1500 Fax: (760)597-1510
Datron World Communications, Inc.
Vista, California


-----Original Message-----
From: David Baldwin [mailto:[log in to unmask]] 
Sent: Tuesday, May 08, 2007 4:04 PM
To: [log in to unmask]
Subject: Re: [DC] Current Capacity of Thermal Reliefs

George:

Nice calculator, good job.  The via though is just a via, 
correct?  It doesn't have anything for thermals.  Like Bill said, I 
can add up the spokes and get the overall width and treat it like a 
trace.  Or I could just forget about the thermals and let the 
assemblers deal with soldering the parts down, not the designers 
problem right;-)

Thanks,

Dave

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