Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, "Dave Mucha" <dave_mucha@y...> 
wrote:
> Hi all,
> 
> I was wondering it there is an introduction to basics of 
schematics 
> and board design?

Me Three :D

 
> Things like what polarity is the square pad on an LED ?
> Why 15 mil traces and not 12.5 mil ?
> how to best isolate an AC rectifier/cap from the rest of the DC on 
> the board ?  things like that.
> 
> are there 'standard' drill sizes that are common for all board 
> houses ?

IFAIK no, just 1/32, 1/16. whatever is the smallest size your parts 
will easily fit into. Probably 1/16th would be the best general 
purpose - though them big diode leads generally won't fit. Need 3/32 
I think.

> I have a friend who says all square pads on 2 pins are ground or 
> negative. Is that rule of thumb or general industry practice ?

Means pin 1 on chips and typically positive on things like caps etc. 
Haven't seen anything stating that this is a law or anything.

> and it seems a 90 degree corner on a 15 mil trace, when etched can 
be 
> undercut to less than 10 mil.

So put a couple 45's in there instead. Check out a trace width 
calculator like 
http://www.geocities.com/CapeCanaveral/Lab/9643/TraceWidth.htm. You 
might be suprised at how much current a tiny bit of copper can 
carry. e.g. a 15 mil trace can carry 1 full amp and only raise five 
degrees C.

The most common reason for larger traces is that they're less likely 
to be botched during home etching.

The second most common is to keep noise down on power traces (ground 
being more important - use bypass caps for Vcc and the traces can 
stay thin).

Third is high frequency. Electrons under high frequency tend to 
travel at the edges of a trace and shy away from the center. There's 
a ratio of how far out from the edge the electons travel and the 
frequency the circuit is at (saw it in passing but didn't pay 
attention). You have to make these traces wide enough so that 
the "used" portion of the trace is just as big as the DC only 
equivilent current would require. Or something like that - like I 
said, I didn't pay a lot of attention.

> Instrumentation inputs may be put inside of a ground ring to as to 
> keep all stray baord voltages away.
> 
> Seems I went to the monkey-see monkey-do school of board design.
> 
> Dave

Hey, who was your homeroom teacher? I might have had the same one!