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Thread: Current on a PCB
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face BY : Mark Willis email (remove spam text)



I do it this way, just in case someone misunderstood me - keep seeing
posts that might indicate that:  I'm a "massive overkill style" sorta
guy <G>

This saves having to make another board when you were in a hurry and
some twerp stole your good test leads and that dang test probe slips and
shorts Vcc and Ground.  (We all heat shrink 90% of the test probes off
our DVM's, or use microclips, I'd bet.  Dingbat DVM manufacturers - who
wants test probes?  Ack.)

You don't get paid for removing copper, you get paid for the final
circuit working <G>

Tightest packing I ever use is 12.5 mil to 15 mil traces on 25+ mil
centers, for SMD CMOS gate to CMOS gate routing.  Usually 25 mil traces
on 100 mil centers (leaving room for a trace between pads where
needed.)  Keep the smaller traces as short as possible i.e. just for 50
mils or less as you go between pads on a DIL I.C., use 10 mils, then
back to wider traces.  Avoid that when you can, as it's a bear to FIX
that *when* that fusible link blows...

I use 35 mil traces at a minimum for Vcc/Ground.  Usually 50+ mils.  Or
just Flood Fill all unused space for a power/ground plane.

(Go 250 mils for a 1.5A power supply, right?  <G>)

If things go wrong, solder a piece (or even several) of wire wrap or #22
phone wire down atop the ground trace, to "enhance it's current carrying
capability somewhat".  It's a pain, almost never have to do that unless
space is really tight - Or, nightmare of nightmares, someone who doesn't
know what they're doing routes a bad PCB.  (Some people think you can
get away with 10 mil ground & Vcc traces that snake all over the board,
and things will work fine.  Well, even when you solder some #32 WW wire
down on that puppy, you still have problems, and the PCB Cad guy's your
worst problem <G>  Educate 'em, gently, on what the consequences are of
that thinking.)

Use despiking caps, large and small, then add more, and add some more
for fun;  Add more on a whim - any whim will do.  Get it working right,
then you can remove some them to lower costs, if the manager wants to
spend the time -  Engineering time costs too, and there's nothing like a
glitch to confuse you in your coding.  For quick prototypes, use SMD
parts when you can, if out you can take a scrap box through-hole part
(one of those with short, cut leads is good <G>), bend the leads and
solder it down SMD style - saves drilling more holes, works well too,
and is fast & cheap.

Put text on the PCB somewhere to remind you what the durn thing's for,
btw.  "Solder" and "Component" side markings (even "S" and "C"!) are
good ideas too, those'll save your sanity.  Also - design the boards so
they're not completely symmetrical, if you can (Ever seen someone turn
the solder side 180 degrees and it FITS?  That can really mess things
up!  Wasn't pretty.  Love learning vicariously on THAT sorta thing.)

Ground rings are good where applicable <G>

I mingle TH and SMD a lot, put LEDs & DE-9 connectors etc. on the edge
of the PCB, for example - it's quicker than drilling and works well;
Learned all that when I didn't have a drill press available for one job,
having just a few pin vices and elbow grease for drilling gets sorta
old.  You don't get extra points for working harder, just for working
smarter and faster.

Finally, cast the spec in quick-setting epoxy and wait for it to cure
before starting work, whenever possible <VBG>  (A spec that isn't set in
stone, is usually a spec that'll run away FAR faster than you can code
and design...)  It's "time to practice and re-train yourself" if they
don't have that set yet, you'll probably have to re-do things (maybe
you'll be lucky & can re-do it all in software, though.)

 Mark

<39355D28.496F9D5F@foxinternet.net> 7bit

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