Homebrew PCBs

Hi all,

I was wondering it there is an introduction to basics of schematics 
and board design?

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 ?

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 ?

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

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

On Sun, 21 Sep 2003 14:24:05 -0000, Dave Mucha <dave_mucha@...> 
wrote:

> Hi all,
>
> I was wondering it there is an introduction to basics of schematics and 
> board design?
>
> 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 ?
>
> 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 ?
>
> and it seems a 90 degree corner on a 15 mil trace, when etched can be 
> undercut to less than 10 mil.
>
> 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
>
>
>
you can design all these things like you want them.
like it makes best sense for you.
track with is determined by current/voltage drop you can accept.
but in many cases the voltage loss due to resistance is so little that you 
rather make it as small as your design process allows to be manufactuered 
securely. (if you have lots of space make it wider).
further you for sure will not calculate the minimum with for each track, 
you simply take "standard with"
0.3mm or so what you like and only the tracks which are higher current 
(maybe supply etc.) you make thicker.

the sqare pad is pin 1 most cases. never thought about cathode/anode on 
diodes. i leave it how the pcb software
makes it.

i don't understand what you mean with isolate the rectifier part...

standard drill sized i don't know because i drill myself.
but 0,6mm, 0,8mm, 0,9mm, 1mm, 1,2mm and maybe 0,4mm if you need i am sure 
any boarhouse in europe offers.
0,85 and 0,95 too.
If you can choose than you select the drill for the wire thickness 
(emphasis on tolerances!).

ground rings around opamp inputs work. but there is also "guard ring".
it puts the "ring" or coax shielding not at ground but on a voltage which 
is the same like the one measured (but low impedance).
this way you get zero losses from bad insulation etc. because there is no 
voltage differential thus no current flow.
some opamp have special guard output but you may often use the other input 
if it is low impedance.
(remember in amplifier configuration the inputs are at nearly same voltage) 
.

monkey-see, monkey-do is not the worst way.
but monkey always should know why he does things.
only doing things for no special reason only because somebody else did it 
is not good.
it may be not adequate for the situation.


st

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>From: "Dave Mucha" <dave_mucha@...>
>Reply-To: Homebrew_PCBs@yahoogroups.com
>To: Homebrew_PCBs@yahoogroups.com
>Subject: [Homebrew_PCBs] basic basics of boards
>Date: Sun, 21 Sep 2003 14:24:05 -0000
>
>Hi all,
>
>I was wondering it there is an introduction to basics of schematics
>and board design?
>
>Things like what polarity is the square pad on an LED ?

Square pads generally indicate pin 1 on a component. I don't think it is 
used much these days, as it takes up too much area.

>Why 15 mil traces and not 12.5 mil ?

For my home made PCBs I go down to 10 mils occasionally.

>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 ?

0.8 mm is the most common drill size, used for ICs, Rs and Cs.


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

It's a good idea to mitre all corners. They etch better and work better at 
high frequencies.

>
>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.

"Sitting by Nellie" is the UK equivalent.

Leon
--
Leon Heller, G1HSM Tel: +44 1424 423947
Email:leon_heller@...
My web page: http://www.geocities.com/leon_heller

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--- 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!

--- In Homebrew_PCBs@yahoogroups.com, "ghidera2000" 
<ghidera2000@y...> wrote:
> --- 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 ?

OK, My friend has been around a long time and has pretty much done it 
all.  Says the square pad on a 2 pin is ground.  That's where it all 
started.

that go me thinking about when I was doing instrumentation schematics 
(as opposed to pcb schematincs)  there is a complete written 
stanadard that details everything.   It would make electronics a 
little easier if were adopted some of those practices, or if they 
existed.




> > 
> > 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.



YIKES !    I use a #78 for resistors (0.015")   0.9mm for thru chips 
and 1/16 for screw terminals.   I just figured that there is 
something more 'standard' than using one of each lot. 



> > 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.


Thans for the link.   (btw, the period on the end makes it a pita to 
use.)


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


Nellie Prymaite.  

She taught us by sitting beside Nellie in the moring and in the 
afternoons, she would have us do what she did.

It sound like we went to different high shcools together.

Dave