Homebrew PCBs

Alan,
That's right, it isn't very high for digital circuitry.  A 16F877 and
a few other supporting ICs will run the count up in a hurry.  That's
why I mentioned that my 215-pin design was discrete components only. 
The design was the IF system in Figure 6.50 of "Experimental Methods
for RF Design", the one using two pairs of J310 JFETs in cascoded
arrangement.  There are a lot of components in that design, and I had
about a 1 square inch open area left on the 3 x 4 Eagle board when I
was finished.  

I manually routed the board as I went, by the way.  I added the
components for a functional section to the schematic, for example the
JFET pairs and associated circuitry for the board signal input from
the crystal filter, laid them out on the board in a logical position,
and routed; then I moved on to the next section, etc., following the
signal flow from the schematic in the figure.

Doing it that way made it pretty easy to tweak the routing when I had
all of the components on the board, without having to unsnarl any
oddball paths created by autorouting.  I found that when I'd rip up a
few tracks to move components around a bit, I could just hit the
autoroute icon, and it would redo the routes pretty much as I wanted.

Originally, I laid the board out with two 12 volt feeds, and some
jumpers from the on-board regulator for Vcc to various points in the
cicuit, but as it turned out, I was able to lay down tracks for most
of the power feeds to the functional sections by the time I was finished.

I did this board with a ground plane for the component side.  To avoid
having to mill clearance holes for non-grounded through-hole component
leads, I exported the board image as a BMP file, and using the layer
feature in Photoshop (freeware Paint.net will do layers, too), I
manually added circular clearance holes for the ground plane side. 
After  I printed the bottom layer tracks and ironed, I drilled a few
holes in through-component lead pads to line up the holes, and then
aligned the ground plane printout by holding the board up to the
light.  Then I ironed the ground plane pattern onto the PCB.

I found that when I reheated the board while ironing on the ground
plane/component side, some of the bottom layer tracks' toner
transferred to the pad I was using as a surface for ironing.  Next
time I do a double-sided board, I think I'll etch the tracks side
(masking the other side with electrical tape), drill my registration
holes, and then iron the ground plane side.  It shouldn't be necessary
to do it that way.  Under my old process of applying the toner to
Press-n-Peel Blue with a copier, that reverse-transfer wouldn't have
happened. P-n-P is so tough when it's applied with a copier, you just
about have to sand it off (or use acetone, as I learned after many
boards :) ).  Now that I'm using my Brother 2040 laser printer for
toner transfer, I have real problems in getting the toner to stick to
the board in the first place, which has been much discussed here in
this group.

73,
Ted KX4OM
www.kx4om.com
 
--- In Homebrew_PCBs@yahoogroups.com, "Alan Marconett" <KM6VV@...> wrote:
>
> Hi Ted,
> 
> That doesn't seem very high.  I easily exceeded that in the little PIC
> design I'm doing.  Connectors eat up a lot of pins I guess.  Digital
> probably has more pins typically then analog.  I'm at maybe 350-400 now.
> But then this is really my first PCB project (that I'm laying out)
in a long
> time. 
> 
---snip---
> > The free version of DipTrace has no limit on board size, but is
> > "complexity limited" by a 250 pin maximum, which is fairly generous.
> > I did a receiver IF system using all discrete components in Eagle,
> > which pretty much filled the board, and the pin count was 215.
> > DipTrace would be especially good for boards that need large foil
> > traces, physical separation and have physically large components, such
> > as power supplies with board mounted transformers and large
> > computer-grade electrolytic caps.
> > 
> > Cheers,
> > Ted  KX4OM