Homebrew PCBs

As a followup to my recent introduction and questions, I've got two
more questions (well, two more for now ... !)

1) How many of you fabricate your own boxes in which to mount your
projects? For the latest board, I fabricated a box out of some thin
sheet steel, snipped to shape with tin snips, bent with a vise and
hammer, and tack welded the seams. It is workable, but crude. I'm sure
there's a better way?

2) Both of my recent efforts with TT have been double-sided layouts.
The first time, I tried to align using the edges of the board; the
second time, I thought I had gotten smart (based on something I read
here or elsewhere), and I lined up the two prints using a bright
backlight, then stapled them together to make a "pocket" into which I
inserted the board. Both methods gave me boards that were usable, but
not as closely aligned as I had hoped -- the second was marginally
better, but still off in some places by maybe .010" - .020". What's
the secret to getting a perfectly aligned double-sided board?

On Sun, 22 Feb 2009 15:30:57 -0000, you wrote:

>As a followup to my recent introduction and questions, I've got two
>more questions (well, two more for now ... !)
>
>1) How many of you fabricate your own boxes in which to mount your
>projects? For the latest board, I fabricated a box out of some thin
>sheet steel, snipped to shape with tin snips, bent with a vise and
>hammer, and tack welded the seams. It is workable, but crude. I'm sure
>there's a better way?

I tend to use aluminum, and since I don't weld, resort to tapped holes
and screws.  The latest project has a case about 8 inches wide, 2 1/2
inches deep, and about 13 inches long.  It's made of 1/8 thick
aluminum angle as a frame, with side panels that bolt on (for
shielding).  Outside is thick plastic for looks and protection.  Case
is yet to be completely done.  Otherwise, I use plastic cases or
aluminum (purchased or salvaged) boxes as needed.
>
>2) Both of my recent efforts with TT have been double-sided layouts.
>The first time, I tried to align using the edges of the board; the
>second time, I thought I had gotten smart (based on something I read
>here or elsewhere), and I lined up the two prints using a bright
>backlight, then stapled them together to make a "pocket" into which I
>inserted the board. Both methods gave me boards that were usable, but
>not as closely aligned as I had hoped -- the second was marginally
>better, but still off in some places by maybe .010" - .020". What's
>the secret to getting a perfectly aligned double-sided board?
>

I haven't managed to do that yet, the perfect part, though.

Several methods.  The nicest one is to etch in two steps.  On the PC
board pattern, place symmetric vias at the corners, smaller than your
normal vias.  Hole size roughly that of a straight (map) pin.  Place
another via off center.  Make no connections to these vias, they're
for alignment.

Place the pattern on the board, transfer, and etch one side.  Self
stick shelf liner will protect the other side of the board.  (do not
run the shelf liner through a laminator...).

Drill out the vias used for alignment.  With a straight pin, punch
alignment holes in the remaining pattern (you can also do something
similar with the silk screen layer, but vias aren't there on that
layer).  Align the board using the pins, tape, and transfer the toner.
Etch, with the previously etched side of the board protected.

Otherwise, you can try the same thing with aligning the top and bottom
to already drilled holes.  May not be as accurate, though.  

Sources of error:

1) paper slipping while laminating.   I feed through in one direction,
reverse, feed through in 180 degrees, etc...

2) always feed the paper in the printer the same way, do not reverse.
This tends to minimize distortion across the width of the paper.

I can get pretty close with this.

Harvey

I use the old reliable method. 

I place 3 fiducials on the board for alignment holes near the corners
of the board. Two are for alignment and the third is a check point.
They should be near 3 of the 4 corners on the pcb. These can be actual
fiducials or they can be mounting holes or component holes near the
corners as well. 

I carefully poke a hole through the center of the alignment holes on
the images for both layers with a pin (fiducials have a cross hair to
make this easy to do). 

Take one layer and lay it on the copper board and mark through each of
the 3 holes with a small sharp center punch (accuracy depends on how
well these holes are marked and drilled.

Drill each hole carefully with a small bit (I use a #75).

Place the top layer image on the copper board and align it by placing
pins through the hole in the image and the pcb in two opposite
corners, the 3rd hole can be used to verify alignment is good or can
be used for alignment if one of the other holes are bad for some reason. 

With the image aligned on the pcb, tack it down with an iron being
carefully that the image is pressed flat to the pcb and doesn't move
while tacking. 

Remove the pins and Iron the image thoroughly to the board. Let board
cool and repeat on the other side (if the board is hot, the image will
start to stick before being aligned).

After both sides are done, soak & remove the paper. This has worked
perfectly for me every time. All of my boards are double sided with
usually 10/10 rules. Vias are a little larger to make drilling easier.

Phil (KA0HBG)



--- In Homebrew_PCBs@yahoogroups.com, "awakephd" <a_wake@...> wrote:

> 2) Both of my recent efforts with TT have been double-sided layouts.
> The first time, I tried to align using the edges of the board; the
> second time, I thought I had gotten smart (based on something I read
> here or elsewhere), and I lined up the two prints using a bright
> backlight, then stapled them together to make a "pocket" into which I
> inserted the board. Both methods gave me boards that were usable, but
> not as closely aligned as I had hoped -- the second was marginally
> better, but still off in some places by maybe .010" - .020". What's
> the secret to getting a perfectly aligned double-sided board?
>

Hi,

 

I use 20mil sewing pins (available at sewing supply stores) which are, I
think, brass coated, steel pins to align 2 layers. These pins are very handy
for all sorts of things - they solder easily and are very sharp. For example
they make excellent probe tips.

 

Anyway, I print two diagonally opposite alignment holes (vias or pads) just
outside the trace area on both layers when designing the board. I then use a
plain paper template and tape it to the copper on one side and drill the two
alignment holes. After printing the toner transfer paper I use a pin to
punch holes in the transfer paper. Using two pins I align the top and bottom
layer. Next the transfer paper is taped to the copper with Scotch tape. Make
sure the copper board is slightly larger than the prints to allow using the
Scotch tape.

 

Using this method, I regularly achieve alignment accuracy approaching +\-
1mil.

 

On a slightly different subject, I noticed in some recent posts that glossy
photo paper is used in laser printers. Am I correct? I have never tried it
because I was told that the melting temperature of the paper coating is too
low and the heat roller in the laser printer gets gummed up.

 

Am I missing something here?

 

Frank  VK4BLF



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