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'[OT:] Homebrew DS pcb - registration'
2004\03\03@035905 by Hulatt, Jon

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Previously I've mostly made SS pcb's at home with very good results. Last
couple have been double sided, but I'm still having problems getting the 2
sides properly aligned. My last effort was "good enough", but I'm still
destroying half of the pad on the opposite side when I drill the really
small pads (like for my 4 vias).

I use presensitised board, and laser printed transparency and a homemade UV
box.

How do other people align the transparency on either side?

jon

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2004\03\03@040701 by Tim ODriscoll

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On Wed, 3 Mar 2004, Hulatt, Jon wrote:
> Previously I've mostly made SS pcb's at home with very good results. Last
> couple have been double sided, but I'm still having problems getting the 2
> sides properly aligned. My last effort was "good enough", but I'm still
> destroying half of the pad on the opposite side when I drill the really
> small pads (like for my 4 vias).

I'm about to branch into the double-sided world too, and I was thinking of
printing out the overlays with cross-hairs out of the boards' area, then
taping (or stapling) the two transparencies together to form an envelope
using the cross-hairs for alignment. Then you could pop the board into it
and tape it so it can't move.

Well, it seems to work OK in my head anyway :)

Cheers,

Tim

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2004\03\03@044929 by Eric Christensen

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That's about what I do.  In fact, I don't even put cross hairs on it.  I simply
use the holes from through hole parts to line things up and then tape the two
transparency sheets together to form the envelope.

The only time this method hasn't worked for me is when I was careless inserting
the PCB into the envelope.  If you imagine the side profile of the PCB, you need
to make sure the tape joint is half way between the top and bottom of the PCB.
It is easy for the transparencies to slide so that the tape joint is at the top
or bottom, causing the layers to be slightly offset from each other.

You could probably minimize that problem by using the biggest transparency size
you can.  I was limited by my 6" x 6" glass sheets that I use to hold the
transparency flat to the PCB during the exposing step.

So yeah, this method isn't guaranteed to be super accurate, but with a little
care it can produce very nice, well aligned double sided PCBs.

Eric





On Wed, 3 Mar 2004 09:08:20 +0000
Tim ODriscoll <spam_OUTtimTakeThisOuTspamSTOKEPOGESWAY.NET> wrote:

{Quote hidden}

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2004\03\03@051252 by Mike Harrison

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On Wed, 3 Mar 2004 08:49:35 -0000, you wrote:

>Previously I've mostly made SS pcb's at home with very good results. Last
>couple have been double sided, but I'm still having problems getting the 2
>sides properly aligned. My last effort was "good enough", but I'm still
>destroying half of the pad on the opposite side when I drill the really
>small pads (like for my 4 vias).
>
>I use presensitised board, and laser printed transparency and a homemade UV
>box.
>
>How do other people align the transparency on either side?
>
>jon

Staple the 2 artwork sheets together to form an envelope, into which you insert the PCB.

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2004\03\03@084447 by Anthony Toft
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> How do other people align the transparency on either side?

Something that came up in the homebrew PCB yahoo group recently (and is
probably the way I will do it) is to get the very thin single side
material, and etch 2 pieces. Drilling afterwards like normal, and then
epoxy'ing them together in the correct registration.

Seems like an easy way to guarantee good results.
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Anthony Toft <.....toftatKILLspamspam@spam@cowshed.8m.com>

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2004\03\03@090908 by Eisermann, Phil [Ridg/CO]

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pic microcontroller discussion list wrote:

> How do other people align the transparency on either side?

Others have mentioned the 'pocket' method. I've used it,
and it works for small PCBs.

The following method may sound complicated, but it
really is not. Hope I explain it clearly enough.
Maybe it will spark ideas for improvement.

I cut two thin sheets of plexiglass such that the width is
always less than my PCB, but the length is always greater
than my pcb (say like 2" by 12"). The two sheets were
taped together. Using a drill press, drill three holes
for 1/4 screws. Now the two sheets can easily be aligned.

One artwork transparancy is attached (taped) to one of
the plexiglass sheets so it cannot move. the second is
loose. Assemble the plexiglass and position the second
artwork so they line up. Secure the second artwork to
the second plexiglass.

Now loosen the screws so that you can slide your PCB
in between. tighten screws, secure the artwork to the
PCB; then remove the plexiglass, leaving the artwork
aligned and attached to the PCB.

This method requires the artwork to be smaller than
the actual PCB, but larger than the plexiglass
frame so it can be attached. It's sufficient
if the length or width of the artwork is smaller;
that's why the frame is long and skinny.

The above procedure could be simplified if the
plexiglass were replaced by glass; I'm thinking of
a plexiglass "frame" that could be aligned as
described above. this frame would hold the glass
in place. Now only the artwork would have to
be larger than either the length or width so
the two sides can be aligned.


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2004\03\03@112747 by Rafael Fraga

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I drill a few holes in the first etched side, and use them to align the
other side. If you drill really small holes, the registration is quite good.
It takes some time and practice, but this is homebrew anyway. During both
etch baths, I protect the "other side" with standard brown parcel tape. And
I don4t sensitize the second side until the first is ready.
My wish list:  real PTH holes, in a not necesarily easy way.

Rafael Fraga



{Original Message removed}

2004\03\03@113410 by Rafael Fraga

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How do you manage plexiglass bending in the middle of the pcb?  The artwork
must be against the pcb to avoid shadows and light from entering under the
traces...

Rafael Fraga

{Original Message removed}

2004\03\03@120111 by William Chops Westfield

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On Wednesday, Mar 3, 2004, at 05:41 US/Pacific, Anthony Toft wrote:

>> How do other people align the transparency on either side?
>>
You COULD drill some of the holes first, put more holes in
your transparencies, and run registration pins/wires through
the whole sandwich.  But I think even with perfect registration,
you're somewhat likely to push the bottomside pads off during
drilling.  Try larger pads (if you're in control of that.  that's
one of the defaults i change all the time in eagle...)?

BillW

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2004\03\03@130442 by Eisermann, Phil [Ridg/CO]

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pic microcontroller discussion list wrote:
> How do you manage plexiglass bending in the middle of the pcb?

that's why the plexiglass is removed afterwards:

>> Now loosen the screws so that you can slide your PCB
>> in between. tighten screws, secure the artwork to the
>> PCB; then remove the plexiglass, leaving the artwork
>> aligned and attached to the PCB.

I use the normal glass/foam frame for exposure afterwards.
I thought it was obvious, having done it so many times.
sorry that this wasn't more explicit. The plexiglass is
only used to align the top/bottom artwork onto the PCB.

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2004\03\03@175512 by Gaston Gagnon

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Hulatt, Jon wrote:

> Previously I've mostly made SS pcb's at home with very good results. Last
> couple have been double sided, but I'm still having problems getting the 2
> sides properly aligned. My last effort was "good enough", but I'm still
> destroying half of the pad on the opposite side when I drill the really
> small pads (like for my 4 vias).
>
> I use presensitised board, and laser printed transparency and a homemade UV
> box.
>
> How do other people align the transparency on either side?
>

What about drilling the holes in the pcb before etching?
Of course it is easier if you have access to NC machine but here is a
possible method if you don't.

1) Print the holes layer of your pcb on a sheet of paper;
2) spray a thin coat of artist glue onto the copper clad and lay
   the sheet of paper without any wrinkles and air bubbles;
3) drill all the holes;
4) Clean the paper off the clad.

I have used this method for folding metal sheet to make prototypes and
it works great.

Let me know how it works for holes.
Gaston

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2004\03\03@200952 by Dwayne Reid

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At 01:49 AM 3/3/2004, Hulatt, Jon wrote:
>Previously I've mostly made SS pcb's at home with very good results. Last
>couple have been double sided, but I'm still having problems getting the 2
>sides properly aligned. My last effort was "good enough", but I'm still
>destroying half of the pad on the opposite side when I drill the really
>small pads (like for my 4 vias).

I use a piece of PCB material cut into the shape of a carpenter's
square.  Think of a large "L" about 6" tall with a 4" leg.  The material is
about 1" wide.  I make mine from the the same type of PCB material that I
am exposing: un-etched double sided board.  I want it to be the same
thickness or just a smidgen thinner (its thinner by the thickness of the
photo-sensitive laminate).

Tape one of the transparencies to both legs of the "L".  Make sure to
position the transparency so as to leave a border of scrap material on the
finished board.  In other words, don't position the transparency so that it
is exactly on the edge of the holder.  I set mine up about 3/16" or 1/4"
inward (the boards are cut about 1/2" larger than the finished size).

Now flip the whole thing over and place it on a light table.  Carefully
line up the other transparency and tape only one edge to the long part of
the "L".  Check the alignment again.  Then make sure the transparencies are
not reversed!

Flip open the holder, drop in the PCB and push it tight into the corner of
the "L", close it and put the whole thing into your exposure frame.

The main reason for taping only one edge of one of the transparencies is
that they stick to the PCB after exposure.  That is - they stick when I
expose boards in my setup (175 W Mercury vapor bulb with outer glass
envelope removed, vacuum frame running at about 15" of mercury vacuum
level).  Since only one edge is taped on one piece, you can peel that one
off the board first, then peel the board out of the holder and off the
other transparency.  The registration doesn't shift if you are careful.

My exposure frame is made from 1/4" glass.  I line the edge of the bottom
piece of glass with a soft, airtight adhesive weatherstrip.  The top piece
has a 3/8" hole drilled in one corner - just inward of the
weatherstrip.  There are rubber feet on both the bottom and top pieces - I
can just flip the whole frame to expose the other side of the PCB.  The
vacuum source is an old beat-up Gast vacuum pump.

My expose distance is 13" and the expose time is about 130 seconds per side
as determined with one of those graduated exposure calculator strips.

Hope this helps!

dwayne

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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2004\03\04@073913 by Hulatt, Jon

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> -----Original Message-----
> From: Dwayne Reid [.....dwaynerKILLspamspam.....PLANET.EON.NET]

> with one of those graduated
> exposure calculator strips.
>

what are they??

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