Homebrew_PCBs

Hello, I was wondering if there is a technique to plot
double sided pcb's with a plotter. I have an HP 7475A
plotter that will never plot in alignment. Any
suggestions would be appreciated! Thanks, Genaro M.




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In a message dated 6/18/2003 1:21:55 PM Central Standard Time,
gmanca101@... writes:

> I have an HP 7475A
> plotter that will never plot in alignment.

That kind of plotter may be fine for plotting graphics on PAPER, but I'd
never consider one for plotting something that required the mechanical resolution
of a PCB, particularly a double- or multi-sided one!

If your need is "professional enough" that you require plotter-drawn artwork,
then you must simply go BUY what you need!

If your need is avocational, and/or only very occasional, the use of
manually-applied Bishop Graphics PC-component patterns to 0.1" grid paper is not yet
considered a mortal sin!

An acquaintance who worked for a large chemical firm GAVE me a like-new 7475
that his employer was "throwing out"! I removed the cover to clean it with
solvent, and dust-out the innards, and I saw that it moved the paper with RUBBER
ROLLERS, and the quill across the page with a RUBBER "timing belt" with
considerable "wiggle and flop". And both of those axes were moved by little DC
servo-motors with "shaft encoders" and custom "electronics". This may have been
an acceptable mechanism for moving a felt-tip pen about on smooth paper, but
NOT for plotting something like PCB art! NO way! Now I grant you, I DID
repeat the buillt-in Demo-Plot on the same sheet of paper, and it repeated all
probably within 0.01", or even better---hard to tell, running a PEN about over
paper! I put an ad on e-bay, and sold it to an artist in a State far, far away
in a few hours! NO regrets!

Jan Rowland


[Non-text portions of this message have been removed]

Genaro,
I have milled a couple of double sided PCBoards. I mounted
two steel pins in the bed of my mill. The two pins sit in the
centerline of the PCBoard. One at the top and one at the bottom. I
used paper instead of PCB material and a pencil lead instead of a
milling bit. After drawing side one, I can then adjust the offset of
the second side. Holding the paper up to the light I can check the
alignment. When I am happy with the alignmet I replaced the pencil
lead with a PCBoard milling bit and the paper with a piece of double
sided PCBoard. I add two alignment holes to the center of the PCBoard
that match the alignment pins. Make sure you leave room for the pins
in your drawing!!! You can also use four pins, one in each corner.
All my blank boards are 4" X 6" If you use different size boards the
process becomes more complicated.

John

--- In Homebrew_PCBs@yahoogroups.com, Jerry <gmanca101@y...> wrote:
> Hello, I was wondering if there is a technique to plot
> double sided pcb's with a plotter. I have an HP 7475A
> plotter that will never plot in alignment. Any
> suggestions would be appreciated! Thanks, Genaro M.

> Hello, I was wondering if there is a technique to plot
> double sided pcb's with a plotter. I have an HP 7475A
> plotter that will never plot in alignment. Any
> suggestions would be appreciated! Thanks, Genaro M.

Hi Genaro,
A search on Google will give you some leads. I remember crossing past some
articles where someone has had great success. You might start with this
link --> http://robots.tonkaland.com/new-archive/2157.html You might try to
contact them direct.
I know I have read others, too. Methods range from taping the board to the
paper to drilling the holes first then aligning the board on a plot you have
already made on the paper.
Good luck with your search!
-Mike

Hi Jerry! I've had great success plotting boards using the 'scratch' method
with my 7475A. I love it! Such fun to watch.

If you want to see a plot of the boards generated, go to groups.yahoo.com,
into the Homebrew_PCB group, click on the "Photos" link to the left, then on
"Eagle Scratch" folder. You'll see two pictures (front and back) of boards I
made with my plotter scratching away the resist.

You can see from the bottom side that my holes didn't line up quite right -
the board that I photographed was one that I goofed on. I did several of
that board that turned out just fine.

The method I use to align the two layers is as follows.

First, I put a piece of paper into the plotter. I then have the plotter draw
a vertical line about two inches from the left hand side of the paper. Then
I take a spare piece of PCB material, maybe an inch wide by 8 inches long
that has one very straight edge. I line the straight edge up with the
plotted line on the paper (which is what the plotter thinks is vertical) and
tape it down. All of the above steps I just do once for all my boards.

Then when I want to plot a board, I will use my software to move the board
plot such that it sits about 1/8" to the right of the vertical PCB that's
taped on the paper. I have the plotter plot the bounding box of the plot
file on the paper, so I know approximately where to put the blank PCB. Then,
before I plot the top side, I draw a thick black line on the vertical PCB,
about in the middle (vertically) with black sharpie. I then tape the blank
PCB down over the outline that was plotted on the paper, pushing the board
up against the vertical PCB tight. Then I tape it down. Then, when the
plotter scratches the top side of the board, it also scratches one
horizontal line on the vertical PCB exactly in the middle of the top and
bottom limits of the board. It also plots a small horizontal line on the
blank PCB at the same vertical location, but in the 1/8" of blank space
between the vertical PCB and where the scratching outline starts on the
blank PCB.

Once the top side is scratched, I have a very sharp horizontal line on the
vertical PCB exactly in the (vertically) middle of the plot, and I have a
corresponding line on my PCB. I take the PCB off the paper (untape it), then
drill a very small hole through the 'reference' scratch. Then, I flip the
board vertically, and line up the hole with the scratch on the vertical PCB.
I again push the PCB up against the vertical (flat) reference PCB, and then
tape it down. When you plot the bottom, it will line up.

Is it close enough to do tiny vias? No. Is it close enough to have .100"
headers line up on both sides of the board - no problem.

The plotter has a resolution of .001", and the head seems accurate to almost
that tight of a tolerance. The problem I currently have is that my
scratching tip is not very securely held by the head, and so drags behind
the head as it's being pulled across the copper and resist. So my lines
aren't always exactly the width that I want them to be. But it works fine
for me - I haven't had any electrical problems with the boards I've made.
(The good ones anyway.)

I hope this helps-

*Brian

> Hello, I was wondering if there is a technique to plot
> double sided pcb's with a plotter. I have an HP 7475A
> plotter that will never plot in alignment. Any
> suggestions would be appreciated! Thanks, Genaro M.

Thanks to all who replied! I just noticed something, when i was first
trying to plot double sided pcbs, I would first plot on paper then
try to lineup the alignment holes i made on to the paper. However,
even though i put the paper down on the holes, the plotter would
always print the holes to the left on the pcb. I am using Eagle Cad
if that helps diagnose my problem.

hi

i have the 7475a also.
and i think it is good enough for making pcbs with components with leads,
and track with 0,3mm.
i have a pen for 0,18mm, this is quite close to the mechanical resolution
you can get with that setup.

you have to tape the pcb down on a piece of paper if it isn't big enough
(also the metal rollers would scratch it ((@Jan Rowland the metal rollers
are the driven ones, the rubber only for counterpressure.)

please don't use a heavy pen (forget about all that ohp pen scrap), use a
short metal tip pen instead.
this results in much more stable movement (at the corners you get "drops"
with ohp pens)
and set to suitable speed.

the alignment with double sided plot is really, really a problem.
i didn't think about it too much until now... (i still have some
photocoated clad here and do not need plotting)

one idea would be to get the pen to stop at a certain position and align
the board then.
should be possible with some hpgl command or transfering the hpgl file in
parts.

the other idea would be to use a real flatbed plotter.. ;-)

third idea:

use ohp transparency to tape pcb to.
then make a good edge on the plotter plastic.
so you can see your "target" through the transparency when aligning.
this is a targed referenced to the plotter frame, so differences in
transparency alignment will change nothing.
just like using a flatbed then i guess.

regards
st

Ahh. What might be happening is that when you 'print' the plot to your
plotter from Eagle, it might be trying to shift the output to the left so
that the left hand sides match up or something like that.

What you need to do is plot the top and bottom to separate HPGL files. Then
load those HPGL files up in a HPGL file viewer on top of one another. Most
likely they won't line up correctly.

To solve this problem, draw a single box of copper (like a wire in the shape
of a box) that completely encases your design. Put these copper boxes on the
top and bottom layers. They will be the largest part of the plot, so even if
the printer driver is trying to align the outputs, it won't have to shift
them any.

*Brian

> Thanks to all who replied! I just noticed something, when i was first
> trying to plot double sided pcbs, I would first plot on paper then
> try to lineup the alignment holes i made on to the paper. However,
> even though i put the paper down on the holes, the plotter would
> always print the holes to the left on the pcb. I am using Eagle Cad
> if that helps diagnose my problem.

At 02:34 PM 6/18/03 -0400, JanRwl@... wrote:

>That kind of plotter may be fine for plotting graphics on PAPER, but I'd
>never consider one for plotting something that required the mechanical
>resolution
>of a PCB, particularly a double- or multi-sided one!
>
>If your need is "professional enough" that you require plotter-drawn artwork,
>then you must simply go BUY what you need!
>
>If your need is avocational, and/or only very occasional, the use of
>manually-applied Bishop Graphics PC-component patterns to 0.1" grid paper
>is not yet
>considered a mortal sin!
>
>An acquaintance who worked for a large chemical firm GAVE me a like-new 7475
>that his employer was "throwing out"! I removed the cover to clean it with
>solvent, and dust-out the innards, and I saw that it moved the paper with
>RUBBER
>ROLLERS, and the quill across the page with a RUBBER "timing belt" with
>considerable "wiggle and flop". And both of those axes were moved by
>little DC
>servo-motors with "shaft encoders" and custom "electronics". This may
>have been
>an acceptable mechanism for moving a felt-tip pen about on smooth paper, but
>NOT for plotting something like PCB art! NO way! Now I grant you, I DID
>repeat the buillt-in Demo-Plot on the same sheet of paper, and it repeated
>all
>probably within 0.01", or even better---hard to tell, running a PEN about
>over
>paper! I put an ad on e-bay, and sold it to an artist in a State far, far
>away
>in a few hours! NO regrets!

Come on now, Jan.

I most respectfully must disagree with you on this topic.

I (we) used our HP 7475A plotter to plot the artwork for hundreds of
layouts - most were plotted at 2X size and reduced to 1X negatives by our
local print shop.

The only problem we had was with the pens clogging midway through the
plot. Accuracy was excellent for boards up to 5" x 7" (10" x 14" plot
size). We used both standard carbide tip and carbide cross-groove pens
plotting onto mylar.

The rollers that move the media are not rubber: they are sintered metal
wheels with *very* sharp teeth. These teeth imprint the media so that it
does not shift about: when you load media into the plotter and start a
plot, the first action the plotter does is move the media from one end to
the other. This imprints the tooth pattern on the media. That pattern
functions somewhat like the perforations on tractor fed fan-fold computer
paper and helps keep the media in the same location.

As you mentioned, repeatability was better than 0.01" over the length of
the plot. Reduce your artwork by 50% and that error drops to about
0.005". This is definitely most acceptable, even by today's standards.

That 'timing belt' that moves the pen holder across the paper is even more
accurate: repeatability is somewhere around 0.002" over the full width.

The main reason we moved over to an inkjet plotter (Pacific Data Protracer)
was because of the pen clogging problem I mentioned. But I keep that old
plotter handy - it gets used several times a year for odd jobs.

dwayne

--
Dwayne Reid <dwayner@...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax

Celebrating 19 years of Engineering Innovation (1984 - 2003)
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`-' `-' `-' `-' `-' `-' `-' `-' `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.

Someone on this group was asking about etch resist ink for inkjet printers.
I made an inquiry to a company I found online (I was looking for etch resist
plotter pens) and got the following feedback. The attached pdf file will
probably not go through, but perhaps it can be put in the file download
area. The response and contact info is below.
-Mike

----- Original Message -----
From: "Branthwaite Giles AG" <Giles.Branthwaite@...>
To: <circuit@...
Cc: <contact@...>; <contact@...>; "Johnson Steve
S - IJPM" <steve.johnson@...>; "James Mark MR - RSCH"
<mark.james@...>
Sent: Wednesday, June 18, 2003 7:30 AM
Subject: Etch Resist Enquiry


> Dear Mr Putnam,
>
> Thank you for your enquiry regarding ink jet printable etch resist sent
> through to our website. We do have a product for this purpose, and I
attach
> a pdf file highlighting the performance characteristics of the fluid for
> your information.
> <<EtchResistflyer2.pdf>>
> Avecia supplies its etch resist fluid commercially through its ink jet
> printer hardware partners to ensure that the ink and printer are fully
> optimised. The leading ink jet printer available for our etch resist at
the
> current time is from an Italian company called New System
> (www.new-system.com).
> We also work closely with New System to determine whether the ink works
for
> your specific required etch and strip processes and substrates - although
> the inks have been designed for what we determined as the most popular
> chemical processes for the PCB market, we do see them working in other
> fields.
>
> The distributor for the West coast of the US is detailed below. For your
> information I have also copied in the Italian office to this e-mail so
that
> they are aware of your possible interest.
>
> Contact:
> Enrique Gomez
> Tel: 626 581 0891
> E-mail: contact@...
>
> I trust that there is enough detail in this e-mail, and the associated
> contacts, to help you evaluate the technology. If you have any further
> questions in the meantime, please don't hesitate to send them my way. I
will
> try to answer and/or redirect as appropriate.
>
> Kind regards,
>
>
> Giles Branthwaite
> Business Development
> Avecia Ink Jet Printing Materials
> Tel: +44 161 721 1578
> Mob: +44 7740 447 972
> Fax: +44 161 721 5810
> giles.branthwaite@...
>
>
>


[Non-text portions of this message have been removed]

hi


i did doublesided experiments here because i am too very interested.


as brian wrote it is good to use a piece of pcb for alignment.


but first some "experiments" with the 7475a:


1) if you use the rotate command in the hpgl file this is a problem because
you would need a3 paper. if you use a4 paper the "roll it all the way up
and down" on initalisation rolls over the edge and looses the paper.
so maybe we can avoid this.


2)
if you plot, lets say two boxes. they are not aligned at all, one is fully
in the other and not touching it.
i make this with 2 layers in my pcb software.
i then export the layers seperately into top.plt and bot.plt hpgl files.
the export tool i set to offset x=100 and y=100.
then always the two boxes ARE ALIGNED AT THE LOWER LEFT CORNER.
you have to try this with your software. depending on the export sowtware
it may behave different.

3)
if you plot "top.plt" and the plotter stops in at the end. the pen stays
still at last x/y pos.
(with my software i manyally add the pen up because the software converter
desiger has "forgotten" to lift the pen after work).
now with the pen osition somewhere in the middle you can send the next
bot.plt (dont raise the paper lever/reset plotter)
and it will come out exactly at the same lower left corner. this is also so
if you use the move pen keys at the plotter panel.
this is also "counted" in the plotter, it stores the absolute coordinates.
this is important later if you want to flip the clad and want to move it in
the open area to tape it down.


so what do we know now?
brian uses one side to align, why may we not use one edge for aligning?
well, you may say: when turning the pcb to plot the other side the edge
rotates and so this won't work.
but what if we also rotate our artwork for the bottom side?


(following i assume your software shows the layers "like if the pcb is of
glass and you see through". when i talk of "upper / right edge" i mean
always the edge like displayed in software, not like turning the finished
pcb around.)


for the top layer all stays the same in your pcb software.
you don't rotate, don't mirror.
but if you want to plot the bottom side the image for the bottom layer has
to be modified.
which was the upper right corner stays the upper right.
which was the upper left corner gets the lower right corner.
the lower left corner stays itself (remember - this is the alignment edge,
it MUST stay the same.)

what does this say us?
first mirror the bottom layer, then rotate it by 90 degree.

i tried this, this really works.



so to the actual doublesided pcb procedure:

a) we need two guiding pcb pieces, one horizontal, one vertical.
but to align them/fix them we have to get reference lines on our page.
best is to do this with your pcb software.
THIS LINES HAVE TO BE AT THE SAME DISTANCE FROM THE ACTUAL PCB BORDER (your
outline box).
this is important to get the images aligned.
(personal suggestion: if pcb is big enough simply print the outline box
with offset not at your preferred value but -20 or -10 on both, x, and y.
the actual value is determined by the borders you want)

when we have this lines plot we fix the pcb guide pieces. please make sure
they are exact

b)export your top layer to hpgl (plot with eagle).
put your pcb well pressed against the guidepieces and tape it down.
plot your top.hpgl file.

c)(make this befor starting plotting top)
rotate your bottom layer, mirror it. to get this right please try first
with paper and think about it.

d) flip the board in the plotter (steer it down with arrow NOT LIFT LEVER,
to keep paper position the same.
simply pivot around the lower left corner.
tape dwn again.

e) plot bottomrotatedandmirrored.hpgl ;-)



enjoy your perfectly aligned board.


(( as long as you don't lift the lever and keep ypur plotter powered you
can reuse the guidepieces and let it fisex to the paper.
you simply have to put in the clad. (for part outline, series of multiple
pcbs))



i know this is a long mail, sorry for all reading and wasting time because
not interested.
but it is very important for all doin gdirect plotting.
it also contains important "experiments" about 7475a behavoir which i didn
know.



if i'm wrong with this please tell me. i have tried it and discovered it
working.


regards
stefan

i tried plotting with eagle now.

seems the described precedure works very fine if you simply set "mirror"
and "rotate" at the cam processor window.


but i don't get a good output - too complicated in some areas and
the drill holes are also not plot open.
PLEASE tell me if anyone has a solution for this.

regards
stefan