Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

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