Libris Britannia 4/science library(b).zip/science library(b)/ELECTRON/H478.ZIP/APCB.ZIP/APCB1.DOC

Text File | 1993-05-01 | 15KB | 306 lines

The information on this file is primarily intended to aid the electronics
enthusiast turn more of his/her ideas into reality by making the manufacture
of home P.C.Bs a little easier and more flexible.
You are free to copy this file for others but I would prefer the whole file
be copied rather than portions of it.
Although the information given below details how to make a single sided
PCB I am (when commitments allow) working on a technique to produce
double sided P.C.Bs. At the present time the main difficulty is in getting
two 'transparencies' to align accurately enough using the tools and skills
available to the average home constructor.
The basic principles detailed below are comparatively straightforward,
however as with most manuals, actually writing the procedure has the effect
of making the technique appear to be more complicated than it really is.

P.D.S.S                                      Mr Adrian Peirson
Winscombe House                              133 runley Road
Beacon Road                                  Luton
Crowborough                                  Bedfordshire
Sussex                                       LU1 1TX
TN6 1UL                                      England
England


   A Simple P.C.B manufacturing process for the electronics enthusiast.

One of the most off putting things about being an electronics enthusiast
is that having thought out a design sufficiently enough to warrant
the building of a prototype, there then comes the problem of making the base
board. This is one of the main causes of project abandonment as far as
the home constructor is concerned.
With simple circuits the use of strip board is perfectly adequate, however
as circuits become more complex, so does the amount of wiring that needs
to be added to the strip board to make all the required connections.

To have a PCB made proffesionally usually costs a small fortune.

The only option remaining for the experimenter/enthusiast is to make
his/her own PCB.
This usually involves placing pads on a copper clad board then adding
tracks etc to complete the circuit.
Whilst this method can produce excellent results it is rather time consuming
and not very versatile, any errors in tracking can result in having to
scrap that board and start again. Another drawback to this method is that
you are only making one board at a time. If its not quite right
a complete board may have to be redesigned.
Alternatively the layout may be made onto transparent 'acetate' sheets,
this is then termed a transparency, using a transparency allows copies of
the board to be made over and over again.
However should the finished design need modification, removal of tracks
and pads etc is to say the least, fiddly.

Detailed below is a comparatively simple technique that will enable the
home constructor to produce medium qaulity P.C.Bs relatively simply and
cheaply.

Try this method with a simple circuit to get the idea.

i      Use a CAD package to draw out the circuit you wish to make
       There are many excellent shareware electronics CAD packages
       available (do'nt forget to register these).
       NB the printout needs to be an inverted copy


       Print this out on a good quality printer, those of you fortunate
       enough to have laser or inkjet printers may print this out on a
       1:1 scale. The reasoning here is that laser and inkjet printers
       can produce a finer, more detailed printout.

       The less fortunate of us will have to print out at
       2:1 scale, this is because sometimes a line may be printed that
       should actually be 1 and a half 'printer pins' wide so the printer
       makes it two 'pins' wide and the line comes out too wide.
       If we print out at 2:1 then that line will be printed out three
       'pins' thick which, in relation to the rest of the layout (at this
       scale,)is the correct thickness.

ii     Photocopy this, reducing it back to 1:1 scale if you've used
       an enlarged printout. You may need to experiment with the
       contrast setting of the photocopier, what you want is a very
       black circuit layout on a very white background, without spots
       or 'blurring' of the tracks which some copiers seem to do.
       A few spots in areas not used might be OK but make sure that these
       blemishes do not occur between tracks or pads as these will lead to
       shorts when the board is manufactured.
       Also some photocopiers are better than others, you may need to
       try several the next time you visit town, if you're unable to use
       the works one.
       The reason for photocopying is to prevent the ink from 'running'
       during the next phase.

iii    You should now have on white paper, the circuit you wish to place
       on a copper clad board.

iv      Buy a piece of Ultra violet sensitive copper clad board of suitable
        size. This should be shielded until required  from sources of UV
        light, eg sunlight, flourescent lights, sunbeds etc

v      Soak the photocopy in a transparent oil such as baby oil, wiping off
       any excess.
       The photocopy is now transparent.
       Those of you with experience making P.C.B s do'nt really need to
       read further, you now have a transparency.


vi     Place the transparent photocopy ( transparency ) on the sensitized
       copper clad board, cover this with a thin sheet of plastic or
       glass and apply pressure, this is to make sure the transparency
       is pressed hard against the surface of the board.
       Perhaps the best method of ensuring good all over contact between
       board and transparency is by using a glass picture frame.
       The reason for the inverted printout is that the circuit tracks
       (or rather, the photocopy 'ink') needs to be pressed hard against
       the board to prevent the UV light 'getting underneath'.
       If the printout were not inverted there would be a layer of paper
       between the photocopy image and the UV sensitive board, thus allowing
       light to expose the board directly underneath the photocopy tracks.
       This will lead to 'blurring' of the track layout.


vii    Expose the board to a U.V. light source. There are many sources of UV
       light, the wifes sunbed for example. Try to ensure that illumination
       is directional.
       By this I mean ensure that only one source of UV light is used for
       the exposure. This will help to reduce 'blurring' of the edges of
       the tracks and pads.

       You'll have to experiment with the exposure time, it depends on lots
       of factors distance, glass thickness etc, try 5-10 mins to begin
       with.

       Although I've never tried it I'm sure with a little
       experimentation with exposure time, the sun could be utilised here.

       Also many hardware stores have small battery operated UV light
       sources, the type used for identifying items marked with UV
       flourescent marker pens.
       A relatively inexpensive one may be obtained in UK for about 6 pounds.

                              *******************


viii   WARNING.  WEAR RUBBER GLOVES, SODIUM HYDROXIDE CAN BE DANGEROUS
                 READ & HEED THE WARNINGS ON THE PACKAGE.


       After exposure, the board should be screened from any strong UV
       source but this doesn't involve working in the dark.
       I usually find that simply closing the curtains to exclude direct
       sunlight is perfectly adequate.

       Wash the board with a little soapy water to remove any oil.
       The board should now be placed in Sodium Hydroxide solution.
       Use a weak solution, test the edge of the board first of all, if
       the solution is too strong it will remove all the UV sensitive
       coating off the board very quickly, you do not want this.
       What you want at this stage is for the board to appear unnafected
       initially, after about 30 seconds you should see the circuit
       layout 'developing' slowly.
       Try initially approx 1 teaspoon per litre of cold water.


       Periodically rub the surface of the board with a finger of the
       rubber glove (that you are wearing) to speed up the development
       process.
       This process may take fifteen minutes or more to dissolve away
       the UV exposed coating and leave only the unexposed portions
       of the circuit.  The exact amount of time depends on the concentration
       of the solution, on the exposure time, on the distance between
       the board and source during exposure, glass thickness etc,etc.

       Hopefully, by this time you should have a copper clad board with
       the actual circuit protected by the remaining coating.

       Wash the board.

       If the track and pad layout appears 'blurred' there are two possible
       reasons for this:

       The UV light source was placed too close to the board during exposure.
       Or the transparency was not in contact with the board.

       A 'blurred' board may or may not be acceptable/useable.


                                ** ** **

       WARNING....FERRIC CHLORIDE STAINS EVERYTHING, THESE STAINS ARE
       VIRTUALLY IMPOSSIBLE TO REMOVE.
       ANOTHER UNUSUAL PROPERTY OF FERRIC CHLORIDE IS ITS ABILITY TO
       MATERIALISE AS AN ORANGE STAIN IN PLACES OF THE HOUSE YOU'RE SURE
       YOU NEVER VISITED THAT NIGHT, AND ON ITEMS OF CLOTHING YOU
       SWEAR YOU WERN'T WEARING AT THE TIME. MOST WIVES ARE RATHER
       SCEPTICAL ABOUT THIS PROPERTY, SO BE WARNED.




ix     Place the board in a solution of Ferric Chloride to etch away
       the unprotected copper areas. This part of the process can take
       an hour or more, periodically the (plastic) container should be
       agitated to aid the etching process. Keep an eye on this stage
       don't bring it out too early as you will only later find short
       circuits remaining. Bring out the board too late and you'll find
       the solution has etched away underneath the protective coating.

       Good P.C.B manufacture by any method is a skill that requires
       experimentation and experience.

x      Remove the board from the solution, washing it in cold water.
       Hopefully by now you have the required copper tracks/pads etc,
       the rest of the copper having been etched away

xi     Scour away the protective coating from the etched board and inspect
       the board for shorts and open circuits.If none, then the board is now
       ready for drilling.



In essence what I'm saying here, for those of you with a little experience
in making P.C.B s is make a photocopy of the printout, soak it in transparent
oil and hey-presto   instant transparecy, now that's not quite what
Mr Daniels would call magic, but it is cheap, quick and if you want more
boards you still have the transparency.
Another advantage is that should the inital prototype require modification
this may be carried out on the C.A.D package rather than by hand.


Should you regularly make P.C.B s and find this method an improvement or
cheaper than the one you would normally use, a small donation ( 5 pounds)
to my favourite charity, namely me, would be very much appreciated.
(Wife and sixteen goldfish to support (more on the way (goldfish, that is))
This is however not a condition of usage, you are free to use the information
and techniques contained in this file and make copies to give to friends
collegues etc without obligation.

A contribution might allow me to replace those nice towels the wife bought,
you know, the ones with the mysterious orange stains.

One final point, many electronics magazines often carry P.C.B track layouts
of their projects for the home experimenter to copy, or alternatively they
offer a P.C.B manufacturing service for these.
Reasonable results can be obtained by photocopying these and using the above
method to produce the transparency required.
The magazines however do not always print them out full scale and so a
scaling will have to be carried out during the photocopying process.
Also they are generally not inverted, this means that UV light can get
under the photocopied tracks sometimes producing a 'blurred' PCB.
This effect may be reduced by placing the UV source further away from
the board (increase the exposure time) but will not completely eliminate this.
Again, experimentation is the key.



                                Good Luck.


*****************************************************************************



                    Printout looking a little pale

This suggestion applies to fabric impact printer ribbons and is not suitable
for plastic printer ribbons.
Generally only a small part of the ribbon is impacted, the middle, look at
an old printer ribbon you'll find it paler than the outside of the ribbon.
You can of course buy reinking spray but I have found earosol oil sprays
a little easier,      and cleaner.
Oil has the effect of emulsifying the ink and allowing the ink remaining
at the edges to be redistributed.
It is of course necessary to open up the ribbon case to spray the whole
ribbon, best results are obtained if the 'rejuvenated ribbon' is allowed
to stand for a day or two befor use. Do'nt soak the ribbon when spraying
use just enough to dampen it.
The result can be a little patchy, this method is probably not suitable
where high quality printout is needed, but for the average home user
it should give reasonable results.
A word of warning here, some oils may have an adverse effect on some types of
plastic, experiment first with a small amount of spray.
The best advice is, use the oil sparingly and carefully, do'nt allow the
spray to contact anything other than the ribbon by doing this away from
the printer, that way the worst that can happen is that you have to buy a
new printer ribbon.














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