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Producing
PCBs from positive artwork and positive resist PCB material
February
2010
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If you have
visited other pages on this website ( www.vk4adc.com
), you will have noted that many of my electronics-style projects
are based around building up the actual circuits on printed circuit
boards. I started doing my first PCBs back in the 1970's when I
first drew the 'artwork' directly onto the copper cladding with
a 'etch resist pen' - these days often called a 'Dalo pen' after
one of the original pen manufacturers. In the years between then
and today, I have used several other methods including negative
and positive resist pre-coated and self-coated material. My circuit
board artwork often took me weeks to create part-time with donut
pads and black adhesive tapes on clear acetate sheets, then cross-checked
a number of times to make sure the components fit in the spaces
and there weren't any layout errors. It was a time consuming process
and wasn't exactly cheap.
These days
the process is a lot simpler and almost anybody can create printed
circuit boards for most of their projects. It still requires patience,
accuracy and practice to develop a schematic out into an actual
PCB layout but the tools are available to almost anyone with a normal
PC in their home plus a few assorted plastic containers for the
developing and etching fluids to be put in.
At this point
in time, read the whole web page to see what the each step is and
what materials ( PCB material, developer, etchant, containers ...
) you will require. Later you can work through the relevant sections
step-by-step as you actually produce your first PCB.
Some of the
images will expand on your screen as you move your mouse over them.....
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STEP
1 :
The first step is to download ExpressPCB from the internet and install
it on your PC (it will run with Windows XP through Windows 7). It
is a free download and is available from http://www.expresspcb.com
. When you go to install it, it will install two applications :
ExpressPCB (which is used for creating PCB layouts) and ExpressSCH
(which is used for drawing electronic schematics) and place shortcuts
on the Windows desktop. Of course the reason that it is free is
that they anticipate that you will do an 'on-line' order for your
PCBs through them and they will make their money that way.
I have no
intention of describing how to use the applications here because
there are help files included even if not totally clear the first
time you look at them. There are also a number of components included
in the supplied libraries but I have found that I have had to create
extra 'custom' components that are more oriented towards my specific
areas of interest, particularly with some of the newer Surface Mount
Devices (SMD) layouts.
The people
at ExpressPCB recommend that you begin your project by drawing a
schematic. While not required, it will save you time when designing
your PCB. Drawing
a schematic with the ExpressSCH program is supposedly as easy as
placing the components on the page and wiring the pins together
but that is only true after you have developed experience with the
program . The schematic can then be linked to your PCB file, so
that the PCB knows what needs to be connected together - but - I
find that I don't necessarily want to draw a schematic electronically
if one already exists on paper - eg in a book, magazine or even
a freehand scribble on scrap paper.
The
people at ExpressPCB : Designing 2 or 4 layer boards using the
ExpressPCB program is very simple. Start by inserting the component
footprints, then drag them into position. Next, connect the pins
by drawing the traces. If you link your schematic file to the PCB,
the ExpressPCB program will highlight the pins that should be wired
together in blue.
As
stated a few paragraphs above, you WILL find that you need to create
custom components for either/both application simply because the
libraries are not expansive. It isn't difficult and once you get
the hang of it then it does seem simplistic. You can also modify
the current library components and save them with your alterations
as 'custom' versions with new names. The
PCB layout illustrated above is beyond the abilities of the simple
home hobbyist BUT if you only require one copper layer ( single
sided ) or maybe two layer ( double sided ) and are prepared to
use wire links to connect parts of the PCB then you can do it at
home - and at a reasonable cost.
Actual
size of this piece of the PCB is just 30mm x 20mm high ( so around
a square inch : 25x25mm)
RED = TOP of PCB
YELLOW = Component overlay layer
This
is a small corner of a new PCB layout that I have just finished
and it uses primarily Surface Mount Devices on the upper side of
a single sided PCB. In actual practice, the PCB material is double
sided and the second side is left as a copper (earth) backplane
but all components are mounted on just one side - on top. There
is no need to use [plated-through] "vias" (connections
through from the tracks on one side of the PCB to tracks on the
other) if the project is not too complex and wire jumpers are used.
One trick that I have used in this particular layout is the use
of SMD 1206 size 0R0 resistors (ie zero resistance links) to jump
across a PCB track or two instead of needing jumper wires.
The
standard size SMD resistors that I use are 1206, most of the capacitors
up to 100nF (and some up to 1uF ) are 0805 size, and RF chokes (220uH
above) are 1210 size. The component X1 is a crystal and the circle
above it is a trimmer capacitor - both of which are custom components.
The transistor Q3 is an SOT-23 case style and the component marked
HC00d is a 14 pin SO14 ' 7400 High speed CMOS series ' quad nand
gate IC. The size of the slice of the PCB shown above is just 30mm
wide x 20mm high in practice and the narrowest track width I used
was 0.25mm ( the vertical lines/traces between the HC00d pins are
that width). You will note that there is mostly red (copper) and
little black (insulating sections) in the above image because most
of my circuits are used at radio frequencies where shielding and
low interconnection inductances are mandatory.
I
use widths down to 0.25mm for fine track work and 0.38mm for general
layouts. If you follow the instructions carefully when processing
the PCBs then you will have no trouble re-producing PCBs with the
finer 0.25mm widths.
Text labels on the parts overlay are normally 0.97mm.
The
whole process is a lot easier if you are already following the PCB
layout in a magazine. If you can extract a high quality full-size
( ie 1:1 ) scale of the PCB then use that. Alternatively use ExpressPCB
to duplicate the layout printed in the article - after all they
have done the hard work in laying out the components for you and
a 1:1 artwork will be the result if you use properly sized components
(eg from the supplied libraries).
These
are my current ( at Feb 2010 ) custom component libraries for PCB
and Schematic (about 16KB each,
right-click & Save As ). You will need to extract them into
typically C:\...\My Documents\ExpressPCB\PCBComponents_Custom
and C:\...\My Documents\ExpressPCB\SchComponents_Custom respectively
( these folders are created automatically when you install the Express
software) to be able to use any of them in your projects.
Top
layer only is laid out using ExpressPCB
Bottom layer for this project is plain copper.
This is the
PCB overlay to suit the above layout.
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STEP
2 :
Let's now assume that you have the layout on the computer and want
to produce the artwork. Note that we require a "Positive"
- ie where PCB tracks are required we have a black mark/outline.
Where fibreglass insulating segments are required (eg between tracks),
the artwork is "clear". ExpressPCB will only print a positive
artwork so that makes it a bit easier.
I have found
that the best quality artwork actually comes off a good inkjet printer
using medium-good quality transparency media. I have tried laser
printers at 600dpi and laser/copier transparency materials and the
resulting prints are not really dark/dense enough. I currently use
Celcast a pack marked IJ21-50 (50 transparencies) and an A4 Epson
inkjet with the media setting at "Ink Jet transparencies"
and print quality as 'Best Photo'. Note that there are no printer
settings within ExpressPCB so you must set the default values for
the inkjet to these BEFORE you start
the ExpressPCB software. Always print the artwork to normal plain
paper first and double check the print quality and for any layout
or sizing errors BEFORE you put any transparency media in the paper
feed of the printer !!!!! Paper is cheap, transparencies are expensive
in comparison.
I always
turn off the yellow overlay view at some stage when I think I am
finished so that I can see any obvious errors on the top layer -
like tracks that aren't actually connected - but this fact is usually
obscured by the yellow lines and text of the component overlay.
My SMD layouts
are usually 'on top' so therefore I almost always print the TOP
layer then print the Overlay on plain paper expanded to fill the
page so that I have the parts layout to work from when installing
the components.
The visual
test when you hold the final positive up to light is that the blacks
are reasonably black and the clear is actually clear.
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STEP
3 :
You will need to have purchased positive resist pre-sensitised
PCB. Probably the most-readily available source Australia-wide
(or world-wide) is the that from RS
Components. There used to be a number of hobby stores like
Jaycar and Dick Smith Electronics that had positive resist board
readily available but the move away from electronics home-construction
probably sounded the death knell for the demand.
On the
RS
Components web site, simply enter " Fotomechanix
" in their search box. The results will show around 20
products starting at sizes around 100x160mm in single sided
up to 305x457mm in double sided. My preference is for the product
coded
397-0053 double sided FR4 positive resist PCB material,
203mm x114mm - about $18.50 + GST = $AUD20.35 (early 2010 pricing).
Simple projects can use 397-0047
single sided FR4 positive resist PCB material, 203mm x114mm
at $17.30 + GST. The good thing about RS Components is that
if you are in a capital city with a Trade Counter, you can just
buy these over the counter rather than having to place a web
order.
This
PCB material is covered on the sensitised side(/s) with an adhesive
black plastic layer so that makes it insensitive to light until
the layer is stripped off exposing a greeny coloured copper
underneath, and that happens just before you expose it to light
through your artwork.
The
plastic layer also make it easy to cut to size because you can
simply use a standard hacksaw with a fine (32TPI) blade (or maybe
even a jigsaw with a fine blade) to trim it and still handle it
in full light conditions. Simply draw a pencil line using a 90
degree square on the plastic and it will leave enough of a mark
to cut along. Just make sure you don't actually flex the PCB material
and cause cracks in the resist layer/s.
Make
sure that the material you cut is larger than your artwork, preferably
about 10-12mm longer and wider (so 5-6mm is available on each
side). Once you get the hang of it then these overlaps can be
reduced but it does make it easier for the beginner if you make
the PCB larger and later trim it to size.
***** Note that you can produce true double-sided
PCBs but you have to take a lot of care with side-to-side registration
and usually expose both sides similtaneously.
This page will not deal with the methodology but some other web
sites do. *****
WARNING
: FROM THIS STAGE UNTIL AFTER ETCHING (Step 7), DO NOT TOUCH THE
PHOTOSENSISTIVE SIDE OF THE PCB - HANDLE ONLY BY THE SIDES.
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STEP
4 :
The next procedure is to expose the sensitised PCB to light through
the artwork.
I use an old contact print frame but if your need is to create
very occasional PCBs then a suitably-sized clean piece of glass
will suffice. { an A4 or A5 picture frame is a good source of
glass this this purpose. } The contact print frame is simply a
rectangular piece of pine to which is hinged a rectangular pine
frame for the piece of glass. The glass is simply glued around
the edges to the rectangular frame with a contact adhesive like
KwikGrip or GelGrip. A nearly-the-same-size piece of thin foam
rubber glued to the backing timber provides a "sandwich effect"
and will stop the PCB material from moving. To use a contact print
frame, the PCB material is placed on the foam rubber, the artwork
centred on the PCB and then the upper frame is closed down by
virtue of the hinges.
The
step I didn't disclose in the last paragraph is that the plastic
layer must be removed off the PCB in subdued light or under yellow
light. Do not do it in bright light or the board will already
be exposed directly rather than through the artwork. I tend to
do the plastic removal in a darkened room (ie. lights off) away
from any windows, immediately position the artwork on the top
of the 'green' PCB, close the print frame and IMMEDIATELY cover
the whole lot with a thick cardboard (I use a manilla folder -
it doesn't have to be black).
I
use a typical exposure of 20 seconds to direct sunlight to expose
the board. I simply take the cardboard ( /manilla) cover off the
print frame for the exposure period while making sure that the
assembly is perpendicular to the sun itself. After the exposure
period, you just pop the cardboard back over it and take it back
inside. If you want to avoid using the sun, you can expose the
PCB to suitable UV light sources (around 360nM) for longer times
but the actual times will be determined by the actual bulb used
and the distance the light is away from the PCB during the exposure.
The sunlight option is easier except on rainy days or if you can
only do it at night.
The
20 seconds has proven accurate for direct sunlight here in Brisbane
Australia but other areas of the world may require different timings.
Note
that even though I generally use double sided PCB material, only
one side is actually exposed and etched for most of my projects.
The second side remains as a full copper earth plane and the adhesive
photo-blanking material is left on it right up to step 8.
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STEP
5 :
The developing of the exposed PCB is next in the process and again
you need to do this under subdued light or yellow light.
I
have had great success developing in a weak caustic soda solution
with a development time of about 10 seconds. You don't need to
buy the RS universal developer product 680-949
unless you want to..
The
caustic soda ( Sodium Hydroxide / NaOH ) that I use is granular
and is commonly used for clearing blocked drains in the kitchen
or bathroom, available from the supermarket or hardware. The product
is sold here under the brand name Mechanix and is supposedly 98%
NaOH ( see
suppliers web page for info ) but of course any high quality
NaOH source could be used. The important thing is that you must
use cold water and slowly add the caustic soda to it - NEVER add
water to caustic. The resulting solution will grow warm as the
caustic dissolves.
The
quantity I use is about 1 teaspoon of caustic granules to 1 litre
of water. The right consistency is when if you wet your fingers
with the solution, rubbing the fingers together will feel soapy
but not unduly "bite-y" (/ burning). This solution is
made up in a rectangular plastic container about 30-50mm deep
and of other dimensions to suit your PCB size. In some cases,
I pour half the solution off before using. Do NOT make the solution
too strong or else it will rip the entire photo layer off the
board. Just remember that you can always add a few extra granules
to the mixture... after first removing the PCB !
Still
under subdued light, uncover the exposed PCB, remove from the
print frame and quickly drop it into the developer. Almost instantly
you will see swirls of purple or green and sometimes almost black
emanating from the surface of the PCB as the exposed-to-light
sections of the photosensitive coating are reacted away. As soon
as there is no more purple / green coming off the board and the
board looks 'finished', wash it under cool running tap water and
inspect the board carefully for any section not completely developed.
If it is ok then you are finished this step otherwise continue
developing.
The
weak caustic solution can simply be poured down the sink after
use and washed through with running water. After all, it IS sold
for clearing kitchen drains !
After developing
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STEP
6 :
The etching and the remainder of the processing is done under
normal light conditions.
The
etchant is ferric chloride (FeCl) and is available from RS (
216-2699 but it is 1Kg at $50+ ), Jaycar, Altronics (etc..)
have it in liquid form at a lower cost ..
Do NOT use Ammonium Persulphate as an etchant : you have been
warned !
Before
you etch, cast a second look at the developed PCB material for
little dust spots or scratches in the resist layer. The dust spots
can be removed from insulated areas using a sharp blade and scratches
and marks on the resist repaired by the use of a Dalo pc-resist
pen. Please allow the resist pen ink to dry properly before you
start the etching process.
This
stage uses two rectangular plastic containers, one that fits comfortably
within the other. The smaller one has the ferric chloride solution
with a depth of about 4-6mm and the larger one has boiling water
from a kettle poured into it. This arrangement allows the ferric
solution to be heated and made more efficient as an etchant without
diluting it. The smaller container basically "floats"
in the hot water bath.
The
developed PCB is simply placed into the etchant and the inner
container "rocked" in the hot water bath, a process
that I do in the kitchen sink (just as well my wife doesn't know
about this !) .
This movement produces a swirling motion of the etchant over the
board and you will see parts of the board exposed from time to
time. The exposed copper quickly becomes a dull copper finish
and as the copper is eaten away by the ferric, the copper colour
changes to a grey and finally to that of the fibreglass itself
where there are no tracks. When the board is completely etched,
the pattern should match exactly that of the positive artwork.
A
word of caution - do not leave the PCB in the etchant for 1/2
hour while you wander off to have a cup of coffee. Very fine tracks
can be side-eaten under the resist layer so it is important to
remove the board as soon as it is completely etched and then wash
it under cool running tap water..
Tip
: Ferric chloride stains everything browny/yellow - fingers, sinks,
table tops... you need to wash it off your fingers or the spills
into the sink quickly with running water ! Dilute, dilute, dilute
are the 3 key words.
Do
NOT pour out the ferric chloride into the drain, just put it back
in the plastic or glass bottle, plastic lid a necessity, for next
time.....
For
my PCBs etched in the heated pre-used ( & therefore old )
ferric chloride solution, the etch time is only about 8-10 minutes.
After etching
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STEP
7 :
Removing the resist layer .
The
RS info sheet on the Fotoboard2 material states : "The unexposed
resist can be left on the board to act as a protecting film if
required. If a solder joint is to be made, it can be done without
removing the resist."
This is an issue for me as I usually use Isopropyl Alcohol to
clean off any residual flux after soldering, and as you note above,
this also softens & cleans off the green resist layer !!!!
The
positive resist green coating washes off with Isopropyl Alcohol
& paper towel - don't buy the RS stripper/cleanoff product
690-855
unless you want to.
I usually just pour a small pool of the Isopropyl Alcohol ( from
Jaycar, Altronics etc) onto the PCB top surface and then use a
paper towel to wipe off the coating.
You
can use a scourer to remove the layer but that is hard work -
and you actually remove some of the copper layer when you undertake
this action.
After cleaning off resist from main side
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STEP
8 :
For single sided material, jump this step.
If you used the double sided material, remove the adhesive protective
layer now and wash off the second resist layer with Isopropyl Alcohol.
Removing protective cover from side 2
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STEP
9 :
Trim the PCB to the size of the artwork.
Use a hacksaw, file etc to cut to the required size BUT make sure
that you use "soft jaws" if you place the PCB in a vice
so that you do not damage the PCB tracks.
The best method is to use a metal guillotine but not too many of
us have one of these at home.
Note : "soft
jaws" are typically either L-shaped 1mm+ thick plastic or soft
wood (eg pine) pieces that are put into the jaws and the 'job' placed
between them. Their job is to protect the object's surface from
damage.
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STEP
10 :
Spray a protective coating of clear lacquer on the exposed copper
layer(/s). This will prevent tarnishing of the copper and give it
a longer lifespan.
I generally
use a PCB coating spray, eg Electrolube Clear Protective Lacquer,
which can be readily soldered through.
If I am in a hurry then I sometimes run a hairdryer or a heatgun
(on low) quickly over the sprayed side to make the lacquer go off
immediately.
PCB after trimming & spraying with clear lacquer
That's all.
Once the PCB's coating is dry you can start drilling / soldering
to make your project complete.
It usually
takes me about 20 to 30 minutes to cut, expose, develop, etch, trim
and lacquer a PCB. Of course the PCB layout takes a reasonable amount
of time but once you have the artwork, producing extra PCBs with
a given design is just so simple.
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The
RS web pages also list a Positive
Resist in a spray can 198-9651
that you can to spray onto clean/prepared standard PCB to make
your own sensitised board.
I have plenty of standard PCB material in both double and single
sided forms and have tried some of the spray, baked it, and done
as per the instructions but it just seems to wash completely off
during the developing process. I have even tried multi-coating
the boards - with the same result.
I really haven't had much success with this product but maybe
you will as it does reduce the cost by making your own......
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These
are further images of the above printed circuit board project
:
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Finally,
the more you make up PCB layouts and then go ahead to produce
the physical PCBs then the easier & faster the process gets
- and the fewer the number of production failures .
i.e. 'Practice makes perfect'.
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feedback would be appreciated :
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Last
edit
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17-Mar-2010
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