Making Your Own Printed Circuit Boards(Using the
Positive Proof Photo-etching system) IMPORTANT!
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Some of the chemicals used in this process can be toxic,
caustic or otherwise just plain dangerous to use! I cannot be held
responsible for any consequences resulting from the use of this
information: It is for convenience only. Eye and hand
protection should be worn while working with these chemicals and also
dust protection if you are cutting Printed Circuit Boards. ·
I cannot gaurantee nor warrantee the accuracy of this
information nor can I be held responsible for any losses (financial or
physical) in your using this information: It is for convenience
only. ·
Disposal of chemicals should be done according to
manufacturers specs. Remember, some of these chemicals, in
particular the etchant, are specifically designed to dissolve large
amounts of metals - including sewer pipes if you just pour it down the
drain! (Read: Pouring etchant down the drain may cause myself
and/or my landlord and/or my parents many, many, fine $$$ indeed and
REALLY mess up my rec room in the basement!) So
how d'ya doit? Ok, what I'll discuss here is the process of positive-proof photo etching. It may seem scary at first but once you try it you'll be amazed at just how easy it is. This process is suitable for both single, and multiple board runs; that is you can make only one of the PCB you designed or just keep making essentially as many of the same PCB's as you like - with ease. The big advantage to using photo process etching to make only one PCB is that you can design the board on computer and print it out on a clear acetate, rather than trying to use rub-on transfers or an etch resist pen, etc... The system cannot be done away with if you plan on running off anymore than two of the same board as once you've made the clear acetate and have the involved chemicals, you can "burn" as many boards as you like from the same acetate. The process can be broken down into
simple steps: 2. Burn a PCB 3. Develop the etch resist 4. Etch the board 5. Drill the board 6. Populate the board! I will also include here a very helpful intermediate step (called a time strip) which you should only need to do once when you first set up your photo processing workstation. What you'll need:
Your best bet financially and
practically is to actually buy a positive-proof photo etching kit.
Mine cost about $50 Canadian and I got it from Active components.
Pretty much any serious electronics supply house will have them. Starting at the bottom left hand corner is the copper clad board, a raw PCB before etching. However this board is covered in a photosensitive coating on top of the copper and a protective plastic layer. A light source (you need to provide
this, it is not included with the kit). Ferric chloride or some equivilent
etchant (should be included with the kit), developer (should be included
with the kit), a tray for developing and etching in (usually the case
the kit comes in doubles as this), rubber gloves (usually included with
the kit), and of course the instruction sheet that comes with the kit. Also shown in the upper middle of the picture are some rub-on dry transfers (available at Radio Shack) which are handy for making your acetate transparencies. Not shown in the picture are the acetates themselves; you can get them at pretty much any business supply house as overhead photocopier/laser printer transparencies but they're kind of hard to find in low quantities and expensive to buy in a box. They don't need to be the high-temp kind (i.e.: laser printer / photocopier transparencies) unless you plan on using a photocopier or laser printer to produce you acetate transparencies. They can be simple thin plastic you merely rub your transfers onto if you're using rub-on transfers. You can also get transparency sheets for bubble jet printers as well, again they are intended for making overheads on your bubble jet. The kit I bought
also included two foam brushes, one shown in the picture beside the box.
These are for gently swishing the chemicals around during processing.
A good item to have as well, which is on top of the box, is scotchbrite
or some scouring pad. Steel wool works okay but sometimes takes
off a little too much copper when you're cleaning the boards. A time strip, or time sheet as it more commonly known as, is a technique used by photographers in the dark room to get a feel for the film sensitivity and light power. Film typically takes at least several seconds to sometimes minutes to be properly exposed. Too long an exposure and the film gets overexposure - it all turns black. Too short an exposure time and the film remains underexposed and everything turns white. A time strip is done by wasting a piece of film; you cover almost the entire film with a piece of card to block any light from getting to it, expose the film through a negative the same way you normally would make a picture, but every five seconds or so you move the card exposing a little more of the film. At the end of the timing you develop the film and you can see precisely what exposure time to use: the exposure has been done in steps. The first strip was exposed the longest, the last strip was exposed only five seconds, and you increment the exposure time by five seconds every strip in between. This is very
useful for what we are about to do because you provide the light
source to expose the PCB. This means that your light source cannot
be predicted by even you: you won't know what exposure time to use.
BUT, you can "ball park" it - good enough, a time strip will
tell us the rest. On light
sources: In the picture you can see the light source I used; a 100
watt light bulb in a clamp-on bed light. Not too high tech.
In actual fact, it was way too powerful but it served the purpose.
I had a distance of about 6 inches and the exposure time turned out to
be about 15 seconds. This is where the time strip helps because
the photo sensitive etch resist on the PCB was far more sensitive than I
had thought and the light much more powerful than I realized. I
fact, I have used ambient room light to expose PCB's before and it
worked just fine - it will depend completely on the sensitivity of your
board and on the light in the room. Setting up:
First
things first: Mix your developer if you need to and put some in
your developing tray, have it standing by. It doesn't need to be
deep, just enough to cover the circuit board by a couple of milimeters.
No need to waste developer. Most developers need to mixed with
water, consult your packaging for the details and DON"T FORGET
GLOVES AND SAFETY GOGGLES!
Make a "dummy" acetate, a clear sheet of plastic with a fake
(or real ) circuit printed on it, fabricated in exactly the same way you
plan to make the rest of your acetates.
The Copper Clad Board is packaged
in a black, light-proof bag. Three guesses as to why! Keep
it in there! Turn the lights out and use only very faint light to
work with when cutting the boards or setting up for a burn. The board has a protective peel-off
plastic coating. I believe this protective layer further shields
out light, but don't assume it does or you might be just ruining a board
for nothing. Save your board - cut off a strip or use a very small
board as it will be wasted in the process of making a time strip.
Remove the plastic covering. It's very difficult to photograph
and consequently very difficult to see but this is the setup during the
test strip burning. I have the Photosensitive copper clad board,
with a dummy acetate circuit on top, a piece of glass on top of that and
finally a piece of opaque plastic to block the light for the time strip
(construction paper or bristol board works Ok). I have premarked
the board into 16 different sections and moved the plastic along to the
next mark every 30 seconds. I would recommend a time interval of
five seconds for a bright exposure like I've done here (e.g.: a 100 W
bulb 6 to 12 inches away), but a thirty second interval for a gentle
exposure like ambient room light. Here you can see the markings I have
made on the board. This is about 3 1/2 minutes into the exposure.
The first strip on the far left has been exposed 3 1/2 minutes,
the one on the far right only 30 seconds. IMMEDIATELY
place the circuit board in the developer when you're finished your
exposure. (WHAT!?! You didn't put on your rubber gloves
yet!? Put them on! Quick!) Watch
the board develop right before your eyes! The developing process will
probably only take a few seconds, but that is dependant on the chemicals
and board you used. Once it is obvious that it has completed
developing, remove the board and immediately rinse it off with water to
neutralize the developer.
The parts of the board that were overexposed turn completely black and
dissolve off the board, the strips that were underexposed will not
develop at all, that is the etch resist will completely cover the board
still. In between there somewhere is the sweet spot you're looking
for, the time where your traces get developed properly and not removed
by the developer, and the etch resist gets removed from all other spots.
You can see in the picture how well the dummy circuit developed (bear in
mind my dummy circuit consisted of a simple black strip down the length
of the acetate with a couple of circle pads thrown in for good measure
).
Remember that the far right of the circuit board (the strip that was
exposed last) was only exposed for thirty seconds. That was the
shorter exposure end. In this picture, I would be trying right
around the tenth mark (barely visible) from the right. That's
about a five minute exposure time (30 seconds per strip, ten strips). Making Your Printed
Circuit Board: Remember that
this is a POSITIVE photo fabrication kit. Here's two pictures of
the same circuit board design, the one on the left is shown in NEGATIVE,
the one on the right is shown POSITIVE. You need a POSITIVE
circuit design printout. Make a transparency with the circuit on it (real size of course). Text & especially part identifiers are a good idea - be professional! Lay the transparency on top of your
photo sensitive copper clad board, lay a piece of glass (or thick, clear
plastic) on top to hold down the transparency, then light burn it for
the appropriate time. Immediately
put the circuit board in the developer after you have finished your
exposure burn. Develop the board and immediately wash the board
off with water. (Did you remember your goggles and gloves!?!?
I hope so!) Etching the board:
The etching process itself is quite simple. Fill your tray with a generous amount of etchant (the most popular and common is Ferric Chloride though I've used some "just add water" crystals which worked really good and went a lot farther I found), enough to cover the board with a good centimeter (3/8") of etchant. As the etchant dissolves the copper off the board it becomes saturated and will only dissolve so much. Place the board
in the etchant, copper side up so you can see. The process should
take somewhere around 20 minutes to a half hour, but the time depends on
the strength of the etchant, the thickness of the copper on the board,
agitation, etc... Some tips:
Warm etchant dissolves more copper faster, but beware of fumes! If
you are warming up your etchant, have it done under a fume hood and be
careful how hot you get it and how you heat it. Gentle
agitation, either by a gentle rocking back and forth of the tray or
gently swishing back and forth with a foam brush greatly helps the
etching process by replacing the saturated etchant at the surface of the
copper with fresh etchant. Once
your board is etched to your satisfaction rinse it off with a healthy
amount of water. Let it dry then scour the copper to shiny it up
with something like scotchbrite! (Important - you'll find out why
in short order if you don't and then you try to solder stuff to your
board!) Easy,
aint it? Now you drill all the little holes out where you want to put wires & parts and solder all your parts into place! Problems & Troubleshooting:
Make sure you thouroughly inspect your board after fabrication. The most common two problems are a short circuit between two traces where some copper didn't get removed or a break in a copper trace where the etch resist failed and the copper was removed. Fixing the break
is easy enough, just solder a jumper wire across the copper. A
short piece of bare copper will suffice nicely. Tin the two
traces, then drop the wire in place and solder it!
Fixing a short is a little more tricky. An exacto knife is
indispensible in this case. Carefull cut the copper foil on either
side where you want to remove it, then carefully lift the copper with
the blade of the knife. If you cut it right, it should peel off
the board with a little bit of a fight.
An entire leg of the circuit was removed - what do I do? Uh oh,
you may have to refabricate it. You might get lucky and it's the
leg of a resistor or something and you can use the extra long wire of
the resistor to jump to it's nearest connection spot. You might
want to check your acetate original to see if that image is opaque
enough. Sometimes you need to run off two acetates and layer them
on top of each other to make it dark enough if you're using a
photocopier or laser printer. Your time strip should've shown you
this though. Maybe try turning down your exposure time just a
little. If your exposure time is too long it can bleed through the
opaque traces and expose the etch resist a little.
Solder won't stick? Did you polish the copper? i.e. did you
scrub it down with extremely fine sandpaper or scotchbrite? Even
commercially bought prefab circuit boards should be cleaned before you
use them. The difference between soldering on clean and dirty
copper is amazing and dirty copper will wreak havoc on your circuit. Have fun, and happy etching!
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