Introduction: Somewhat Complete PCB Fabrication
There are a lot of guides to individual aspects of making PCBs, but I have tried here to create a complete "start to finish" guide, in the hope that it is useful.
A long, long time ago, I made PCB's by converting my schematic to a PCB layout on paper, using a pencil, then I would transfer the design to copperclad board using carbon paper and centre punch where the pads would go, then I would use rub-down dry transfers to make tracks and pads over the marks I'd made, and finally etch and drill the board. It worked quite well, but what a time consuming and tedious process!
Fortunately we have computers now, and easy access to materials which make the whole process much quicker and easier!
I've made a few PCB's over the last few years using CAD software, and have moved from doing toner transfer to using dry film, and from doing single sided boards to double sided boards, from using steel drills to using tungsten carbide ones, and to giving my boards a nice shiny green coat.
One of the nice things about moving from toner transfer to dry film is that a laser printer is not needed any more. In fact some people report better results by using an ink-jet printer.
I have started to learn about through hole plating. I haven't attempted it yet, but will share what I have learned so far.
What this instructable covers:
- Selection of PCB material
- Board preparation
- Through hole plating (in theory)
- Use of dry film photo-resist
- Toner transfer (but not in depth, there are plenty of other guides)
- Applying solder resist
All these subjects are covered to a greater or lesser extent in other places, so there is plenty of information out there!
Plenty of other people have described how to
- make PCB drilling machines of one sort or another
- how to do toner transfer by various methods, so I only touch on that
- Also there are a few designs around for UV light sources. I based mine on the design in another instructable, but I changed the power source - schematic provided in the first step :)
Step 1: Tools and Extra Hardware
PCB making is easily accomplished using primitive tools and "kitchen sink" methods, but there are some extras which can make life a lot easier.
The basic tools are:
- Fine "wet and dry" type paper, around 1000 grit for cleaning boards
- Hacksaw with a fine blade, eg 32tpi
- File, to clean up board edges
- Pencil, to mark where to cut boards
- Square, so you can mark straight
- Something to clamp boards whilst you cut them, such as a vice
- Craft knife - often handy but not required for any specific step
- Plastic containers for liquids
- Your chosen means of creating a design
- Your chosen means of applying etch resist.
- A drill suitable for drilling PCB's
I built a version of this UV light-box but changed the design so that it has 3 sets of LED's connected in series and uses a voltage limited (but not regulated), triple constant current supply.
The supply needs a little explanation. It runs directly from rectified mains power, and has 3 LM317's connected as constant current sources, which should work on their own, however the mains voltage fluctuates considerably, so a voltage limiting circuit is provided to stabilise the DC. This looks like a linear regulator, but it is not - the MOSFET acts as a switch controlled by the peaks in the rectified DC waveform, to shut off the supply to the main capacitor when the voltage gets too high.
I use the box to expose dry film photoresist, and also UV curable solder resist. Both of these can be exposed using sunlight, but the box is more consistent if you live in a variable climate.
I also made a PCB drill press. It uses a motor taken from a printer and a cheap brass collet chuck from an eBay seller, a base made out of scrap wood and guide posts made from pieces cut from the legs of an old bird-cage stand I found.
You can drill quite satisfactory holes in a PCB using a hand held mini-drill and I did so for a long time, however using the press is a world apart and if you don't have a manufactured solution, it is well worth the time and effort to build one and there are many good designs here on Instructables.
Step 2: Selection of PCB Material
I'm going to assume you are, like me, on a budget and looking for the cheapest way to do printed circuit boards. This naturally excludes pre-sensitised photo-resist. If you do use it, you can skip the "board preparation" and "dry film" parts of this instructable.
I don't know if there's a standard way to do through hole plating using pre-sensitised board, although I would imagine doing it by "pattern plating" using the photo resist, and then using tin as the etch resist.
There are lots of different types of board available, however the two most widely used are:
- Phenolic (FR2) also known as SRBP (Synthetic Resin Bonded Paper)
- Glass fibre (FR4). This is made of layers of glass fibre mesh bonded with epoxy.
The "FR" designation refers to "Fire Resistance" and is a rating given to all types of PCB material. So the glass fibre board has a higher resistance to catching fire than SRBP.
The main advantages of SRBP are:
- a softer material to work with - doesn't blunt carbon steel drill bits so quickly.
The main disadvantages of SRBP are:
- brittle - you tend to get a cockled edge when you cut it, and corners can break off
- lower fire resistance
The main advantages of glass fibre are:
- greater fire resistance
- stronger material.
The main disadvantages of glass fibre are:
- it blunts your drill bits very quickly
- it's more expensive.
There are also various different "weights" of copper available, so you might get 1/2oz copper if you plan to do through hole plating, or 1oz copper if you don't, or a heavier weight if you want a board that can handle lots of current, be super-reliable, whatever.
You can also get other types of base material, such as boards having an aluminium core for heat-sinking, or thin flexible boards, or exotic base materials such as teflon or ceramic for special purposes.
So, back to what is familiar:
I get nearly everything cheap, which often means without data, so all I know about the boards I have is that I have some single sided FR2 and some double sided FR4, and the copper on the FR4 seems very thin compared to the copper on the FR2!
I discovered the probable reason for this is that the double sided FR4 is manufactured with the intent that it will be through-hole plated, and as the FR2 is single sided there is no reason it would ever be through-hole plated.
Step 3: Laying Out of Your Design
The use of various kinds of software to do this is very well covered elsewhere, so I'll only touch on it here.
I use a free program called DesignSpark. It's easy to use but lacks some features you might find you want if you get more advanced, such as "fiducials" and holes that aren't round. The main caveat however is that you have to register it in order to be able to save your work, and it needs an internet connection when you launch it, otherwise it runs (eventually) as "unregistered".
If you are making several boards for one project, it's not a bad idea to lay them out together on a single piece of PCB, to save etching time. See photos in the toner transfer step.
I've included a picture of a board done in a very rough and ready way, drawn directly onto the board using an OHP pen. This is known as the Sharpie method, after the famous make of permanent marker.
Let's look at the old fashioned way first. That is to say, the pencil and paper approach.
If you've never designed a PCB before, this is a good way to understand how the process works. The same design principles apply to using a CAD program - including the putting it away and getting a drink parts!
It's well worth getting some 0.1" grid graph paper for the final layout, as most components are based upon this spacing.
Ask yourself the questions:
- What do I want this layout to do for me?
- Do I need to make special considerations for high frequencies, high voltages, high currents, digital signals, sensitive circuits, mixed analogue and digital signals, live circuits, noise (coming or going), placement of panel controls, or anything else?
- How am I going to fit in large or bulky components like transformers, large capacitors, heatsinks etc?
- What size and shape board do I need? How will it fit in the box?
- How am I going to mount the board?
- Do I need special grounding arrangements, such as a ground plane, or star ground?
Use the answers to these questions to inform how you place your components.
Re-draw your schematic so that components which are affected by this process are in approximately the places they need to be on the board. Draw it so that associated components are in sensible places and there are no really long connections that don't need to be long. Also re-draw any ICs so they resemble the physical device, ie group together gates etc instead of them being all over the place.
Put it away, get a cup of something, forget about it for a while.
Get it out again and look at it. Change the stupid things you did that you didn't spot before. Check that the schematic is still the same circuit and you haven't redrawn it so it's changed into something else that won't work!
If you are happy with the schematic, it's time to turn it into a PCB layout.
Draw the outline of the board, and draw the placements for the biggest components in the correct orientation and with their actual pins (show the symbol inside if you need to), and any which have critical placements. Leave room for any wide traces you will need. Also leave room for heat to escape from any parts that will get hot. Now is also a good time to draw in your mounting holes, if any, or space to fit other mounting arrangements.
Re-draw the schematic again, but inside the board outline drawing, ensuring that any lines which cross are now passing under components instead. Draw wire links for any that you really can't fit easily. Be wary of trying pass lines between the transistor pins, they tend to be pretty close together. Same goes for IC's.
Taking it in small stages, rub out each component symbol and draw in an outline of the actual device. You may find it easier to re-draw the whole thing again. Tweak and adjust placements as you go.
Turn the schematic lines into wider ones which represent the PCB tracks. Again, tweak and adjust.
When you think you're happy with it, put it away and get another cup of your favourite hot beverage. When you come back you are bound to find things you need to change.
If you are using a photo-sensitive film to make the board (or a pre-sensitised one), trace it onto a transparency sheet. You can use dry transfers for this, special tape, etc.
The assumption is that you have been doing the design as though you are looking down through the component side with the copper underneath.
If you are transferring it direct to the PCB, unless you've been clever and created a copper-side drawing to start with, you need to create a mirror image. You can do this with tracing paper, and flip it over, or with carbon paper, placed carbon side up to the back of the drawing. Either way, trace over the drawing and use the mirrored version.
Next, lay a piece of carbon paper on the board, and lay the design on top and tape it down. Centre punch the hole positions. Trace over the design so there is a carbon paper impression of it on the copper. Then you can lay the pads and tracks using dry transfers or a suitable pen. The carbon paper impression is easily rubbed off, so treat it with care.
Whilst searching for photos of dry transfers, I discovered they are getting hard to obtain. Presumably this is because of the prevalence of other easy prototyping methods. I also discovered decal paper, which may possibly provide a way to make your own.
Using a CAD program
You will need to find out for yourself the details of how to use your chosen program, such as using parts libraries, transferring the circuit from schematic part of the program to the PCB layout part, manipulating tracks, and other software niceties.
With DesignSpark, you select "Tools" > "Translate to pcb", and follow the prompts. You then end up with a lot of components stacked in un-helpful places and a mess of lines called a rats-nest. The connecting lines are known as rat-lines, and you need to replace these with lines which will eventually become your copper tracks.
Move each component into the approximate position you want it. Refer to the schematic, as the placements probably won't be too different, and use the length of the rat-lines to inform the placement. Try to keep them short, and decide which ones you really need to be short and which ones don't matter so much. Leave room for heat to escape from any parts that will get hot.
Double click (in DesignSpark anyway, your software may be different) each rat line to turn it into a track, which you can then move and format as required. Simple :)
Print your design to use for your preferred fabrication method.
Step 4: Cutting Out
Mark the board stock using a test print as a guide, which I have found to be more reliable than measuring. Make it a bit bigger than the final size. Use a square so that your edges are at right angles. If you use a pencil to mark, it's easy to change if you make a mistake. Although a scriber makes a mark which is much easier to see, if you get it wrong you're completely banjaxed.
It's easiest to cut board when it's clamped in a vice, using a hacksaw. Professionals use a guillotine, but when I tried to cut a piece in a domestic guillotine it was a disaster, so I'm not suggesting that!
Clamp the board between pieces of wood, as close to where you are going to cut as possible, and clamp it all in your vice. You can lay it flat this way but it's a bit more awkward. If you don't use the wood, the board will wobble, bind on the saw blade, and shriek dreadfully.
If you find you need to clamp the boards so they just hold the copperclad at one end, such as when filing down the edge, use a scrap piece of board as a spacer to keep the boards parallel.
Don't cut right up to the line, it's too easy to go over it.
Step 5: Board Preparation
The process varies a bit depending on whether you are doing a single, double or a plated through board, so I'll highlight the differences as we go along.
The assumption is that you are going to use the dry film method, however if you prefer to use toner-transfer, the board preparation is exactly the same.
- your PCB layout printed on transparencies or whatever you prefer to use
- sticky tape
- dry film photo resist
- copperclad board
- a source of ultraviolet light
- sodium carbonate, (soda crystals, aka washing soda)
- sodium hydroxide (caustic soda, aka lye)
- very fine wet and dry abrasive paper (1000 grit or thereabouts)
- liquid soap (dish soap, washing up liquid etc)
- a laminator, though you can use a clothes iron
- cardboard the same thickness as your copperclad, if the board is small
- a brush such as a small, stiff paintbrush.
If you are doing a double sided board, lay one of your transparencies on the board and choose a couple of holes which will act as registration holes. Choose holes which will give you an off-set diagonal, ie near the corners but by different amounts. Harder to mix them up, that way. Centre punch and drill these two holes, . You can also include dedicated registration holes in your design, if you prefer. Do one about 1/2 way along 1 edge, and the other about 1/4 of the way along the opposite edge. If you are using transparency film, drill the corresponding holes in it because pushing a pin through distorts it too much.
The other way to do double sided boards, which I haven't tried but is probably better, is to make the two transparencies into a pocket with a board-thickness spacer between, and put the board in.
Get your abrasive paper nice and wet, and squeeze a tiny drop of liquid soap onto the copperclad board. Using small circular motions, clean the copper until it is has an even finish all over. Run water over it so all the soap is washed off. Does the water form a solid sheet all over the board, or do you have some droplets/patches? If it's not a solid sheet, get back to cleaning, and try again! If double sided, do both sides. Some people recommend further cleaning the board with acetone. Don't do this! Acetone can leave greasy residue that causes problems. If you really feel you need to use a solvent, use alcohol or ispropanol. If you are not doing through hole plating, skip the next step.
If you are doing through hole plating, you also need:
- the layout (or, better, just the pads) printed on a paper label
- a tank for plating - a cut off 5L container for example
- anodes. Some lengths of copper pipe or thick copper wire connected at right angles to another length of copper work well. You need two of these
- a hefty low voltage power supply. Plating takes quite a lot of current
- the plating solution. This consists of sulphuric acid, copper sulphate, a tiny bit of hydrochloric acid and an organic additive
- conductive ink. Others have reported success with homemade inks, proprietary inks, silver paint and car de-mister repair paint. So you takes your choice...
If you intend to do plated through holes, stick your paper label on the board and drill all the holes, remove the label and clean off any residual adhesive, and then clean the board.
Step 6: Plated Through Holes
If you are not plating through holes, skip this step
The alternative methods are:
- Use hollow copper rivets (such as the "copperset" system) which you put through the holes and pinch up with a punch. These are very secure and you can put component leads through. It is an expensive system though.
- Use snap-off through hole pins which you solder in place. These are tapered and have a flat head, and work well, however they are hard to obtain.
- Save bits of component leads which you solder into the holes. They are no more reliable than a normal connection but doesn't cost you anything.
- I have seen instructions for variant of 3. where a drill was fitted with a rounded tipped tool and this was used to burnish the ends of the wires to form a very reliable rivet.
This part exists in theory, I haven't actually tried it yet!
The assumption is you are going to do panel plating, not pattern plating, as this is easier for us amateurs. For this reason you should design your PCB layout to remove as little copper as possible.
If you are going to attempt pattern plating, you should apply and expose photo-resist (as per the next step), but with a positive image so the parts you intend to etch away are protected by the resist, which will prevent these areas from being electroplated. After plating, dip the board in tin-plating solution, ensuring the insides of the holes are also plated, then strip off the photo-resist. The tin plating is now your etch-resist. I can only say good luck because I have no idea how well this works.
You need a power supply that can deliver quite a lot of current. The voltage is very low, but the current can be 10's of amps
The plating solution consists of sulphuric acid copper sulphate, hydrochloric acid, and an organic additive in the ratio:
- 41 parts de-ionised or distilled water
- 30.5 parts concentrated copper sulphate solution
- 29 parts 35% sulphuric acid
- 1 part polyethylene glycol (the original patent for the process states coffee was used, but not how much)
- 0.0133 parts hydrochloric acid
There are superior proprietary products to PEG, but these are also more expensive as they are intended for commercial/industrial use and are sold in appropriate quantities. PEG is ok for us amateurs.
The Think & Tinker website gives this in quantities for 100L of solution - a bit much for amateur use! So I've given it in parts instead. The site also gives ratios for if you are using dry copper sulphate rather than pre-dissolving it.
The quantity of hydrochloric acid given is 0.5ml of 35% strength per US gallon of plating solution. For the rest of us this works out as 0.5ml per 3.7854 liters.
This mix is designed to produce a thicker layer of plating inside the holes than on the surface of the board, though I'm not sure how well it works. The ability to do this is called "throwing power".
You also need a tank to hold it in. Make copper anodes out of whatever you have available. They need to give an even distribution on either side of the board. You also need a power supply which can deliver a fairly hefty amount of current, although the voltage is very low. The chap who made the video made bags to catch crud that falls off his anodes, keeping the solution clean. I don't have the experience to comment on this.
You should empty the solution out of the tank and keep it in a sealed container when not in use.
The solution should last indefinitely although it needs to be adjusted from time to time. When the time comes to dispose of the it, DON'T pour it down the drain! It is laden with copper which is toxic to wildlife. Take it to a suitable disposal facility.
- In the last step you should have drilled the holes.
- Get your conductive ink and squeegee it all over the board, making sure it goes well into all the holes but as little as possible is left on the surface of the board.
- Pull the ink through with a vacuum cleaner from the other side.
- Wipe away excess ink, without disturbing what's in the holes. Examine the holes to make sure the ink has fully coated the inside of each and meets the copper, and is not causing any blockages.
- Give the board time to dry
- Clean the board with a green (or whatever colour) dish scrubber. Use the same test as before: does the water form a sheet over the board? That's what you need.
- Now you need some crocodile clips. These should be connected to the negative of your plating supply, and the anodes should be connected to the positive. Clip the board to the clips, suspend it in the tank, ensuring it is a reasonably even distance from all the anodes and at least an inch (25mm) below the surface, and turn on the supply. Let it plate for the time you calculated.
- Turn off the supply, take out the board and clean it. Examine the holes to make sure they are plated through properly.
- Clean the board again with your green scrubber and a little dish soap.
Step 7: Using Dry Film Photoresist
Dry film photo-resist is to toner-transfer as toner-transfer is to the Sharpie method. It's like going from video tape to DVD, and then DVD to Blu-Ray.
There are a few tutorials around that treat the use of this wonderful material, so there are plenty of sources for more information. I just give here what works for me, however you may find differences according to manufacturer of your film.
Cut out a few test pieces of film first so you get the method right. Apply to a small piece of board and do test exposures. You don't have to etch them, just clean off the film and try again until you get nice traces with clean edges.
The exposed parts of the film harden and become etch resist, whilst the unexposed parts remain soft and can be cleaned away. Therefore you need a negative transparency, ie the tracks are left clear and the spaces between are filled in.
- Print your transparencies first. Print the bottom one not-mirrored, and if doing the board double sided, the top one mirrored. This way the printed side goes next to the film and gives you better resolution. Handle and store them carefully to avoid scratching the print.
- If you are doing a normal board, print the design with holes in the pads. If you have plated through your holes however, you need to print with the holes filled in. This way film will cover the holes and protect the through plating from the etchant, a process known as "tenting"
- For small boards, cut a piece of cardboard to a size that will go easily through the laminator but takes a good portion of it's width, and which encloses your board. It needs to be the same thickness as the PCB.
- Lay your piece of copperclad on the cardboard, and cut round it with a sharp knife, so when you take out the piece, the copperclad fits snugly in the middle of the cardboard. Take the board out for cleaning. Switch on the laminator to warm up. Put it on the lowest setting. If you are using a clothes iron, turn it on. Don't have any water in it! Put it on a low temperature, such as the wool setting. If it's too hot to touch, it's too hot.
- When it's nice and clean, fit the board into the cut-out in the cardboard. Exclude daylight and light from fluorescent sources. You really only want light from incandescent or LED lights. Try to keep light levels low in general.
- Cut out a piece of film a bit bigger than the board. Stick a piece of sticky tape on each side of one corner of the piece, cross-wise, so you can begin to separate them. You should get a piece of clear film stuck to one piece and the dry film stuck to the other. Unless it is quite small, don't pull them completely apart.
- Wet the board before applying the film. It will work dry, but you may get small tears in the resist.
- Line up the edge you have started to peel with one edge of the board, press it down.
- The professional way to do this is to peel back the protective film as the board is fed into the laminator so that air is not trapped. Lucky for you I'm not a professional. The method is ok for larger boards, but if you are doing a small one it is simply too fiddly.
- Peel the backing, smoothing the film down as you go. If it's double sided, do the other side the same way. If you are using a carrier, the film will help hold the board in place.
- Now just feed the whole thing through the laminator. If you are using a clothes iron, place a sheet of paper over the board or the iron will mess up the film. The carrier helps keep the iron level, so your task is easier.
- If you are using a carrier, keep the board in it until it's exposed.
- Put it somewhere dark until you're ready to use it.
- Put the transparency on the board - it should be printed side down, though if you are using transparency film it doesn't matter so much. Tape it down.
- If doing double sided, protect the side you are not exposing from UV light.
- Peel off the top protective layer of plastic from the film and wash the board in a solution of sodium carbonate (soda crystals, washing soda, (or natron if you're an ancient Egyptian)). Use a brush to help clean off the unexposed film. I find rubbing the board between thumb and forefinger under running water every so often helps too.
- When the board is etched, clean off the exposed film using sodium hydroxide (caustic soda, or lye). Just wait for it to wash off, you don't need to do anything to it. Don't get it on your fingers, it will try to turn them into soap!
Step 8: Etching
There are a few etchants you can use, however the old standard is ferric chloride, which I'll describe here because I'm familiar with it.
The more modern solution to use is a mixture of hydrochloric acid and hydrogen peroxide, however I've never tried it, and it is well described elsewhere. There are other solutions but these are not really suitable for the hobbyist.
I don't use anything sophisticated. I pour my ferric chloride into a plastic takeaway container, cover it (leaving a vent) and heat it up in the microwave, chuck the board in, swish it about with my fingers and turn it round occasionally until it's etched, shake off as much as I can, then wash it under the tap and wash my hands.
For larger boards, such as the one in the example, I use an old washing up bowl and don't heat it up.
Be very careful heating ferric chloride in the microwave - it gets very hot at the top and can melt the container.
The swishing about is important as it removes dissolved copper from the surface of the board, which would otherwise inhibit further etching, both slowing down the process and making the etch uneven.
A bubble tank is much better as you get a more even etch, however I haven't got round to making one yet.
Now I know the health and safety brigade are going to wring their hands in despair at this casual handling of a harmful substance, so lets get a bit of perspective here.
Ferric chloride, in it's pure form, is a naturally occurring salt, found in soil, and is fairly innocuous. It becomes gradually more toxic as its burden of dissolved copper increases with use.
The worst it will do to your skin with short exposure is stain it yellow. I don't know what the effect of the dissolved copper is due to long term exposure but in this case it is quite brief and I would expect nothing bad to happen.
The amount being washed off the board is tiny. If you were doing a lot of boards every day you would definitely need a cleaning system which keeps the used etchant out of the drain, but that is simply not a practical option for occasional "one off" boards.
Exhausted etchant needs to be disposed of in a sealed container. It is now full of dissolved copper, making it toxic to wildlife, and must never, ever be poured down the drain. You can also get additives which turn it into a solid lump for safe disposal.
If you are doing a standard board, now is the time to clean off the etch resist apply solder resist if you are using it, and drill the holes.
Step 9: Toner Transfer
This method of making PCBs is extremely well covered elsewhere, so I will describe briefly my own experience here rather than try to provide full instructions.
I have not tried any of the "cold" transfer methods.
There are two significant drawbacks with toner transfer:
- Large areas are difficult to create complete and with a good finish
- boards of more than about 5" or 12cm to a side suffer from problems due to thermal expansion, which leads to cracks and wrinkles in the toner
Prepare the board as described in the previous steps.
You need paper with a shiny surface. Many people have reported success with magazine pages. I have also tried using flyers and glossy inkjet photo paper, but eventually settled on using cheap yellow "toner transfer paper" from a Chinese eBay seller, which gave the best results as it has a very thin plastic layer which helps seal the toner.
Others have also reported success using shiny backing paper from stickers, tracing paper, and home-made dextrin coated paper. I tried my hand at making dextrin coated paper, but you need to treat the paper with silicone water proofing spray, which is expensive.
The worst kind of paper is normal office printer paper, as it is porous and absorbs the toner.
The problem I found with paper which has been printed on is that the print to some extent affects the transfer, making it look a bit wobbly.
Normal types of paper will come off if soaked and rubbed away, but toner transfer paper and photo paper can just be peeled away with no soaking.
I haven't tried any of the methods which add extra material to the toner, eg press 'n' peel, or vinyl foil.
My method is to do a test print on A4, then cut a piece of transfer paper which will cover the test print and tape it down at the edge which will go first into the printer. This way the test page acts as a carrier.
The way the transfer works is that the toner melts when heated and sticks to the copper. I found that my laminator simply isn't up to the job, and have always used a clothes iron.
If using an iron, don't be tempted to turn the temperature right up. I found a temperature somewhat less than the steam setting to be satisfactory. If it's too hot you will experience two problems:
- the toner will squash out and you will get wobbly lines.
- the toner will bleed into the paper, so you get less on the board.
After the doing the transfer it is a good idea to "colour in" any large areas such as ground planes and wide tracks with permanent ink as they will almost certainly have patchy coverage. Examine the board carefully and correct any faults either by touching them up with ink or scraping off toner with a fine blade.
Because the layout is normally created as viewed from the component side of the board, it is already the correct way when you print it, so there is no need to print the design as a mirror image, unless you are doing a layout for the top of the board, which you will need to mirror.
When you have etched the board, you can clean the toner off with acetone, although this does tend to leave a slight residue of toner on the substrate. It also however leaves a slight greasy residue on the copper which helps prevent oxidation if you don't apply either solder resist or a conformal coating.
If you find you are not happy with the transfer and need to do it again, you can also clean it off with acetone, however you will need to clean the board again with detergent afterwards as acetone can leave a greasy residue which stops the next attempt from sticking properly.
Step 10: Solder Resist
This is a UV curable epoxy resin coating which seals the board, prevents the copper from oxidising and stops solder going where it isn't wanted. It makes it much more difficult for solder bridges to form.
I was astonished by just how easy it is to apply green (or some other colour) solder resist. In the bad old days before UV exposure was used it had to be silkscreen printed on, but now it's easy for anyone to use.
If you don't want to go to the trouble of using solder resist, you can spray the board with conformal coating. This protects and seals the board, but you coat the entire board surface and solder through it, which is very easy.
You need a piece of thin plastic such as you find used as the window on a cake box, the thinner the better (but it must keep it's shape - cling film is no good and polythene is not much better!). You need this to prevent oxygen reaching the resin, which prevents it from curing, and to give a nice glossy finish. It also makes it a lot less messy!
You also need a piece of cardboard you can tape it all down to.
You can see from the first photo that the demo board has a matt finish. This is because I used a cut-out piece of polythene bag when I found the piece of plastic I had been using proved to be too thick.
The resist doesn't harden completely straight away, which is why part of it was able to be torn away when I pulled the polythene away. It also enabled me to scrape the resist off with a woodworking chisel the next morning and re-do the coating because I was really unhappy with the finish. Once it is fully cured the resist is very tough and difficult to remove.
You can see in the photos spots where air got under the plastic, which stopped the resin from curing so it got cleaned off with the pads.
Print another transparency, this time just the pads, without the holes. If you need any other places without resist, if your software won't allow you to add them, align the transparency on the board and draw them in with black permanent marker, or print another transparency with them on.
If you did plated through holes, I imagine plugging them with something before applying the resist, but have no experience to know if this is appropriate.
Put blobs of resist all over the board and lay your piece of plastic over it. Press it gently with your fingers so the blobs join up with no air pockets. Some people say you should make a line of resist at one edge of the board, but I found this to be more difficult. Roll it with your rubber covered roller to spread it out evenly. You need to take some care doing this as it really wants to make thin and thick bands, which is not what you want! If the resist is too thin it doesn't really matter, though it may not do it's job properly, however if it's too thick there is a problem as it may not cure and come off the board when you clean the holes (see photo). Rolling gently all over for a long time seems to be the best way. Don't let your roller stop anywhere, it will make a stripe in the resist and you will be a long time rolling again.
Flip it over onto some cardboard and push sewing pins through the registration holes you drilled so they pierce the plastic. Take the pins out and turn it back over. Squash out any air you just let in. Put the pins through the holes you drilled in the transparency and insert them in the holes you just made in the plastic sheet and the relevant holes on the board, tape it together, remove the pins, and then expose in the same way as the dry film. Peel off the plastic and clean with acetone, which will remove the unexposed resist from the pads.
Repeat for the other side if double sided.
Step 11: Drilling and Cleaning Up
There are two types of twist drill in common use:
- carbon steel
- tungsten carbide
Carbon steel bits are cheap, but they wear out quickly. They work well for FR2, but will only drill a few holes in FR4 before becoming too blunt to use.
Tungsten carbide bits are more expensive, but last a long time. You should really invest in a set of these if you are going to drill a lot of holes, especially in FR4. They need to be treated more carefully than carbon steel bits however as they are brittle.
You can get them in sets of various sizes, just shop around for the best price.
In the bad old days I drilled hundreds of holes using a hand-held mini drill, which worked well enough even though the holes were never quite perpendicular, but it's much easier if you have a drilling machine.
I've been making a semi-automatic PCB drill but 2 years after I started and it's not really usable. I've included some pictures of the hand operated one I made, which has given good service. The chuck is a type available from several on-line sellers, and the motor came from an old printer, though a faster motor would be better. The only bit requiring real care is making sure the motor spindle size matches the mounting hole in the chuck.
I haven't included the design because there are already plenty of good designs out there. The only really unique thing about it is that the operating lever presses a microswitch to turn on the motor when it's pulled down.
It's worth taking care to ensure the drill is lined up properly with the pad. If nothing else, getting a hole mis-aligned is really, really annoying. Also it can make a component not fit properly. At worst it can cause the pad to not be connected to the track.
With care you can make slotted holes for components that have flat leads. Drill two holes which correspond to the ends of the slot, and carefully "mill" the space between them by sliding the board back and forth whilst lowering the drill by very tiny amounts. Be super careful doing this, I broke a drill bit doing it.
This is also where you find any mistakes you missed when checking your layout. You can see I got some pads joined together, and also got a component placed so it overlaps the edge of a heatsink. I cut between the pads by milling the copper away with the drill, but the mis-placed component will have to go on the underside of the board.
The copper is really shiny and will reflect light in the most inconvenient way possible when you come to drill holes. I have found that this is greatly improved by using circular motions at the cleaning stage (so the copper actually looks dull), and is improved much more by using solder resist as it makes the board much simpler to look at.
It doesn't matter if you clean the edges before or after you drill, what's best for one board may not be so good for another. Don't do it using any kind of power tool unless you have dust collection. The dust from either type of board is unpleasant to breath if not actually harmful.
Clamp the board between some pieces of wood, close to the edge you are going to clean. It needs the most support at the back. File the edge down to the board outline and smooth off the corners. Repeat for all edges.
Something that I have found is that sometimes you can get a film of solder resist covering the pads, too thin to see but enough to make soldering difficult. In these cases I scrape the pad clean with the tip of a craft knife.
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