Dr Nathan Scott · July 2002
Let's suppose you have developed a terrific circuit using your breadboard and you now want to make a more permanent version, perhaps to put in a box or a robot. There are several ways to do this. The quick and dirty way is to transfer all your components from the breadboard to a piece of "Veroboard" which is a printed circuit board with straight tracks on it:
[picture of veroboard]
Note that the tracks may run the whole way along the board, you may have to cut some of them by scratching the copper away with a hacksaw blade, to avoid unwanted connections between parts of your circuit. There are several types of veroboard and some already have cuts across the tracks, making them very much like a breadboard.
The components and wires go on the side of the board that does NOT have conductors (the non-copper side) and their legs stick through the holes. Solder is applied to each leg to make a semi-permanent connection.
[picture of legs and soldered legs]
It is possible to de-solder components if you make a mistake or if a component fails. This is not easy with multi-legged chips as you can't easily melt all the solder at once. Therefore use an IC socket for each of your ICs, this makes it very easy to change them, and the sockets only cost a few cents each:
[picture of an IC socket on a board]
IC sockets are available in all sizes and shapes, always use the correct size for the chip you are using. Note that the socket has a little mark at one end which is meant to correspond to the mark on the end of the IC near pin 1. These conventions are very important, your fellow engineers expect you to follow them!
Veroboard is great but has its limits. It is not so good if
Aside: PCBs as physical devicesSeveral PCB substrates are available, for example phenolic resin (cheap and nasty) or glass filled epoxy resin (top stuff). The glass-filled epoxy is machinable and reasonably tough (although ultimately it is a brittle material). The photographic process used to make the electrical tracks - etching - can produce very detailed and accurate shapes. These shapes can guide subsequent drilling and sawing operations. With some careful thought and design work a PCB can be both an electronic device and a physical device. Here is an example of a PCB I developed for an electromechanical device. Its physical shape incorporates a rotary axis, a latch, a detent, stops, two switches and a spring - in addition to having electrical functions! I am sorry I can't tell you more about this project but at the time of writing it is a commercial secret.  The point here is that, as always with design work, you have to exploit the manufacturing processes to get as much function as you can out of each component. 
A 1986 vintage computer mouse PCB. Note how the PCB has physical cutouts to allow the ball to touch the encoder axes, and how it supports the encoders themselves. One encoder spindle has been removed so you can see the locating holes and the slot where the wheel goes. |
The process of making a PCB is
Each of these steps will be considered in turn.
You can draw artwork by hand using black pens and/or special rub-on transfers. However in this age of computers it is probably better to use a computer drawing package. I use a commercial package called Adobe Illustrator(TM). Why? Mainly because I know how to use it. It does not provide any high level features to assist the creation of the artwork, it is just a general purpose drawing package. There are some special-purpose packages out there, even some free ones, but I have no experience with them.
There are some important conventions for the artwork:
1. You must decide at the start of each project whether you are going to create artwork as seen from the conductor side of the PCB or from the "top" or component side. One is a mirror image of the other! Because we will print on transparent film, the film can be flipped over before the photographic transfer is done, so that is not an issue. The only question is which method you are most comfortable with.
|
Method |
Component side view |
Conductor side view |
|
Advantages |
The pinouts on DIL (caterpillar-like) chips match what you see in the data sheets and on the breadboard i.e. the top view. Produces final results of slightly higher quality because the laser printer prints on the TOP surface of the transparent film, and this then goes directly against the PCB (see the Transfer section below). This makes the sharpest contact print. |
Makes it easier to work with "surface mount" (SMT) devices, which are becoming more and more common. Text on the artwork can be in the usual non-mirror form. |
|
Disadvantages |
Easy to make a mistake with SMT devices, remember these go on the conductor side of the PCB, and thus appear in mirror image when working with the "component" (non-conductor) side. You must remember to put all text on the artwork in mirror image form. |
The pinouts on DIL chips do NOT match what you see in the data sheets and on the breadboard, you have to stay sharp or you may make a wiring error. The contact print on the PCB won't be quite as sharp as for the component side approach. However this is not a big issue. |
2. The lines and "solder pads" must be the right thickness, size, shape and so on.
Through trial and error, and with some help from some friends in an electronic workshop, I have fixed on the following standards for my own artwork. Note that " means INCHES so 0.023" is 23 thousandths of an inch. The spacing between IC legs is 0.1" so it is convenient to work in inches here.
There is a great temptation to make the lines thinner or to have very narrow gaps between them. But we have to respect the properties of the photographic and etching processes. If there are delicate tracks or narrow gaps this can mean that either (a) the tracks become open circuit because the etching process has made them too thin (eaten them away!) - or (b) there are short circuits between tracks because the etching process did not eat away the metal between the tracks properly. These problems can happen anyway but having clear, well defined artwork will help prevent them.
So what do you do if the topology of your circuit requires more than one track between a pair of pads? I add some extra pads and use "jumper" wires to complete the circuit at the soldering stage. This is not as nice as having all the conductors on the artwork but sometimes there is no choice. If you have a very complex circuit then you might need to consider having a double-sided, through-plated PCB made (see below).
Aside: the black art of track layout1 High frequencies If your circuit only works with DC or slowly changing voltages, it does not much matter how you draw the tracks. But if your circuit has rapidly changing voltages, you may encounter some unwanted effects. Two conductors close to each other behave like a capacitor and a small amount of energy can be stored in an electrical field between them. This means that a rapidly changing voltage on one wire can cause a voltage change on the wire next to it - which, if you think about it, may not be a good thing! As the frequency increases this effect becomes more and more important until, at radio and microwave frequencies, it is one of the main qualities of a circuit board. This is an advanced area and I don't yet know enough about it to do design such circuits. I want you to be aware of the problem, though, so that you will recognise it when it happens to you. 2 High currents If you follow my proposed track width convention (above) your tracks will be able to carry up to 1 Amp (approx.). If your components draw more than 1 Amp then you may need to take extra precautions so that the tracks do not heat up and burn out. For example
|
The first time I did this I printed on clear overhead transparency film. However my technicians asked me to re-print the artwork on tracing paper. It seems that they had established the exposure time for their ultraviolet light box with tracing paper as the standard. You will have to establish your own conventions and this may require some experimentation.
I always try to print directly on the tracing paper, rather than using a photocopier, because I want the darkest, sharpest original I can get.
Some laser printers do not print large black areas well and you may need to go over these areas with a thick black felt pen and colour them in. Remember we want to use the tracing paper in a photographic process so it is very important that black areas are very black and the other areas are of uniform clarity.
Special photographically sensitized PCB blanks are available. Like photographic print materials and film, they are supplied in opaque (light proof) bags. The copper side of the PCB blanks is fully covered in a special photo-sensitive substance that is easily washed away, but only if it has been exposed to ultraviolet light.
The sensitive PCB blank is placed with the sensitive side up. The tracing paper is put on top of the sensitive layer, facing so that the text is "non mirror" as seen from above. Then a bright ultraviolet (UV) light is thrown onto the tracing paper. Some of the UV light passes through the parts of the paper which are clear, or at least which are not black. The sensitive layer is exposed in these areas.
Then the exposed PCB is rinsed with water to wash away the exposed parts of the sensitive layer.
This simple description has left out the all-important detail of exposure time. This is something you will have to determine by experiment because it depends on so many variables - the type of PCB blank, the temperature, the strength of your light source, etc. I recommend that you try to use a UV light source indoors rather than trying to use the Sun. The Sun is too powerful and it is hard to accurately time the exposure. Also sunlight, for all its marvellous properties, is not a perfectly constant thing. Even a slight haze could throw your calibration out of whack, let alone the effect of Winter. A "blacklight" fluorescent tube mounted inside a box is the way to go. It may be necessary to have a frosted glass diffuser a few inches above the artwork.
The exposed PCB is placed in a bath of etching chemicals, typically nitric acid or ferric chloride. The etchant can eat the copper conductors but only where they are exposed. The conductors that are still covered in sensitive material can't be easily attacked so they are left behind.
In Australia the necessary plastic tubs and chemicals for PCB etching can be obtained quite cheaply from an electronics hobby store.
I use a 1/32" (about 0.8mm) drill and a drill press set on its highest speed. The drill is so small that not all chucks will grip it. You may need to buy a special small chuck called a pin vise, which can grip down to practically zero diameter and which can itself be held in a normal chuck.
The PCB pad pattern (see template above) includes a "white" or "etchable" dot in the middle and this tends to help guide the drill, making it easy to get the holes in the centres of the pads.
If you do a lot of electronics then one day you will want to create a very advanced circuit board, like the one inside a mobile phone or a modern computer. These circuit boards have such dense connections on them that one side of a PCB is not enough. Both sides of the PCB have conductors and have been photographically exposed and etched using advanced, high-resolution processes. Not only that: these very dense boards have been drilled and through-plated, which means that metal has been deposited onto the substrate inside the drilled holes. This metal connects the top and bottom conducting tracks whether or not solder is applied. I do not know how the through-plating is done, it seems miraculous to me. Anyway all the double-sided techniques are probably beyond what can be done by amateurs like you or I. If you ever need this technology it is probably best to pay an expert to make the boards (and perhaps to put the components on too).
There are companies on the internet who will accept PCB artwork in a special computer format, and a credit card number, and will produce small or large quantities of PCBs for you. They will even deliver the finished PCBs to your door. This service is not as expensive as you might imagine, especially if you order a large number of PCBs. The cost per board comes down very quickly.