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One of the neatest ways of building up a circuit is on printed circuit board. In this special feature, guest writer Don Keighley shows what a printed circuit board is and how to make your own.
PRINTED CIRCUIT BOARDS--PCBs--are used extensively because they provide a neat and reliable way of holding components and of making the necessary connections between them. In the days before PCBs, circuits were built up underneath a large metal chassis with the components strung be tween connection points--tag strips, they were called--in a seemingly haphazard manner. You only have to look inside an old valve radio or TV to get the point. All electronics manufacturers must have breathed a sigh of relief when PCBs were invented.
PCBs aren't used only in industrial equipment--they are equally suitable for the hobbyist. The difference between commercial/industrial PCBs and those we make at home is really only in the techniques used.
Figure 1 shows the PCB of the Intelligent NiCad Charger project from last month's issue, with the components drawn in to show where they go; Figure 2 shows the same PCB viewed from the bottom. It is quite easy to make a board such as this at home--and shortly, we will explain how it's done.
Now that you know why a PCB is used, let's backtrack a little and see just exactly what it is. Any PCB starts life as copper clad board--a thin base-board (about 2 mm) of insulating material (such as fiber-glass or resin-bonded paper) with an even thinner (about 0.25 mm) copper coating on one side.
Figure 3 shows a cross-section of a piece of copper-clad board.
Some highly specialized circuits may be built on boards with copper on both sides--called, naturally, double-sided PCBs.
To turn this copper-clad board into a PCB we have to remove all excess areas of copper, leaving behind only those areas (which make up the foil pattern) that we want. After this it's a simple job to drill the holes and to insert and solder components in their places.
By this time you'll be wondering, no doubt, how you 'remove' the areas of copper which aren't wanted--do you need a laser gun to blast them off the face of the earth? Or perhaps Superman might fly along to save the situation and burn them away with his infra-red vision? The answer is nothing quite as sci-fi as that I'm afraid; in fact it's all down to pure, old-fashioned school chemistry. You see, copper is a metal and metals are dissolved by acids. So all we have to do is place the board into acid and the areas of unwanted copper will be rapidly dissolved (etched) away. But now we have another problem--surely the acid will etch away the wanted areas as well? Well, it will if we let it. But, if we cover up the wanted copper, in the shape of the required foil pattern (as shown in Fig. 4), with acid-resisting material before putting the board into the acid, then those areas will remain un-etched when the rest has disappeared (see Fig. 5).
Down To the Nitty-Gritty
By now, you'll have realised that the production of any PCB goes through three stages:
• design of the PCB foil pattern to suit the circuit
• application of resist in the shape of that foil pattern
• etching of the PCB until unwanted copper areas are removed, subsequent removal of etch-resist and drilling of the component holes prior to circuit make-up.
I'm now going to unravel the mysteries of each stage and show you how to set about designing and making your own PCBs.
Designing Your PCB Foil Pattern
The only things you'll need at this stage are: pencils, a rubber, some paper (plain, graph and tracing) and either: a selection of one of each type of component used—i.e. one resistor, one transistor, one IC, etc--or their physical dimensions. This is because you need to know how big each component is so that, when you begin the layout of the board, you know what distance to leave between component lead holes. If you attempt more than just a couple of PCB foil pattern designs you'll begin to re member the sizes of the more common components and you will only need to measure the unusual ones.
Imagine that the sheet of paper is your PCB. All of the component fit into the 'board' from the top, therefore the 'foil pattern' is underneath and you can't see it directly.
Don't be afraid to position components close together: a spacing of 2 'rim between components is ideal.
Draw the remainder of the circuit components on the paper and draw in the remaining tracks to complete your foil pattern.
Turn over the tracing-paper, so that you can see the 'underside' of the board--the black connection pattern corresponds to the actual foil pattern layout required on the copper surface of the copper-21ad board.
Transferring Your Foil Pattern On To PCB
Now that you have the foil pattern, you need to transfer it on to the copper-clad board using etch-resistant materials (resist). This resist can take many forms: the simplest can be ordinary household enamel or gloss paint, applied with a fine brush (messy); the most complex is photographic resist, developed on the circuit board much like your summer holiday snapshots are developed on film. The one you use will depend on how much you're prepared to pay. You can expect to pay about (gulp) £60 for a basic photographic system: this will let you make first class PCBs, but it won't do your pocket a lot of good--it's the system which is likely to be used by schools and colleges where large numbers of one-off PCBs are needed.
The poor electronics hobbyist doesn't normally have that sort of money to spend on his pastime, so a cheaper way of doing the job is preferable--but it must also produce boards of good quality. One of the best resists for hobby use (the method is not too expensive and can produce excellent quality boards) comes in the form of rub-down transfer sheets. All the shapes you need, i.e. holes, IC pads, transistor pads, straight and curved track, are available and a number of companies produce their own versions. Most electronics hobby shops will stock at least one variety. An outlay of about a fiver should get you all of the shapes you need to let you make a number of PCBs.
Putting the foil pattern on the copper board is easy:
When you finish this task your board is ready to be etched.
Etching Your PCB
The etchant used to etch PCBs is not actually an acid--it's ferric chloride a horrible messy fluid--but it does its job well. Most local component stockists will sell it bagged as crystals or granules and you simply mix water with them to give an etchant solution.
The etching process takes place more rapidly when the ferric chloride solution is warm and you can buy specially built containers or 'baths' to hold the ferric chloride. These baths may be fitted with heaters to maintain a high temperature, with agitators to keep the solution moving (vital to stop the build-up of copper oxide on the surface of the copper, which prevents further etching). They are, however (you guessed it!) expensive.
At the other end of the scale, a plastic photographic-type dish can be used as a bath to hold the ferric chloride during etching, but this method is fraught with hazards--ferric chloride is a very dangerous fluid and a strong solution of it will merrily eat its way through your clothes, the carpet, or even the stainless steel kitchen sink (try explaining that to the missus!). Until recently there was no real alternative to the these two methods but a new, low-cost, etching system seems to be the answer to the hobbyist's dreams. It's the subject of an HE Reader Offer, on page 51 and I think you'll agree that it offers excellent value for money.
The HE Reader Offer
PCB Etching System is supplied in an expanded polystyrene box which is internally formed to hold the various bits before and after water has been added to the etchant granules. This polystyrene box, therefore, lets you store the system safely and tidily between use. Enough granules sup plied to allow a number of PCBs to be etched. Most readers will find they use it only once or twice a month, so this way of storing it will be very convenient. All etching is done within a polythene bag and no mess is involved when making a PCB. The way to use this system to etch a printed circuit board is illustrated in the following pictures. Start by unpacking the bag from its leak-proof box and making sure everything you need is to hand before commencing.
It will hold 250 ml when full to within an inch of the top.
(adapted from: Hobby Electronics magazine, Feb. 1982)
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