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.
Board Stiff
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).
Figure 1. The printed circuit board of the Intelligent NiCad Charger
in the January 1982 issue of Hobby Electronics.
Figure 2. A view of the underside of the board shown in Fig. 1. Figure
3. Cross-section of a piece of copper-clad board.
Figure 4 Cross-section of copper-clad board with acid-resistant material
applied in the shape of the foil pattern.
Figure 5 After etching, the unwanted copper is etched away and those
areas in the shape of the foil pattern (with acid-resisting material)
remain.
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.
1. Begin by displaying the circuit diagram in front of you, along with
a sheet of plain paper. Make sure all components in the diagram ape numbered
so that as you lay out the foil pattern design on paper you can mark
each component's location with its number.
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.
2. Choosing a central component, say an IC, pencil in its approximate
shape and mark where its connection leads enter no the paper 'board'.
Now, draw another component which is connected to the first component.
3. When you have drawn a few components on the paper, you can begin
to connect them by penciling-in 'tracks' on the paper, between holes.
Don't worry if you get stuck, or even make a m stake; that's why you
have the rubber! Rub-out the error and start again.
Don't be afraid to position components close together: a spacing of 2 'rim between components is ideal.
4. Remember that the finished PCB will have its foil pattern on the
other side to its components, so tracks can actually pass underneath
components. There is no reason why two, or even three tracks cannot pass
underneath a component, as long as the tracks don't touch either each
other, or the component lead holes.
Draw the remainder of the circuit components on the paper and draw in the remaining tracks to complete your foil pattern.
5) Place a sheet of tracing-paper over a sheet of graph paper and tape
the two together along the edges. Now, using the grid of the graph paper
to help align components, redraw the layout on the tracing paper. Draw
component shapes in one color of pencil (red) and tracks in another colour
(say, black). In this way you can instantly tell which is the 'top' and
which is the 'bottom' of the board. Check, as you redraw the foil pattern,
that it is correct with regard to the circuit diagram.
6) The tracing-paper copy of your PCB foil pattern is now your reference.
Cut, or pull off, the tape holding it to the graph paper.
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:
1) Lay the pattern on the copper surface of the board and, with a sharp-pointed
instrument la scriber or the point of a pair of compasses), prick through
the pattern in to the copper at every component hole location. The foil
pattern can be your own design or one taken from HE.
2) At every prick-mark in the copper, rub down a transfer circular hole.
3) Following your foil pattern, interconnect the holes with transfer
tracks until the resist on the copper has the same overall shape and
connections as your foil pattern drawing. Be careful as you do this job,
because any small cracks or breaks in the resist will mean that a corresponding
crack or break will occur in the copper when the board is etched. If
you make any mistakes at this stage, they can be removed with the corner
of a small piece of sticky tape--but once you have etched the board your
mistakes are permanent.
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.
1. Holding the bag flat, make sure that the bottom clip is in place.
2. Remove the top clip and add 250 ml of warm water. If you have lot
got a measuring jug, use the polythene bag supplied.
It will hold 250 ml when full to within an inch of the top.
3. Re-seal the top of the bag. Remove the bottom clip, mixing the water
and the granules. Rock the bag backwards and forwards to dissolve the
granules.
4) Seal the solution in the bottom of the bag unseal the top and insert
the PCB then reseal the top.
5) Release the bottom clip then, holding tie bag flat, rock it so that
the etching solution flows evenly across el parts of the board Check
every few minutes to see if the etching process is complete.
6) Hold the bag by the bottom and turn it upside-down; holding onto
the PCB, turn tie bag right way up and squeeze any remaining solution
down into the bottom of the back.
7) Seal off the solution in the bottom of the bag, then remove the top
clip. Leaving the PCB inside the bag, rinse off with cold running water.
9) All that's left to do is to clean the etch-resist off the copper
tracks and to drill the holes for the component leads. One of the many
12 V hand-held drills is suitable for the job, or a hand drill can be
used if one of the powered variety is not available. The best sized drill
bit is 1 mm.
10) The completed PCB! The board is now ready for the components to
be inserted and soldered in place. Provided the PCB layout land all the
components) are correct, there is now very little that could--or should--go
wrong)
(adapted from: Hobby Electronics magazine, Feb. 1982)
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