PCB DESIGN AND PREPARATION MANUAL

BY C.W.MURRAY I.Eng AMIED Copyright 1992.

(Technical Services Unit 4AB21)

Chapter 1 Introduction.

This manual, intended for students of Electrical / Electronic Engineering, serves as a guide to the procedures involved in designing Printed Circuit Boards, (PCB's) as may be used in student projects for example.

Why use PCB's ?.

The Printed circuit board represents one method of connecting components to form a circuit. It has the advantage of being easy and cheap to mass produce, and consequently it is widely used in industry. It also has the advantage that, given a component layout, components can be quickly and easily fitted, this means that the board can either be hand assembled by a relatively unskilled operator on a production line, or assembled automatically.

From a student point of view, using a PCB allows a student to take advantage of, and hence gain experience in using Computer Aided Design (CAD) packages like those they may encounter in industry. However it is also important to realise that is not the only way to construct a circuit and, in some cases not the best way either.

When is it best to use a PCB ?.

Let's start with a review of the various ways of constructing circuits.

Chassis & Tagboard.

A metal frame is constructed which carries large components such as transformers, some capacitors etc. This technique was popular when valves were used since many of the components were large and required firm fixings, valve holders were also fitted to the chassis, and the components fitted to tag strip underneath, with interconnections made using short lengths of wire.

This technique is still useful when constructing high power circuits, it is also worth bearing in mind that valves are still used in some applications where they have advantages over semiconductors, for example in high radiation environments.

When to use Chassis & Tagboard

High current circuits. (I.e. Power Supplies) Where currents in excess of 10A are used PCB tracks need to be fairly wide, hence it is more convenient to use wire instead of tracks, also components operating with currents in excess of 10A tend to be designed for chassis mounting although there are exceptions. This construction technique also has advantages when constructing RF circuits since it is easier to control stray capacitance and inductance as well as being free of dielectric losses caused by the PCB substrate.

Wire wrap.

This technique became popular with the advent of the Integrated circuit and was widely used for prototyping digital circuits. It has the disadvantage that at high clock speeds cross coupling between adjacent wires in a bunch can occur causing data corruption, also wires tend to behave as transmission lines due to the effects of capacitance and inductance, so reflections due to incorrect termination can also cause problems.

Special wire wrap pins, IC sockets and connectors are fitted into a perforated board. Connections are made by stripping and wrapping insulated wire around pins to be connected using a special tool, in some instants the wire used need not be stripped, the tension in the wrap is such that the sharp edges of the pins cut through the wire's insulation thus making contact.

When to use Wire wrap

If it is anticipated that the circuit is to be built in stages and may need some alterations, and provided that the circuit being constructed operates at relatively low frequencies i.e. up to 1 MHz for logic then this technique can be used. However it must be borne in mind that the wire used breaks easily, and, if the insulation displacement wrap is used, contacts must be made correctly otherwise they tend to become unreliable. When making alterations it may be necessary to unwrap existing connections which can lead to problems, also there is a limit to the number of connections that can be made to an individual pin.

Dead Bug !.

So named because devices such as IC's are glued face down onto a sheet of copper clad laminate (unetched PCB material) with their legs facing upwards hence resembling a dead bug with it's legs in the air.

This technique has many advantages and is particularly useful when constructing RF circuits since components can be mounted close together, it is constructed over a ground plane and problems due to dielectric losses in materials used can easily be controlled. Small components can be accommodated by gluing small pads of PCB laminate to the base material to act as termination points.

When to use Dead Bug !

It is best suited to RF and Microwave circuits constructed using discrete components, It becomes difficult to use when there are relatively few components compared to the number of connections i.e. where several IC's are used. In Microwave circuits it also has a considerable cost advantage since PCB substrates such as Duroid, Teflon, Alumina, etc., are very expensive and can be difficult to process.

StripBoard.

This consists of a sheet of laminate, perforated - typically at 0.1" intervals both vertically and horizontally with 1mm dia holes, and has parallel rows of copper tracks at the same pitch on one side. There are many variations of this basic idea, prototyping boards for PC's, Eurocard, etc., being examples. Circuits are formed by soldering components to the board and using the tracks and wire links to make connections, the tracks being broken using a Stripboard cutter or drill bit where required.

When to use StripBoard

Useful as a general purpose prototyping system, provided that components are not fitted too close together then modifications may easily be made. This technique is not very good at high frequencies and relies on components with leads suitable for use with the hole pitch and size of the stripboard selected.

Printed circuit board.

This is probably the most popular construction technique in use today. There are two ways of mounting components on a PCB - Surface and Through board. Surface mount is a relatively new technique and has the advantage of producing smaller, more compact circuits. Surface mount components may also be used in combination with conventional through board components. A problem with surface mount is that the components are difficult to handle due to their small size, and special techniques are needed for assembly. Because of the smaller pad and track sizes, the artwork for PCB production needs to be of the highest quality, being either photographically produced by reduction (typically 2:1 or 4:1) or photo-plotted. For this reason it is recommended that surface mount techniques are avoided unless there is no other alternative. To make a PCB an artwork detailing the component connections as a track pattern is produced, this is then used to make a PCB, onto which the components are fitted. (Later chapters of this manual describe this process in more detail).

When to use Printed circuit board

PCB's can be used for a variety of applications, they are better suited to small signal and logic applications although they can be used up to microwave frequencies provided the right substrate is used. Where currents in excess of 10A flow continuously it is better to 'hard wire'.

The main disadvantage of PCB's is that the circuit is difficult to modify and it is easy to make mistakes so it is important to check the design and PCB layout prior to producing the board.

Of course a combination of the above techniques can be used, so it really is a case of considering the various techniques available and selecting a technique or techniques suitable for your particular application.

Chapter 2 Producing a PCB.

Before any work can be commenced on a PCB design, you need to carefully consider the following points:

  1. Check the design, Is the circuit diagram up to date, and accurate? It may help to run a simulation of the final circuit at this stage using SPICE to verify the design if a breadboard prototype has not been built and thoroughly tested first.
  2. Check availability of components, obtain connection and pin out details of all components, this may be a convenient time to find out if all the components you intend to use exist in the library of the CAD package you will be using to design the PCB. If not then use the library editor supplied with the CAD package to create symbols for the extra components. (see below for further details).
  3. What material is to be used for the PCB, are there any special requirements - check to see if this is available, if necessary estimate size of final board and order suitable material at this stage.
  4. Are there any special processes to be considered in producing the board, ie through hole plating, profiling special plating, multi layer etc. Check to see that the requirements can be met within your cost constraints and that facilities exist to carry out these processes.

Once the above points have been satisfied you are now in a position to start designing the PCB. There are a number of different approaches, these are -

  1. Schematic capture,autoroute,tidy,check,plot
  2. Netlist entry,autoroute,tidy,check,plot
  3. Manually route,tidy,check,plot

Each CAD package has it's own specific features, however at least one of the above approaches can be used in most cases. While it is not possible to give examples of how to use each different package, or give instructions on how to use them, I have decided to base my examples on a CAD package known as "Boardmaker" using approach listed above. Information on this and other CAD packages can be found by referring to the appropriate manual(s).

This is the quickest and easiest package to learn about and use, although in it's most basic form does not offer autorouting. (note that "Boardmaker2" and "EasyPC professional"(Which is similar to boardmaker) do offer advanced features such as schematic capture, netlist and autorouting).

Firstly a note on Autorouting.

(not necessarily true in 1994)
Autorouters ,even the latest rip-up and reroute types are no match for a skilled PCB designer using manual layout techniques. Often more time is taken routing tracks that the router failed to route, and generally sorting out to obtain an acceptable routing scheme, than would be taken manually laying out the PCB for all but the most complex of boards.

Considering this, I have chosen to concentrate on manual layout because this highlights the problems of finding paths around components and leads to an appreciation of what a autorouter needs to consider while working. Also it gives good practice at solving routing and layout problems.

Example 1. Audio Power Amplifier.

Under construction

written by C.Murray@ee.surrey.ac.uk

Last modified: 2004 May 26th.