PRINTED CIRCUITS

Reproduced from Machinery Lloyd and Electrical Engineering (Vol. 36, No. 26 - 19th December 1964) with kind permission of the publishers – The Certificated Engineer June 1965.

Printed circuits are used in large quantities in many industries, particularly for radio, television, electronics, telecommunications and automobile equipment, and many other industries are carrying out development work with these materials. Circuits, made of metal bonded to an insulating material, outdate the conventional wire connections and offer many important and far-reaching advantages.

Copper-clad bakelite laminates consist of a synthetic resin laminated base to. which is bonded copper foil. Extensive research into the properties of these materials, combined with experience in producing all types of laminates, has enabled manufacturers to develop a high-quality electrical product possessing several distinctive features. The excellent bond between the copper foil and the base material permits flow soldering and other production processes to be used without danger of the copper lifting or moving.

The most widely used method of producing printed circuits is to print the shape of the required connections on the copper surface of a sheet of copper-clad lamina ed material and to remove the areas of unprinted copper by etching. The success of such circuits is determined by the quality of the copper-clad laminates from which they are produced, and a range of materials is available which ideally fulfils the most extracting technical and commercial requirements.

Printed circuits made from copper-clad bakelite laminates are already being used in a wide range of commercial and service installations, including radio and television receivers, LF and transistor amplifiers, and radio and electronic testing equipment, and in the motor car industry.

The preparation of printed circuits from the copper clad sheet is carried out either by the end-user of the printed circuits or by companies who are specially equipped for this type of work. Individual techniques have been developed, but the manufacture of printed circuits involves six basic production stages.

Copper clad bakelite laminated is supplied mechanically cleaned and normally with only mild degreasing, e.g. 30 seconds in hot trichloroethylene vapour, is necessary to obtain a satisfactory surface for printing. If the copper tarnishes in storage or in handling it may be recleaned by using a scrub brush with a pumice cleaner or using a chemical cleaning agent.

The acid-resisting coating may be applied either by the photoresist method, the silkscreen printing process or the offset litho printing process. In the case of the latter method, it is advisable to apply a layer of bitumastic powder to the 'wet' ink and to adhere this coating either by heating at approximately 230°F (110°C) or using trichloroethylene vapour.

The etching is carried out in a spray or splash type machine using a ferric chloride solution. The etched panel should be washed thoroughly in water immediately after production.

After the resist coating has been removed using the appropriate solvent, the copper pattern is mechanically or chemically cleaned before soldering. Ultrasonic cleaning methods are now much in favour of this work.

A flux coating (e.g. rosin alcohol type) should be applied to the copper as soon as possible after cleaning. Usage of solder can be reduced by selective soldering which is accomplished using a solder resist applied by silkscreen printing to those areas where no solder take is required.

For dip soldering a bath temperature of approximately 240°C and a dipping time of approximately 5 sec are typical. In static solder baths, it is necessary to wipe solder dross aside before dipping the circuit board, but special 'flow wave' solder baths have been developed to overcome this problem.

Printed circuits can be plated with a variety of metals to improve their corrosion and abrasion resistance. Indium, gold, silver and nickel are among the metals which are frequently used. It is necessary to observe certain precautions when electroplating copper clad laminates.

One of the advantages of using printed circuit arises from the fact that they permit the layout of components to be so arranged that all points of interconnection are in one plane and can be connected simultaneously by dip soldering.

This technique eliminates the considerable time spent in manually soldering connections and it reduces the number of tags, eyelets, wires and screws required. In mass-producing radio receivers, for example, it is estimated that use of dip soldered printed circuits increase assembly line output by approximately 300 per cent. As a result, labour costs are lower and overhead costs per unit of output, including supervision and factory space, are also reduced.

Since, when printed circuits are used, the position of each component in a unit is rigidly determined, assembly methods are simplified and the danger of wrong connections being made - even with spot soldering-is eliminated. Printed circuits thus facilitate the production of small and miniature units.

In assemblies which incorporate printed circuits, the non-conducting laminated base material is normally used as the chassis, and this considerably reduces the amount of component insulation required. Sleeving and insulated resistors are unnecessary.

The elimination of many ancillary components and much of the insulating material necessary in conventional assemblies, together with the fact that the laminated base of the printed circuit forms the chassis, results in a considerable reduction in the weight of an assembly, and this, in turn, reduces the problem of securing the assembly in its cabinet. It has been estimated that the weight saved varies from 1 lb on a small sound receiver to 4 lb on a television set.

Another development which printed circuits have stimulated is the possibility of automatic printing of components such as resistors, capacitors, and inductors on the circuit itself. Such a development would make possible a considerable reduction in the size and weight of many types of electronic units.

For certain applications, the copper pattern should be flush with the surface of the base laminate. A typical example is a coding disc Over which a sliding contact revolves, and when the copper stands proud of laminate excessive wear occurs. A special grade of material suitable for this type of work has been developed.

Bakelite Ltd has developed epoxide/glass fabric laminates clad on one or both surfaces with copper foil. These laminates maintain their properties elevated temperatures and are ideal for application demanding very high mechanical strength combined with excellent water resistance. They have good electrical properties, and, in general, their resistance to chemicals is very good, but ketones should be avoided. These materials are finding increasing uses where operations under arduous conditions are involved. Typical uses are in television cameras, computer equipment, guided missiles, aircraft electrical equipment and many other applications being developed.

Bakelite Ltd also produces resistance metal-clad laminates for use in the motor car industry where it is suitable in dimmer switch equipment and have also introduced copper-clad polyester film for flexible circuits, cable housing and connecting printed circuit on different planes.

The latter, Grade DZ82, is copper foil bond d to polyester film and has potential uses in many fields including automobile wiring, computer wiring and memory units, telephone and switchboard wiring, radio and domestic electrical appliances. Printed wiring produced from Grade DZ82 may be coiled and folded and need not, therefore, be limited to a single plane in the same manner as circuits produced from the rigid copper-clad laminated sheet. This feature could enable the size and weight of an assembly to be considerably reduced. Circuits may be prepared in the normal way, using any of the known printing and etching techniques. Special processing is not required, but soldering should be carried out at a lower temperature than is generally used for rigid copper clad laminates.

Polyester film is noted for its toughness, flexibility and stability over a wide range of temperatures. It also has high tensile strength, excellent electrical properties and resistance to most chemicals, oils and moisture. The transparent nature of the base film is an advantage in printed circuitry since the circuit pattern and components are visible from the reverse side.