EASA PART 66 Module 4_electronic Fundamentals

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JAR 66 CATEGORY B1 MODULE 4 ELECTRONIC

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FUNDAMENTALS

Index

1 SEMICONDUCTOR DEVICES... 1-1 1.1 RECTIFIER DIODES... 1-2

1.1.1 Circuit Symbols & Identification ... 1-2 1.1.2 Operating Characteristics... 1-3 1.1.3 Parallel & Serial Arrangements of Diodes ... 1-4 1.1.4 Rectification... 1-5 1.2 SIGNAL DIODES... 1-10

1.3 ZENER DIODES... 1-10

1.4 LIGHT EMITTING DIODES... 1-11 1.5 PHOTOCELLS... 1-11

1.5.1 Photoconductive Cells... 1-11 1.5.2 Photovoltaic Cells... 1-12 1.6 PHOTODIODES... 1-12

1.7 VARACTOR DIODE... 1-12 1.8 SILICON CONTROLLED RECTIFIER... 1-13

1.9 TRANSISTORS... 1-13

1.9.1 NPN Transistor... 1-14 1.9.2 PNP Transistor ... 1-16 1.10 TESTING SEMICONDUCTOR DEVICES... 1-18 1.10.1 Testing Diodes ... 1-18 1.10.2 Testing Transistors... 1-19

2 OPERATIONAL AMPLIFIERS... 2-1

2.1 THE PERFECT AMPLIFIER... 2-1

2.2 OP AMP SPECIFICATION... 2-1 2.3 POWER REQUIREMENTS... 2-2

2.4 PIN OUTS & CIRCUIT SYMBOL... 2-2

2.5 OPERATION... 2-3

2.5.1 Negative Feedback ... 2-3 2.6 OP-AMP COMPARATOR... 2-5

2.7 OP AMP SUMMING AMP... 2-6

3 PRINTED CIRCUIT BOARDS... 3-1

3.1 BASE MATERIAL... 3-2 3.2 CONDUCTOR MATERIAL... 3-2

3.3 BONDING OF CONDUCTOR MATERIAL... 3-2

3.3.1 Inspections & Tests ... 3-3

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FUNDAMENTALS 3.5 CIRCUIT ARTWORK... 3-4 3.6 PRINTING OF CIRCUITS... 3-5 3.6.1 Etching Process ... 3-5 3.6.2 Additive Process... 3-6 3.6.3 Inspection ... 3-7 3.7 SOLDERING METHODS... 3-7 3.7.1 Hand Soldering ... 3-7 3.7.2 Mass Soldering ... 3-7 3.8 SOLDER SPECIFICATION... 3-9

3.9 FLUXES & THEIR APPLICATION... 3-9

3.10 SOLDER RESISTS... 3-10

3.11 PLATING OF PRINTED WIRING CIRCUITS... 3-10 3.11.1 Through-Hole Plating ... 3-10 3.12 ORGANIC PROTECTIVE COATINGS... 3-11 3.13 FLEXIBLE PRINTED WIRING CIRCUITS... 3-11 3.14 HANDLING OF CIRCUIT BOARDS... 3-12

3.14.1 Electrostatic Discharge Sensitive Devices ... 3-12 3.14.2 Removal & Installation of ESDS Printed Circuit Boards 3-15 3.14.3 Removal & Installation of Metal-Encased ESDS LRU's 3-16

4 SYNCHRONOUS DATA TRANSMISSION... 4-1

4.1 DESYNN SYSTEM... 4-1

4.1.1 The Basic Desynn ... 4-1 4.1.2 Slab Desynn ... 4-4 4.2 SYNCHRO SYSTEMS... 4-4

4.2.1 Synchro Types ... 4-5 4.2.2 Synchro Schematics... 4-7 4.2.3 XYZ Synchro system... 4-9 4.2.4 Synchro Supplies ... 4-9 4.2.5 Torque Synchro System... 4-10 4.2.6 Electrical Zero ... 4-13 4.2.7 Differential Torque Synchro System... 4-14 4.2.8 Control Synchro System... 4-16 4.2.9 Differential Control Synchros... 4-20

5 SERVO SYSTEMS... 5-1

5.1 CATEGORIESOFSERVOSYSTEMS ... 5-1 5.1.1 open loop ... 5-1 5.1.2 closed loop ... 5-2 5.2 REMOTE POSITION CONTROL SERVOMECHANISMS... 5-3

5.2.1 Positional Feedback ... 5-3 5.3 TYPES OF INPUTS... 5-5

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FUNDAMENTALS 5.3.2 Ramp Input... 5-5 5.3.3 Accelerating Input... 5-5 5.4 SYSTEM RESPONSE... 5-6 5.5 DAMPING... 5-7

5.5.1 Frictional Forces which Produce Damping ... 5-7 5.5.2 Velocity Feedback Damping... 5-9 5.6 VELOCITY CONTROL SERVOMECHANISMS... 5-11

5.6.1 Residual Error ... 5-11 5.6.2 Velocity Lag... 5-11 5.7 A.C. SERVOMECHANISM COMPONENTS... 5-12

5.7.1 E & I Bar Transducer... 5-12 5.7.2 A.C. Tachogenerators ... 5-13 5.8 PRACTICAL SERVO SYSTEMS... 5-15

5.8.1 Direct Servo Current System... 5-15 5.8.2 Alternating Current Servo System ... 5-16

6 OTHER TRANSDUCERS ... 6-1

6.1 LINEAR VARIABLE DIFFERENTIAL TRANSFORMER... 6-1 6.2 ROTARY VARIABLE TRANSFORMER... 6-2

6.3 INDUCTIVE TYPE TRANSDUCERS... 6-2

6.3.1 Induced EMF Type ... 6-2 6.3.2 A.C. Current Control... 6-3

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FUNDAMENTALS

1 SEMICONDUCTOR DEVICES

The early discoveries in the field of electricity made by Volta, AmpHata! Yer işareti

tanımlanmamış.ere, Gains, Faraday, Hertz and others raised fundamental

problems concerning the nature of matter.

The first breakthrough came in 1897, when Sir J.J. Thompson discovered the electron, a discovery soon verified by other investigators. In 1913 Bohr evolved the basic theory of atomic structure, and that theory has been developed to our present concept of the nature of matter.

The electrical characteristics of an atom are determined by how tightly the nucleus holds on to its outer electrons. If the outer electrons are easily removed from the atom, the material will conduct easily and is known as a conductor. If the outer electrons are difficult to dislodge from their orbits, the material is known as an insulator.

The material used in diodes and transistors is known as 'semi-conductor' material. One of the attributes of this material is that the number of free electrons in any given area can be fixed during the manufacturing process.

Interest in semi-conductors began in 1873, when it was discovered that the

resistance of rods and wires of selenium decreased as they were heated. This was surprising because the resistance of metals normally increased with an increase in temperature. Furthermore, some lowering of resistance was noted when the rods were exposed to light. Later investigations found similar effects in other materials, but the change in resistance was so small that no practical applications could be found.

By 1906 a number of crystalline semi-conductors were being used as radio signal detectors, but the introduction of thermionic valves put an end to them. The valves were more reliable and had the advantage of being able to amplify the signal as well as detect it.

During the development of radar systems in WWΙΙ, it was discovered that valve type mixers would not operate at the high frequencies being used. Research turned to semi-conductor type mixers, and silicon proved the most successful. After the war, the peculiar properties of Germanium and Silicon were rigorously investigated, and a germanium diode detector was made and used extensively in radio and television. During development of the Germanium detectors an important discovery was made. It was found that when two very close contacts are made with a piece of germanium, the current flow through one of the contacts affects the amount of current flow

through the other.

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Bell Telephone Laboratories latched onto this phenomena and eventually in 1948 they announced the manufacture of the first solid-state amplifying device, the

transistor. This triggered renewed interest in semi-conductor diodes, resulting in the development of a huge variety of semi-conductor devices that we now take for granted.

1.1 RECTIFIER DIODES

A rectifier diode is the electrical equivalent of a one way valve, it is a semiconductor device which allows current to flow in one direction but not in the other.

When conducting, the diode is said to be 'forward biased'. Under these conditions the diode offers little resistance to current flow.

When opposing current flow, the diode is said to be 'reverse biased'. Under reverse biased conditions the diode has a high resistance.

1.1.1 CIRCUIT SYMBOLS & IDENTIFICATION

The various symbols used for diodes are shown below.

Whether the triangles are filled or unfilled depends only on the drawing office

preference. Where it is considered necessary, it is possible to show that one of the electrodes is connected to the case of the device by adding a dot to the symbol, but this is not often used. In every symbol, the arrow indicates the direction of

conventional current flow.

The base of the triangle is the end where conventional current enters the diode, this end is called the anode. The end through which current leaves the diode is the cathode. In some cases the arrow symbol is marked on the diode, where it is not, the cathode is identified by a band or distinctive shape as shown above.

Two identification codes are used for diodes. In the American system the code always starts with 1N and is followed by a serial number, i.e. 1N4001. In the continental system, the first letter gives the semiconductor material; A for

germanium; B for silicon, and the second letter identifies the use; A - signal diode; Y - rectifier diode and Z for zener diode. To complicate the situation some

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FUNDAMENTALS 1.1.2 OPERATING CHARACTERISTICS

Most semiconductor diodes are made from silicon or germanium, these two

materials have different operating characteristics, although the principle of operation and circuit symbols are both the same.

1.1.2.1 Biasing

A diode is said to be 'biased' when a voltage is applied between the terminals such that the diode operates as required.

An external voltage applied so that the anode is positive and the cathode negative is called 'forward bias'. There are many ways of achieving this, for example:

• Connect the anode to +3V and the cathode to 0V. • Connect the anode to +1V and the cathode to -1V. • Connect the anode to -50V and the cathode to -52V.

So far as the diode is concerned, it is the voltage of the anode with respect to the cathode which determines the bias.

If the voltage is applied so that the anode is negative with respect to the cathode, the diode is ‘reverse biased’, again, there are many ways of achieving this.

The forward voltage required to make the diode conduct depends on the material it is made from. Germanium diodes require a voltage of approximately 0.1 to 0.2 volts and silicon diodes 0.6 to 0.7 volts.

1.1.2.2 Forward Voltage Drop

Ideally a diode should have zero resistance when conducting and should cause no voltage drop, unfortunately this does not happen. Germanium diodes create a voltage drop of approximately 0.6V and silicon diodes a drop of approximately 1.1V. This needs to be taken into account when doing circuit calculations.

1.1.2.3 Reverse Leakage Current

When a diode is reverse biased, it should ideally have infinite resistance and no current should flow. Unfortunately when a diode is reverse biased, a small current called 'reverse leakage current' flows, generally this is too small to be of

significance, however, it should be noted that the value of this current increases with an increase in diode temperature. The reverse current of silicon diodes is much smaller than that of germanium diodes, (approx. one thousandth), therefore silicon diodes can be used more successfully at high temperatures (150º - 200ºC) than germanium diodes (80º - 100ºC).

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1.1.2.4 Reverse Breakdown Voltage

If the reverse bias voltage is increased, eventually the diode breaks down and current flows in the wrong direction through the diode. This causes permanent damage and the diode has to be replaced.

The breakdown voltage can have any value from a few volts, up to 1000V for silicon diodes and 100V for germanium, depending on the construction and forms of

material used.

The maximum reverse voltage is an important diode characteristic. Under normal conditions this value should not be exceeded.

1.1.2.5 Graphical Representation

Shown below is a graphical representation of the operating characteristics of a typical silicon and germanium diode.

1.1.3 PARALLEL & SERIAL ARRANGEMENTS OF DIODES

It is possible to operate silicon rectifier diodes in parallel or in series to provide respectively, higher current or higher voltage capabilities.

1.1.3.1 Parallel Arrangements

In parallel arrangements used for higher currents, some method must be used to ensure that the current divides equally through the individual diodes. This is difficult to do.

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FUNDAMENTALS 1.1.3.2 Series Arrangements

Series arrangements can be used if the applied voltage is greater than the

maximum rated value of a single diode. Some method must be used to ensure the applied voltage divides equally among the individual diodes. Resistors or capacitors in parallel can be used in an effort to achieve this.

1.1.4 RECTIFICATION

Rectifier diodes are designed to convert ac to dc. To do this effectively and efficiently they must have:

• Low resistance to current flow in the forward direction. • High resistance to current flow in the reverse direction.

Almost all semiconductor rectifier diodes are silicon, junction types. The symbol used in circuit diagrams can be any of those shown earlier in the notes.

1.1.4.1 Basic Rectifier Circuit

A basic rectifier circuit is shown below. The diode is inserted in series between the a.c. supply and the load.

The diode only passes current when forward biased. Thus when an a.c. signal is applied, pulses of uni-directional (d.c.) voltage are developed across the output load resistance.

Note from the diagrams that the d.c. polarity can be reversed by reversing the diode connections.

If the average value of ½ wave rectified a.c. is calculated it will be found to be 32% of the peak value of the output voltage.

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1.1.4.2 Centre Tap Full Wave Rectifier

In full wave rectification, both halves of every cycle of input voltage produce current pulses through the load resistor.

In the circuit shown above, two diodes D1 and D2 and a transformer with a

centre-tapped secondary are used.

During the positive half cycle of the input waveform, A is positive with respect to O and D1 conducts, the current flowing top to bottom through the load resistor. During

this time diode D2 is reversed biased and does not conduct.

During the negative half cycle of the input waveform, B is positive with respect to O and D2 conducts, the current again flowing top to bottom through the load resistor.

During this time diode D1 is reverse biased and does not conduct.

In effect, the circuit consists of two half wave rectifiers working into the same load on alternate half cycles of the input. The current through R is in the same direction during both half cycles and a fluctuating d.c. is created across R.

The average value of this full wave rectified a.c. is 64% of the peak value of the voltage across the load resistor R.

The output frequency is double that of the input frequency.

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1.1.4.3 Full Wave Bridge Rectifier

The circuit of a Full Wave Bridge rectifier is shown below. The rectifier has 4 diodes as opposed to 2 and does not have a centre tapped transformer.

During the positive half cycle diodes D1 and D2 conduct, the current flowing top to bottom through the load RL. During the negative half cycle D3 and D4 conduct, the

current again flowing top to bottom through the load. The output from this rectifier is the same as that obtained from the centred tapped transformer type. The average value again being 64% of the peak voltage across the load resistor.

It should be noted that in this rectifier, the peak voltage across RL is equal to the

whole of the secondary transformer output voltage, whereas in the previous rectifier, the peak voltage across RL is only half the transformer secondary voltage.

1.1.4.4 Smoothing

The rectifier circuits previously discussed produce pulsating d.c. outputs. A smoothing circuit changes these outputs into a steady d.c. voltage level.

1.1.4.4.1 Half Wave Rectifier

The diagram below shows a simple half wave rectifier with a reservoir capacitor, C, connected in parallel with the load RL. The capacitor charges towards the peak

value of the input voltage whenever the input voltage is greater than VC and the

diode is conducting. When the input voltage is less than VC the diode cuts-off and

the capacitor discharges through the load.

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This results in a mean d.c. output level less than the peak of the input, with a ripple superimposed at the input frequency.

1.1.4.4.2 Full Wave Rectifier

The diagram above shows a centre tapped full wave rectifier with a reservoir capacitor. The charge is now topped up twice during each cycle of the input waveform which results in:

• A lower amplitude ripple, at twice the frequency of that from the half wave rectifier.

• A higher mean d.c. output than that from a similarly loaded half wave rectifier.

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FUNDAMENTALS 1.1.4.5 Ripple Factor

A measure of the amount of ripple present at the output of a d.c. supply is given by the ripple factor, which is usually expressed as a percentage and defined as:

Ripple factor = Hata! × 100% 1.1.4.6 Peak Inverse Voltage

The peak voltage across a rectifier diode in the reverse direction is known as the 'peak inverse voltage'. In a half wave rectifier with a reservoir capacitor, the peak inverse voltage is twice the amplitude of the peak voltage across the load. i.e. mean d.c. level to maximum negative peak. The diode must be able to withstand this voltage without breaking down.

1.1.4.7 Voltage Regulation

Voltage regulation is a measure of the ability of a power supply to provide an increased load without a fall in output voltage.

Regulation = Hata! × 100% 1.1.4.8 Filter Circuits

Smaller ripple factors and improved voltage regulation is obtained by using R-C and L-C filter circuits across the output of the rectifier.

1.1.4.9 Ripple Frequency

The ripple frequency on the d.c. output from a half wave rectifier is equal to the supply frequency. For a full wave rectifier, the ripple frequency is double the supply frequency.

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FUNDAMENTALS 1.2 SIGNAL DIODES

Signal diodes are used to detect radio signals (a process similar to rectification in which radio frequency a.c. is converted to d.c.), because of their very low

capacitance. A capacitor passes a.c. The higher the frequency of the a.c. and the greater the capacitance, the less opposition it offers. At radio frequencies, a normal diode would be of little use as a detector because of its large junction area. The large junction area resulting in a large capacitance value and little opposition to current flow.

A point diode type signal diode has a very small junction area resulting in a low value of capacitance and a large opposition to current flow.

Germanium is used for signal diodes since it has a lower 'turn-on' voltage than silicon, and so lower signal voltages start it conducting in the forward direction.

1.3 ZENER DIODES

In an ordinary diode, if the reverse bias is increased, the diode breaks down and the diode suffers permanent damage. A zener diode is designed to be used in the breakdown region. The zener diode looks like a rectifier diode, the cathode often being marked by a band. Its symbol is shown above.

From the characteristic graph, it can be seen that the reverse current is negligible as the reverse bias is increased until the breakdown voltage is reached, then it

suddenly increases. The breakdown voltage is called the zener or reference voltage.

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The important thing is that the voltage across the diode remains almost constant over a wide range of reverse currents. It is this property of a zener diode that makes it useful in stabilised power supplies.

To limit the reverse current at breakdown and prevent overheating, the power rating of the diode must not be exceeded. This is achieved by using a resistor in series with the diode.

1.4 LIGHT EMITTING DIODES

A light emitting diode is a specially constructed and doped diode type device which emits light when operated in the forward bias condition. The colour of light emitted depends on the semi-conductor material used.

Gallium arsenide phosphide - red light Gallium phosphide - green light

Symbols used are similar to the photodiode.

Unless an LED is of the constant current type, which incorporates an integrated circuit regulator, it must have an external resistor connected in series to limit the forward current which typically may only be 10mA. The voltage drop across a conducting LED is about 1 to7 volts.

In seven segment LED displays, each segment is a separate LED and depending on which segments are energised, the display lights up the number 0 to 9. Such

displays are usually designed to operate from a 5V supply - each segment needs a separate current limiting resistor and all the cathodes or anodes are joined together to form a common connection.

1.5 PHOTOCELLS

Photocells change light into electrical signals. There are two basic types, Photoconductive cells and Photovoltaic cells.

1.5.1 PHOTOCONDUCTIVE CELLS

The resistance of certain semiconductors decreases as the intensity of light falling on them increases. They are therefore light sensitive resistors and sometimes referred to as light dependent resistors.

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FUNDAMENTALS 1.5.2 PHOTOVOLTAIC CELLS

When illuminated, a photovoltaic cell produces a voltage. If an external circuit is connected to the cell, current flows through it. The source of energy is the light.

The voltage available depends on the material used, the intensity of the light and the amount of current drawn from the cell. For a silicon cell in full sunlight the voltage on open circuit is 0.45V. With a maximum current of 35mA for each square cm of cell. Only about 10% of the light is turned into electrical energy.

1.6 PHOTODIODES

Photodiodes are operated under reverse bias conditions. The leakage current increasing in proportion to the amount of light falling on the device. Photodiodes are used as fast counters and light meters.

1.7 VARACTOR DIODE

A varactor diode is a special type of diode constructed to act as a voltage controlled capacitor. It is also known as a varicap diode. The diode is operated under reverse bias conditions, with an increase in bias decreasing the value of capacitance. The circuit symbols are as shown below.

There are 3 main uses for varactor diodes:

• As remotely controlled capacitors in RF tuned circuits. • As variable capacitors in amplifiers.

• As variable capacitors in frequency modulator circuits.

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1.8 SILICON CONTROLLED RECTIFIER

Silicon controlled rectifiers (SCR's) are now more commonly known as thyristors. They are semiconductor devices which rectify a.c. and control the power supplied to a load in a way that wastes very little energy. They are commonly used in

household lighting dimmer switches. The general symbol is shown below, together with the symbol for 'P' and 'N' types.

SCR's normally block the flow of current in both directions, but can be triggered so as to allow current to flow in the forward direction as in a normal diode, whilst still blocking current flow in the reverse direction. In the triggered condition the characteristics are similar to rectifier diodes.

An SCR will continue to conduct until the load current is reduced to zero, or until it is reverse biased, when it automatically returns to the blocking state.

The SCR is triggered by applying a pulse to a third terminal called the gate. The duration of the pulses can be extremely short.

1.9 TRANSISTORS

Transistors are the most important device in electronics today. Not only are they made as discrete components, but integrated circuits may contain several

thousands on a tiny slice of silicon. They are 3 terminal devices used as amplifiers and as switches, and are classed as active devices.

Hundreds of different transistors are available. The same identification code is used as for diodes, but in the American system transistors always start with 2N followed by a number. In the continental system the first letter gives the semiconductor material and the second letter gives the use:

• C indicates an audio frequency device. • F a radio frequency device.

• S a switching transistor.

An example being BC108, a silicon audio frequency amplifier device.

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The two basic types of transistors are: • The bipolar or junction transistor. • The unipolar or field effect transistor.

In this element of the course we concentrate on bipolar transistors, of which there are two basic types. The NPN and the PNP, both of which are active devices having three terminals labelled; Base, Collector and Emitter.

1.9.1 NPN TRANSISTOR

NPN transistors are made from 3 pieces of semi-conductor material joined together in a manner similar to two diodes, as shown in the diagram below. Also shown is the circuit diagram with each terminal identified.

If the base is made positive with respect to the collector, the diode, or junction as it is called, is forward biased and current flows (conventional current flows from base to collector).

If the base is made positive with respect to the emitter, again the junction (diode) is forward biased and conventional current flows from base to emitter.

If the collector is made positive with respect to the emitter, or the emitter is made positive with respect to the collector no current will flow, because in either direction one of the junctions (diodes) is reverse biased and will prevent current flow.

The last three paragraphs should be noted, as their contents is invaluable when it comes to determining the terminals and testing transistors. This will be discussed later.

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FUNDAMENTALS 1.9.1.1 NPN Transistor as a switch If the NPN transistor is

connected as shown, it can be used as a switch.

In this diagram the transistor is being used to turn on a lamp, it could however be used to operate any type of d.c. device such as a relay, solenoid, another transistor or an LED.

When the base is made positive with respect to the emitter, the junction is forward biased and current flows in through the base and out of the emitter. The flow of current from base to emitter creates a reaction in the transistor that causes the reverse biased collector/base junction to break down and conduct. Current can then flow from the battery positive terminal, through the lamp, through the reverse biased collector base junction, through the forward biased base emitter junction and back to the battery, illuminating the lamp.

When the base is made sufficiently positive with respect to the emitter (approx. 0.6V for silicon, 0.2V for germanium) so that current flows from collector to emitter

through the transistor, the transistor is said to be switched or turned 'ON'.

If the base / emitter potential is reduced below the switch 'ON' potential, or removed totally, the collector / base junction will return to its reverse bias condition and will prevent current flowing around the circuit through the lamp. Under these conditions the transistor is said to be switched or turned 'OFF'.

If should be noted that it may be necessary to limit the current through the transistor when it is switched on, this can be achieved by a series resistor as in the LED circuit.

1.9.1.2 NPN Transistor as an amplifier

When the base is made positive with respect to the emitter so that the transistor is switched 'ON', the amount of base emitter current required is very small. If the base / emitter current is increased slightly, by increasing the base emitter voltage, the transistor will turn 'ON' more, its effective resistance will decrease and the collector / emitter current will increase.

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If the base / emitter current is decreased slightly by reducing the base / emitter voltage, the transistor will turn 'OFF' more, it's effective resistance will increase and the collector / emitter current will decrease.

The transistor can therefore be likened to a variable resistor. As the base / emitter bias increases, the resistance of the transistor effectively decreases and more current flows from collector to emitter. The change in current and resistance causes the output voltage to decrease.

As the base / emitter bias decreases, the effective resistance of the transistor increases and less current flows from collector to emitter. The change in current and resistance now causes the output voltage to increase.

When set up correctly, millivolt changes across the base / emitter junction produce changes at the output of 10's or even 100's of volts, depending on the collector voltage.

If a small sinusoidal a.c. signal is applied to the base / emitter junction, the bias will vary sinusoidally as will the resistance of the transistor and the output voltage, however the output voltage will vary sinusoidally 10's of volts for millivolt changes in the input signal. (Using the example voltage in the diagram).

It should be noted, that although the changes in output voltage are much greater than the changes in input voltage, the bipolar transistor is a current device. Small changes in base / emitter current result in large changes in collector / emitter current. It is these changes in collect / emitter current that produce the large output voltage swings.

1.9.2 PNP TRANSISTOR

PNP transistors are made in a similar manner to NPN transistors, except the direction of the junctions is reversed.

If the base is made negative with respect to the collector, the diode, or junction is

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If the base is made negative with respect to the emitter, the junction is forward biased and current flows.

Current cannot flow between collector and emitter, because irrespective of the bias applied, one junction will be reverse biased.

Again these three statements are worth remembering when it comes to determining the terminals and testing transistors.

1.9.2.1 PNP Transistor as a switch When connected as shown, the PNP transistor can also be used as a switch, however, for the transistor to be tuned 'ON', the base must be made negative with respect to the emitter. For a silicon transistor the base needs to be about 0.6V

negative with respect to the emitter, for a germanium transistor 0.2V negative.

Once turned 'ON', conventional current flows from the emitter to the collector, which is in the opposite direction to that in the NPN transistor.

1.9.2.2 PNP Transistor as an amplifier

The PNP transistor can also be used an amplifier. It operates in a similar manner to the NPN transistor except the transistor must be turned 'ON' by making the base negative with respect to the emitter, as seen above. If the base / emitter potential is increased by making the base more negative with respect to the emitter, the

transistor turns 'ON' more, its effective resistance decreases and more emitter / collector current flows. If the bias potential is decreased, by making the base less negative with respect to the emitter, the transistor turns 'OFF' slightly, the effective resistance increases and less emitter / collector current flows.

A small sinusoidal signal applied to the base will vary the effective resistance of the transistor and produce much larger changes in the output voltage as with the NPN transistor. Again it must be realised that the transistor is a current device. The small changes in base emitter bias potential created by the input signal results in small changes in base emitter current, resulting in large changes in collector / emitter current.

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1.10 TESTING SEMICONDUCTOR DEVICES

1.10.1 TESTING DIODES

Diodes only conduct in one direction, it is therefore relatively easy to determine the terminals and serviceability using a multimeter, however, 2 points need noting: • When AVO's are used on a resistance range, the black terminal is positive with

respect to the red terminal.

• The potential difference between the red and black terminals of a digital meter may be insufficient to forward bias a silicon diode (remember: requires 0.6V). This would indicate that the diode was non conducting in both direction leading to the false assumption that the diode is unserviceable.

1.10.1.1 Determining the Terminals

When forward biased, a diode has a resistance of approx. 1kΩ. When reverse biased the resistance is in the order of megohms. To determine the terminals of a diode, it is simply a matter of connecting the meter across the diode to see if it will conduct, if it will not, the terminals should be reversed to confirm conduction and serviceability. When conducting, the black terminal of an AVO, or the red terminal of a digital meter, is connected to the anode (flat end of symbol).

1.10.1.2 Confirming Serviceability

The serviceability of a diode is determined by ensuring it has a resistance in the order of 1KΩ in one direction and a resistance in the order of megohms in the opposite direction. Remember the points made about the two types of meter.

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FUNDAMENTALS 1.10.2 TESTING TRANSISTORS

As we have seen, transistors basically comprise 2 back-to-back diodes, therefore the process of confirming the serviceability and determining the terminals is similar to that used for diodes.

1.10.2.1 Determining the Base

The base of the transistor can be found by considering the transistor as two back-to-back diodes, and using a multimeter set on ohms.

1.10.2.1.1 NPN Transistors

Connect the positive terminal of the meter to one of the three transistor terminals. Measure the resistance between this terminal and the other two. If both indicate a low resistance then the positive terminal is connected to the base. If the resistance to the other two terminals is not low, the positive terminal is not connected to the base. Connect the positive terminal of the meter to another terminal and repeat the process until the base is determined.

1.10.2.1.2 PNP Transistors

The procedure used to identify the base of a PNP transistor is the same as that used to determined the base of the NPN transistor, except that the negative terminal of the meter is connected to each transistor terminal in turn, and it is this negative terminal that indicated the base.

1.10.2.2 Confirming the Serviceability

Both types of transistor are serviceability tested by confirming that each forward biased junction (Diode) has a low resistance, and each reverse biased junction a high resistance. The high resistance between collector and emitter should also be confirmed. Remember the points made about AVO's and Digital meters, otherwise incorrect conclusions may be drawn from the observations.

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NPN Base to Emitter - forward biased - low resistance Base to Collector - forward biased - low resistance Emitter to Collector - reverse biased - high resistance PNP Emitter to Base - forward biased - low resistance

Collector to Base - forward biased - low resistance Emitter to Collector - reverse biased - high resistance

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2 OPERATIONAL AMPLIFIERS

Operational amplifiers are integrated circuit devices designed to be a close approximation to the perfect amplifier.

2.1 THE PERFECT AMPLIFIER

Although a theoretical device, the specification of a perfect amplifier would be as follows:

• Gain infinitely high. This has to be controlled in some way otherwise the smallest input would result in maximum output.

• Input impedance. Infinitely high so as not to load the source.

• Output impedance. Zero, so that the amplifier can be connected to any load without the output voltage being affected.

• Bandwidth. Infinite, so that signals from d.c. to infinite frequency are all amplified by the same amount.

• Supply voltage. The amplifier should be unaffected by variations in the power supply voltage.

2.2 OP AMP SPECIFICATION

The following specification is for a SN72741 operational amplifier. This is a very popular operational amplifier generally simply referred to as a 741 op-amp. • Gain - 200 000 voltage gain (106db approx.)

• Input impedance - 2MΩ. • Output impedance - 75Ω • Bandwidth - d.c. to 1MHz.

• Supply voltage - The op-amp will operate with a supply of plus and minus 5 to 15 volts, and take a quiescent current of about 2mA. The output voltage will change less than 150μV per volt change in supply voltage.

It can be seen that the 741 Op Amp approximates the specification of a perfect amplifier.

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FUNDAMENTALS 2.3 POWER REQUIREMENTS

Operation is most convenient from a dual balanced d.c. power supply giving equal positive and negative voltages (+ Vs) in the range +5V to +15V. The centre point of the power supply, i.e. 0V is common to input and output and is taken as their voltage reference.

The input signs on the circuit symbol for an Op Amp should not be confused with those for the supply polarities.

An op-amp can be operated from a single power supply. The voltage difference available from, for example, a 0V to 18V supply is the same as that from a +9V to 0V to -9V one, however, if a single power supply is used, extra components are required.

2.4 PIN OUTS & CIRCUIT SYMBOL

The circuit symbol and pin outs of a typical operation amplifier are shown below.

Most of the terminals are self-explanatory or will be explained in the course of these notes. Terminals 1 and 5, the offset null terminals however require further

explanation.

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If the same input signal is applied to the input terminals 2 and 3 the output (terminal 6) should be zero, in practice it is not. For d.c. amplification this not acceptable. The output is zeroed by connecting a resistor between terminals 1 & 5 as shown, and adjusting it until the output falls to zero. For a.c. amplification a coupling capacitor in series with the output removes any unwanted d.c. offset.

2.5 OPERATION

An operational amplifier has one output and two inputs as seen on the circuit and pin-out diagrams. The two inputs are referred to as, the non-inverting input, marked with a +, and the inverting input, marked with a -.

If the voltage applied to the non-inverting input (+) is positive relative to the other input, the output voltage is positive. If the voltage applied to the non-inverting input is negative relative to the other input, the output voltage is negative. That is, the non-inverting input and the output are in-phase.

If the voltage applied to the inverting input (-) is positive relative to the other input, the output voltage is negative. If the voltage applied to the inverting input (-) is negative relative to the other input, the output voltage is positive. That is, the inverting input, and the output are anti-phase.

Basically an op-amp is a differential amplifier. It amplifies the difference between the two input voltages.

There are 3 cases:

• If V+ > V- the output is positive • If V+ < V- the output is negative • If V+ = V- the output is zero

In general to output is given by V0 = A0 × ((V+) - (V-)) where A0 is the gain.

2.5.1 NEGATIVE FEEDBACK

As already mentioned, and as can be seen from the transfer characteristic to the left. There is only a very small range of input values giving an output that is directly proportional (A to B). It takes very little input to drive the amplifier into saturation due to its extremely high gain.

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Assuming a gain of 105, the maximum input voltage swing (for linear amplification) is ±9V/105_{= ±90μV. This is of little practical use.}

To reduce this gain and allow larger input signals, requires the use of negative feedback. Part of the output is fed back to the input in such a way that it produces a voltage at the output that opposes the one from which it was taken. This basically means taking part of the output and feeding it back to the inverting input. (Feedback applied to the non-inverting input would be positive and would increase the output). The application of negative feedback also gives greater stability, less distortion and increased bandwidth, it also becomes possible to exactly predict the gain of the amplifier. The relatively small loss in gain is far outweighed by the advantages obtained.

A simple feedback network is shown in the diagram of an inverting amplifier below.

The signal to be amplified is applied to the inverting input via the resistor, the output is therefore antiphase with respect to the input. The non-inverting input is

connected to ground. Negative feedback is provided by resistor Rf, called the 'feedback resistor', it feeds back a certain amount of output voltage to the inverting input.

Using this arrangement the gain can be calculated from; -Rf/R1 if Rf = 1MΩ and R1 = 10kΩ

the gain A = Hata! = -100 and,

an input of 0.01V will cause an output change of 1.0V.

It should be noted that the gain depends entirely on the values of resistors Rf and R1, and is totally independent of the parameters of the operational amplifier.

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FUNDAMENTALS 2.6 OP-AMP COMPARATOR

If both inputs of an Op Amp are used together, then V0, the output voltage, is given

by:

V0 = A0 × (V2 - V1)

Where V1 is the inverting input and V2 the non-inverting input. (Note: no feedback is

used).

The difference in voltage at the input terminals is amplified and appears at the output, however the gain is so large, that about 90μV difference in the two inputs will cause the output to fall or rise to the +ve or -ve supply voltage.

When V1 > V2 the output is almost -Vs, when V1 < V2 the output is almost +Vs. The

op-amp basically behaves like a two state switch, switching 'high' or 'low' depending on the difference in the inputs.

By connecting a reference voltage to the inverting input and a signal to the non-inverting input, the output will swing to +Vs when the signal is greater than the

reference voltage and to -Vs when the signal is smaller than the reference signal.

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Hata!

2.7 OP AMP SUMMING AMP

When connected as multi-input inverting amplifier (see previous topic on feedback), an op amp can be used to add a number of voltages, either a.c. or d.c.

In the above circuit, 3 input voltages, Vin 1, Vin 2 and Vin 3 are applied through resistors R1, R2 and R3 respectively.

Hence: = Hata! + Hata! + Hata!

V0ut = - Hata!

Thus the input voltages are added and amplified if Rf is greater than each of the input resistors.

If R1 = R2 = R3 = Rin, the input voltages are amplified equally and V0ut = Hata! (Vin 1 + Vin 2 + Vin 3)

If R1 = R2 = R3 = Rin = Rf

then V0ut = (Vin 1 + Vin 2 + Vin 3)

The output voltage is the sum of the input voltages but is of opposite polarity.

This device can be used as a digital to analog converter by making R2 twice the size of R1, and R3 twice the size of R2. If a 3 bit digital word is then be applied to the resistors, with the least significant bit applied to R1 and the most significant bit applied to R3, the output will be the analogue equivalent of the binary word.

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3 PRINTED CIRCUIT BOARDS

The assembly of the various circuits which form part of the units employed in aircraft electronic systems, necessitates the interconnection of many components by means of electrical conductors. Before the introduction of printed wiring, these conductors were formed by wires which connected to the components either by soldering, or by screw and crimped terminal methods.

In the development of circuit technology, micro-miniaturisation, rationalisation of component layout and mounting, weight saving, and the simplification of installation and maintenance become essential factors; and as a result, the technique of printing the required circuits was adopted.

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In this technique, a metallic foil is first bonded to a base board made from an insulating material, and a pattern is then printed and etched on the foil to form a series of current conducting paths, the pattern replacing the old method or wiring. Connecting points and mounting pads, for the soldering of components appropriate to the circuit, are also formed on the board, so that, as a single assembly, the board satisfies the structural and electrical requirements of the unit which it forms a part. If the circuit is a simple one, the wiring may be formed on one side of a board, but, where a more complex circuit is required, wiring is continued on to the reverse side, which also serves as the mounting for components. In addition, complex circuits may be incorporated in multi-layer assemblies.

3.1 BASE MATERIAL

The base material, or laminate as it is sometimes called, is the insulating material to which the conducting material is bonded. The base material also serves as a

mounting for the components which comprise the circuit. The base material is commonly made up either of layers of phenolic resin impregnated paper, or of epoxy resin impregnated fibre glass cloth which has been bonded to form a rigid sheet, which can be readily sawn, cut, punched or drilled. The thickness of the base material depends on the strength and stiffness requirements of the finished board, which, in turn are dictated by the weight of the components to be carried, and by the size of the printed conductor area.

3.2 CONDUCTOR MATERIAL

The most commonly used conducting material is copper foil, the minimum purity value of which is 99.5%.

3.3 BONDING OF CONDUCTOR MATERIAL

For the manufacture of a typical circuit board, the base material and copper foil are cut into sheets, and are then inspected and assembled inside a clean room in alternate layers with stainless steel separator plates (known as cauls) interposed between the layers, as shown below. The steel plates, which are accurate in thickness to within 0.001 inch, are very hard, and have a delicately grained surface which is imparted to the finished boards.

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The layered sheets (the assembly) are then passed out of the clean room to be bonded in a hot press. During the pressing operation, the heat melts the resin in the base material , so that it flows and fully wets out the material and the copper foil. The pressure applied is adjusted so as to exclude all air and vapour from any residual volatiles. As polymerisation of the resin mix proceeds, each layer of the base material reaches the fully cured state with the copper foil firmly bonded to it.

After cooling has taken place, the individual copper-clad boards are trimmed to the required size, inspected, and packed in sealed polythene bags.

3.3.1 INSPECTIONS & TESTS

After manufacture, all boards are inspected, and tests are carried out on selected samples, in accordance with the relevant specifications. Tests will include:

• Inspection of appearance • Checks on thickness

• Measurement of bow and twist

• Measuring the peel strength of the foil • Checking the heat resistance by solder • Measurement of pull-off strength

• Electrical tests

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FUNDAMENTALS 3.4 MACHINING OF BOARDS

All boards require machining, e.g. guillotining, sawing, punching, and drilling during the various stages of production.

Guillotining is one of the quickest and most economical methods of cutting sheets

of copper-clad laminates into strips and panels, and it is frequently employed in conjunction with subsequent punching operations. Correctly performed guillotining results in a clean, burr-free edge, with no wastage of stock.

Cutting with a circular saw is superior to guillotining as it gives a cleaner edge,

especially so as the thickness of the laminate increases. Wood-cutting machinery is satisfactory for laminates.

The type of resin, base material and the degree of cure, are the main factors affecting the drilling characteristics of a laminate. All laminates are abrasive particularly those with glass fibre base material, and drilling techniques should be adapted to suit.

Where large quantities of laminates are required, and cost of tools is acceptable, punched parts can be produced by conventional pierce and blank methods, such methods are most commonly adopted for copper-clad phenolic / paper base laminates.

3.5 CIRCUIT ARTWORK

The quality of a printed wiring board is, in the fist instance, dependent on the production of master artwork which must show precisely the circuit conductor pattern required, where components are to be located, circuit module designations and other essential references. Artwork production requires the use of

dimensionally stable base materials, and the application of skilled drafting

techniques, because, unlike conventional electrical drawings, which are used as a guide to the build-up of an assembly of wiring and connections, a printed wiring board is an actual reproduction of the original artwork produced for it.

Human error in drafting can be reduced, and, in certain cases, eliminated, by the use of numerically-controlled drafting machines. These are accurate X and Y co-ordinate plotting machines which are capable of automatically plotting a point, or line, on a surface whether it be on a film or glass base.

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FUNDAMENTALS 3.6 PRINTING OF CIRCUITS

The printing of circuits is carried out using either an etching process or an additive process. Both of these processes, are briefly described in the following paragraphs. 3.6.1 ETCHING PROCESS

In this process the copper foil is first cleaned, either chemically or mechanically, and is then coated with a photo-sensitive solution known as a 'resist', which has the property of becoming soluble when exposed to strong light.

A photographic positive of the circuit artwork is then placed over the sensitised board and time-exposed in a special printing machine. After exposure, the resist is washed away to leave unprotected areas of copper around the circuit pattern. The board is dried by a clean, oil and water free air blast. The complete board is then inspected to ensure that no resist has been removed from any part of the conductor pattern itself, and that no resist particles are present in areas which are to be etched away. The board is then placed in a bath which contains an etching solution, such as ferric chloride or ammonium persulphate, which etches away all the unprotected copper.

When the etching process has been satisfactorily completed, the board is thoroughly washed in water in order to remove all traces of etching solution, and is then dried and given a final inspection.

As printed circuit boards with the same circuit pattern are often required in large numbers, the simple 'print and etch' process is generally superseded by a screen printing process.

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FUNDAMENTALS 3.6.2 ADDITIVE PROCESS

In this process, copper is deposited only in the areas where conductors are

required. To achieve this the base material is pre-coated with a suitable adhesive, the circuit holes are pre-fabricated, and the board is sensitised with a photo-resist solution. A negative of the circuit pattern is then screen printed onto the board so that the exposed areas define the conductor network. These exposed areas are chemically activated, and the board is then immersed in an electrolyses copper plating solution. After a period of time consistent with the deposition of the required thickness of copper, the board is removed from the bath. The major advantages of the additive process are: no chemical etching takes place, thereby eliminating

wastage of copper, the thickness of the deposited copper can be reduced and made more uniform, the conductor widths and spacing are less restricted, and the hole diameter can be reduced, thereby increasing the board area available for routing of conductors.

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FUNDAMENTALS 3.6.3 INSPECTION

After printing, circuit patterns are inspected with particular attention being paid to the following:

• Dimensional Accuracy and Condition of the Edges of Conductors • Condition of the Pattern Surfaces

• Particles of Copper in Unwanted Areas • Insulation Areas

• Lack of Resin Bond in etched Areas

3.7 SOLDERING METHODS

There are two main methods of soldering employed in connection with printed circuits boards, (a) hand soldering and (b) mass soldering.

3.7.1 HAND SOLDERING

This method is used for soldering joints separately, e.g. in limited batch production, and when a component or a wire is replaced after a test or a repair has been carried out. This method involves the use either of electrically heated hand irons, or of resistance type hand tools when the use of these is permitted.

3.7.2 MASS SOLDERING

In this method, all joints of a finally assembled board are soldered simultaneously, by bringing the board into contact with an oxide-free surface of molten solder, which is contained in a special type of bath. Mass soldering may be carried out in any one of five different ways:

Flat or Static Dipping - one edge of the board is first lowered on to the solder and

the other edge is then lowered slowly to allow flux and solvent vapour to escape.

Wave Soldering – solder is pumped from the bottom of the solder bath through a

narrow slot, so that a symmetrical 'standing wave' of solder is produced across the width of the bath. The circuit board after being fluxed, is then either manually or automatically passed against the crest of the solder wave by a conveyor.

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Weir and Cascade Soldering systems are of the moving solder type, the solder

flowing down a trough by gravity, and then being returned to the main bath by a pump. In weir-soldering (diagram (a)) a circuit board is lowered on to the solder; while in cascade soldering (diagram (b)) a board is conveyed across the crests of solder waves in a direction opposite to the solder flow.

Reflow soldering is an automated process also known as 'heat cushion' soldering.

It is applied particularly to circuit boards on which microcircuits and associated devices are to be assembled. These efficient but costly components require a special soldering technique, so that their full potential as surface-mounted devices can be realised. The reflow technique is generally recognised as the best method, since the soldered joints are easier to inspect and to remake when a faulty

component has to be replaced. In addition, soldering times and the risk of

overheating sensitive components are reduced, and distortion of leads is prevented. The sequence of reflow soldering is shown in the diagram on the following page. The leads of the circuit or component and the relevant lands on the circuit board, which have been pre-tinned by such methods as wave soldering or dip soldering, are first brought into contact with each other and accurately aligned. The sequence is then initiated by lowering the electrode on the lead to be soldered.

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Shortly before the electrode makes contact with the lead, the pre-set heating power is automatically switched on. The electrode is then pressed on to the lead under a load which gradually increases until the pre-selected value is reached. The solder melts, and in reflowing, it forms a 'cushion' through which the lead is pressed against its corresponding land of the circuit board. As soon as the cushion is

formed, the timing device cuts off the heating supply. After a 0.75 second delay, an air blast is delivered to cool the soldered joint, this accelerates the completion of the soldering process, and also improves the quality of the joint. At the end of the cooling period, the load is relieved, and the electrode is automatically raised ready for the next operation.

3.8 SOLDER SPECIFICATION

For the mass-soldering of printed wiring boards, solder complying with BS 219 Grade K (60/40 tin/lead) is the one most commonly used, since it has a free-flow characteristic which permits good joint formation in the short period during which boards are in contact with the solder.

The solder temperature is chosen for each individual combination of board and types of material being processed, but it should normally be within the range 220°C to 260°C.

3.9 FLUXES & THEIR APPLICATION

To assist in the wetting of surfaces by molten solder, a flux must be used both to prevent oxidation during joint formation, and to dissolve the thin oxide films which may already be present on the surfaces which are to be joined, and on the solder itself.

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FUNDAMENTALS 3.10 SOLDER RESISTS

There are organic coatings which are designed for use on both rigid and flexible printed circuit, to mask off those areas where soldering is not required. Some important advantages of the use of solder resists are as follows:

• Elimination of bridging and icicling between closely spaced conductors and mountings.

• Protection is afforded against corrosion and contamination during storage, handling subsequent life of the circuit.

• Flexibility of circuit patterns is maintained since a resist flexes with the conducting material.

• The surface resistance values of the circuit patterns are improved.

• Minimising of solder contamination from large surfaces of copper and other plated materials, thereby maintaining a high level of solder purity and an extension of bath life.

• Heat distortion is minimised, since a resist acts as a heat barrier.

3.11 PLATING OF PRINTED WIRING CIRCUITS

Plating finishes for printed wiring circuits are used as aids to the performance of circuits under specific conditions of use, and are not intended to be decorative. The choice of finish is, therefore, governed strictly by the functional and environmental conditions in which the circuit will be used. In many cases, the different parts of a circuit may be subjected to different conditions of use, and provided there is cleat demarcation between these parts, they can be plated with the appropriate finishes. A typical example of this differential plating method, is a circuit that is tin/lead plated for solderability over the component area, and nickel/gold plated for durability on edge-connector finger contacts.

3.11.1 THROUGH-HOLE PLATING

Through-hole plating is a process which is widely employed to provide a conducting surface in the holes of single-sided and double-sided boards, and also to provide a land or pad for the connection of components.