In this technique metallic foil is bonded onto a base board made from insulating material, and a pattern is printed onto the foil and chemical etching on to the foil forms a series of current conducting paths. The components are then mounted to the board and soldered to the appropriate points to make-up the circuit required.
The boards are usually made-up of layers of phenolic resin impregnated paper, or epoxy resin impregnated glass-fibre cloth.
The thickness of the boards depends in the strength and stiffness required. The boards are manufactured in three basic configurations:
1. Single layer -
These boards contain all printed conducting paths on one side with the components mounted on the opposite side
2. Multilayer -
These have printed conducting paths on both sides and the components may also be mounted on both sides
3. Multilayer sandwich -
These boards are many thin boardslaminated together with the components mounted on one or both external sides
The most commonly used conducting material is copper foil. To bond the copper to the board, copper foil sheets are cut to the size of the board and steel separate plates are interposed between the layers as shown in figure 154.
Figure 154: MAKE UP OF PCBs
The layered sheets are bonded in a hot press. The heat during the pressing operation melts the resin in the base material so that it flows and fully wets the material and the copper foil. As polymerisation of the resin mix proceeds, each layer of base material reaches the fully cured state with the copper foil is bonded to it. When cooled each board is trimmed to the required size, inspected and packed in polythene bags.
Next a master diagram must be produced to show clearly the conductor pattern (a sort of wiring pattern) required and where the components are to be located. This is usually done by computer aided design techniques.
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The printing process may be by the etching or additive process. In the etching process the copper foil is cleaned and coated with a photo-sensitive solution known as a 'resist', this solution has the property of becoming soluble when exposed to strong light. The master diagram is then placed over the board and exposed for a time in a printing machine. The resist is washed away to leave the resist etched away around the circuit pattern. The board is then placed in a bath of ferric chloride to etch away all the unprotected copper.
Figure 155 ETCHING PROCESS
An alternative process is the additive method. In this process the copper is deposited only in the areas where conductors are required. Again the board is coated with a photo resist solution. A negative of the master diagram is then screen printed onto the board, exposing the areas for the conductor layout. These exposed areas are chemically activated and the whole board is immersed in a copper plating solution, when the required thickness is obtained the board is withdrawn from the solution.
Figure 156: ADDITIVE PROCESS
The components are soldered to the board by two main methods (a) by hand, (b) mass soldering.
In mass soldering all joints are soldered simultaneously by bringing the board into contact with an oxide free surface of molten solder, which is contained in a special bath. The solder specification for mass soldering is 60/40 tin/lead. To prevent oxidation a flux is used and in the automated mass soldering system a fluxer unit is incorporated, removal of any flux residue is by solvents.
Figure 157: TYPICAL DOUBLE SIDED PCS
Figure 158: BOARD COMPONENTS
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Flexible printed wiring circuits are available and usually serve as a means of interconnecting units and are basically copper foil conductors bonded to a base of thin flexible insulator (polyester, epoxy glass cloth and polyimide) and covered with the same material.
Printed circuit boards are widely used in components on a modern large transport aircraft. When removing or replacing these boards strict precautions must be observed.
The reason for this is that the static electricity or charge that we have in our body can cause serious damage to the software of the components on the boards. The table shows typical electrostatic voltages that may be developed.
ELECTROSTATIC VOLTAGES RELATIVE HUMIDITY (%) MEANS OF STATIC GENERATION
10 TO 20 65 TO 90
WALKING ON CARPET 35,000 1,500
WALKING ON VYNIL FLOOR 12,000 250
WORKING AT BENCH 6,000 100
VINYL (PLASTIC) DOCUMENT
ENVELOPES 7,000 600
‘POLY BAG’ PICKED FROM BENCH 20,000 1,200
CHAIR PADDED WITH
POLYURETHANE FOAM 18,000 1,500
Figure 159: TABLE OF TYPICAL ELECTROSTATIC CHARGES
If we were to touch the edge connectors or some other exposed metal part then a surge of current due to the difference in potential between our body and the PCB would cause damage to the components. The following table lists static sensitive devices and voltages that can cause damage. These devices are often referred to as ESD's (electrostatic sensitive devices).
TYPE OF DEVICE RANGE WHERE DAMAGE CAN
OCCUR (V)
MOSFET 150 TO 1,000
CMOS 250 TO 1,000
BIPOLAR TRANSISTOR 4,000 TO 15,000
SILICON CONTROLLED RECTIFIER
(SCR) 4,000 TO 15,000
THIN FILM RESISTORS 150 TO 1,000
Figure 160: TABLE OF VOLTAGE SENSITIVITY
To identify components fitted with ESD's a symbol is used on the line replacement unit (LRU) and associated documentation, transport bags etc.
To overcome the static discharge problem the person removing the PCB must use a conducting wrist strap which is connected to a convenient grounding point on the aircraft and the person, to initially discharge any energy within the body.
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Figure 161: BSD DECALS AND WARNINGS
When removing an BSD PCB (or any PCB for that matter) electrical power is removed, the wrist strap is connected to the ground (there is usually a convenient point nearby on the aircraft), attach strap to your wrist and remove the PCB using the extractors provided. Place the PCB immediately into a special conductive bag (designed for ESD components) and identify with a label, do NOT use staples or adhesive tape. Remove wrist strap if not immediately refitting a new PCB. Do not forget any documentation such as JAA form 1 etc.
If you are removing the complete LRU then it is important you do not touch the connector pins and place dust caps on all connectors.
REMEMBER STATIC DISCHARGE CAN CAUSE DAMAGE!
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