4.2 Printed Circuit Boards Flashcards
Since the 1970s, Printed Circuit Boards (PCBs) have been a popular medium for the installation of electronic components
most common materials for printed circuit boards (PCB’s) are:
Copper laminated paper treated with phenolic resin used mostly in radio and TV technology.
Copper laminated paper treated with epoxy resin used for its flame resistant properties.
Copper laminated glass silk weave. Laminate materials reinforced with fibreglass are strong when subjected to mechanical stresses.
Laminated paper for additive processes used for circuit boards that have circuits printed on both sides.
(PCB materials)
Polyester, Teflon and ceramics are also used for specialised applications. According to the respective requirements, the board is completely laminated with copper on one or both sides.
The first step in producing a PCB is called ‘patterning’. This is the process of converting a circuit diagram into a pattern of connected conductors and spaces for the components. This is done using Computer-Aided Design (CAD) techniques.
The next step is printing. This is the process of transferring the pattern to the PCB and there are two principal methods of production:
Etching method - The surface begins as a complete layer of conducting material, such as the copper layers referred to above, and a pattern is removed by dipping the surface into an acidic solution.
Addition method - The surface begins as a complete layer of an insulator and a pattern is added by dipping the surface into a copper plating solution.
Single Sided Boards
If the circuit is a simple one, the wiring may be formed on one side of a board and the components mounted on the reverse side.
Double Sided Boards
Where a more complex circuit is required, wiring is continued onto the reverse side and components may also be mounted on both sides. This allows the conducting paths to cross one another without connecting, which is necessary as circuits become more complex.
Double Sided Boards
Interconnection is via plated-through holes, eyelets or funnelets inserted into the holes. Components are normally mounted on one side of the board; leads are through the hole.
Multi-layer Board
Where additional complexity of such circuits demands it, additional layers of connections can be incorporated into the construction of the PCB. Similar in construction to a double layer.
Multi-layer boards utilise one or more internal conductor planes.
The disadvantage of this is with the soldering process, as internal planes act as heat sinks.
Flexible Circuits
flexible circuits are not as common as the rigid type and have conductors laminated to one or more layers of a flexible insulating base (Mylar or Kapton). Flexible circuits are usually custom-designed for a particular application.
Component Attachment
Soldering by hand is adopted where joints between components and board are to be made separately, e.g., in limited batch production, and when a component is replaced after a test, or a repair has been carried out
In the automatic or mass soldering method, all joints are made simultaneously by bringing them into contact with an oxide-free surface of molten solder contained in a special bath which is part of a soldering machine.
Surface Mount Technology (SMT)
or sometimes known as planar mounting is a way of attaching electronic components to a printed circuit board. The solder joint forms the mechanical and electrical connection. Bonding of the solder joint is to the surface of a conductive land pattern. The connection does not use through holes or terminals.
The advantages of SMT are…
-Smaller parts
-Denser layout
- Cheaper PCBs (no holes to drill)
- Improved shock and vibration characteristics
- Improved frequency response
- Easier to shield from EMI/RFI (Electromagnetic -Interference/Radio Frequency Interference
- Easier to automate manufacturing
The disadvantages of SMT are
- More heat generated
- Small clearance makes cleaning difficult
-Visual inspection difficult
-Good joint formation important for the mechanical reliability of the assembly
-Harder to hand assemble
Offset Pad Mounting is a variation of the surface mount. The lead is passed through a hole and is bent so that it can be surface mounted. The join can be some distance away from the hole.
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Conformal coatings are transparent liquid films applied to the conductors of a printed circuit board
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conformal coatings provide a multitude of protection against things such as
moisture, humidity, shock, vibration, and corrosion, and provide electrical isolation. These are typically done as the last assembly step and are applied via spray, dip, brush, or robotic selective systems.
Once applied the conformal coating must be allowed to cure via heat, UV, or room temperature cure mechanisms.
By providing a protective layer over the conductors, water molecules cannot migrate to metallic surfaces.
They also protect against dust, dirt, contaminants, and a large host of other elements that the PCB may encounter in its end-use environment.
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.
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. In the aircraft maintenance environment, PCBs are called cards e.g., engine fire detection card.
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There are three ways PCBs or cards are utilised on aircraft, these are
- Fitted inside boxes or components
- Fitted into rack assemblies or card files
- Fitted into computer rack assemblies
Historically PCBs have been mounted in boxes which are Line Replaceable Units (LRUs) which in turn have been mounted in avionic racks in the aircraft.
Now in some aircraft types, PCBs within these boxes are also LRUs. These PCBs can be replaced, without replacing the complete box. They can be accessed via a removable front or top panel of the box. The aircraft’s maintenance manual will give a guide to where this is applicable.
In the latest version, PCBs are now installed into a computerised rack assembly. This is the equivalent of a Graphics Card mounted to the motherboard in a desktop home computer.
In the quest for weight reduction, the Boeing 777-8/9 will have rows of circuit breakers mounted on a PCB with relays mounted on the rear side. These boards are then installed in power panels, saving space and weight.
why are PCB’s often made with epoxy resin
it has good flame resistant properties
