Printed Circuit Boards (4.2) PDF

Printed Circuit Boards (4.2) Learning Objectives 4.2 Recall the function and use of printed circuit boards (Level 1). 2022-11-10 B1-04 Electronic Fundamentals Page 112 of 163 CASA Part 66 - Training Materials Only ...

Printed Circuit Boards (4.2) Learning Objectives 4.2 Recall the function and use of printed circuit boards (Level 1). 2022-11-10 B1-04 Electronic Fundamentals Page 112 of 163 CASA Part 66 - Training Materials Only  Printed Circuit Boards Introduction to PCBs A Printed Circuit Board or PCB for short, is a flat insulating surface upon which printed wiring and miniaturised components are connected in a predetermined design and attached to a common base. The image below shows a typical printed circuit board. Aviation Australia Double sided PCB - front (left) and back (right) Note that various components are connected to the board and the printed wiring is on the reverse side. With this technique, all interconnecting wiring in a piece of equipment, except for the highest power leads and cabling, is reduced to lines of conducting material (copper, silver, gold, etc.) deposited directly on the surface of an insulating circuit board. Since printed circuit boards are readily adapted as plug-in units, the elimination of terminal boards, fittings and tie points, not to mention wires, results in a substantial reduction in the overall size of electronic equipment. 2022-11-10 B1-04 Electronic Fundamentals Page 113 of 163 CASA Part 66 - Training Materials Only  Schematic and PCB Layout Preceding Technology Before the advent of the PCB, circuits were constructed through a laborious process of point-to- point wiring. This led to frequent failures at wire junctions and short circuits when wire insulation began to age and crack. A PCB is found in almost every electronic device. If you have electronic components in a device, they are mounted on a PCB, big or small. Besides keeping the components in place, the purpose of a PCB is to provide electrical connections between the components mounted on it. As electronic devices have become more complex and require more components, the PCB has become more densely populated with wiring and components. 2022-11-10 B1-04 Electronic Fundamentals Page 114 of 163 CASA Part 66 - Training Materials Only  Basic PCB Construction Printed circuit boards can be made from a variety of substances. They are usually a flat laminated composite made from non-conductive substrate materials with layers of copper circuitry buried internally or on the external surfaces. PCB without components The PCB can be as simple as a layer of copper on one side of the board and components mounted on the other, more complex circuits require two layers one one each side, or in higher density applications they can have fifty laminated layers or more. The flat composite surface is ideal for supporting the components that are soldered on or attached to the PCB, while the copper conductors connect the electronic components to one another. 2022-11-10 B1-04 Electronic Fundamentals Page 115 of 163 CASA Part 66 - Training Materials Only  Base or Substrate Material The base material of a printed circuit board, referred to as the substrate or laminate, is made of phenolic paper, epoxy paper and epoxy glass. The metal foil, referred to as the cladding, is usually made of copper but other types of metal may be used. The substrate board is produced first and then the cladding is bonded to it. PCB cutaway (single, double and multi layer) Conductor Pattern The substrate of the board itself is an insulating and non-flexible material. The thin wires that are visible on the surface of the board are part of a copper foil that initially covered the whole board. In the manufacturing process this copper foil is partly etched away and the remaining copper forms a network of thin wires. These wires are referred to as the conductor pattern or the tracks and they provide the electrical connections between the components mounted on the PCB. A basic printed circuit board has the copper circuit pattern or foil on one side of the board. Holes are drilled through pads or terminals in the foil and board. The component leads are pushed through the holes from the other side of the board. The leads are then soldered to the copper foil to complete the circuit connections. PCB conductor pattern 2022-11-10 B1-04 Electronic Fundamentals Page 116 of 163 CASA Part 66 - Training Materials Only  Solder Mask and Silk Screen What gives the PCB its green or brown colour is the solder mask. This is an insulating and protective coat that protects the thin copper wires from corroding and prevents solder from attaching outside the connection points for the components. Solder mask A silk screen is printed on top of this coloured mask. This consists of text and symbols (often white) printed on the board to label the locations for the different components that are to be mounted. The silk screen is also referred to as the legend. PCB silk screen used to label components 2022-11-10 B1-04 Electronic Fundamentals Page 117 of 163 CASA Part 66 - Training Materials Only  Through-Hole Technology The components that are mounted on one side of the board while its legs are soldered on the opposite side are called through-hole PCBs often referred to as Through-Hole Technology (THT). Such components take up a large amount of space and require a hole to be drilled in the PCB for every leg. Through-hole components Hence, their legs occupy space on both sides of the board, and the connection points for them are also fairly large. On the other hand, THT components are fairly good mechanically connected to the PCB compared to surface-mounted devices, which will be discussed below. Connectors for cables and similar devices also have to withstand mechanical stress and are usually THT. Aviation Australia Through-hole technology 2022-11-10 B1-04 Electronic Fundamentals Page 118 of 163 CASA Part 66 - Training Materials Only  THT Component Mounting To fasten the components to the PCB, their legs are soldered to the conductor pattern. On the most basic PCBs (single-sided boards) the components are located on one side of the board and the conductor pattern on the opposite side. This requires holes in the PCB for the component legs to penetrate the board. Component mounting Hence the legs are soldered to the PCB on the opposite side of where the components are mounted. The top and bottom side of a single sided PCB is therefore respectively referred to as the 'component side' and 'solder side'. PCB soldering (bottom side) 2022-11-10 B1-04 Electronic Fundamentals Page 119 of 163 CASA Part 66 - Training Materials Only  Surface-Mounted Technology The legs of components that are made using Surface Mounted Technology (SMT) are soldered to the conductor pattern on the same side of the PCB on which the component is mounted. This technology therefore does not require a hole in the PCB for every leg of the component. Surface-mounted components could even be mounted on both sides of the PCB directly underneath each other. SMT components are also much smaller than THT components. This makes PCBs with SMT components much more dense compared to similar PCBs with THT components. Today, SMT components are also cheaper than THT components. It is therefore no surprise that most components on main boards nowadays are SMT. Since the connection points and component legs are so small it becomes very hard to solder on an SMT component manually. Considering that machines do almost all assembly, this issue only becomes important when repairs have to be done. Surface Mounted Technology 2022-11-10 B1-04 Electronic Fundamentals Page 120 of 163 CASA Part 66 - Training Materials Only  PCB Terminology Often PCBs have distinct labelling. The foil pattern takes various shapes, depending on function of circuit. Heat sinks are used to dissipate heat from components and require large areas. Voltage and ground lines or planes are long and slender and follow a path so as to provide power to components on board. Terminals or pads are points drilled with holes to accommodate component leads. Conductors or runs are thin foil strips between components. Edge connectors are part of foil that connects circuit board to a special plug. Aviation Australia PCB with some components labelled 2022-11-10 B1-04 Electronic Fundamentals Page 121 of 163 CASA Part 66 - Training Materials Only  PCB Additional Components Component Sockets If a component needs to be removable from the PCB after it is manufactured, it is mounted on the board with the use of a socket. The socket is soldered to the board while the component can be inserted and taken out of the socket without the use of solder. Component socket Various components sockets 2022-11-10 B1-04 Electronic Fundamentals Page 122 of 163 CASA Part 66 - Training Materials Only  Edge Connector To connect a PCB to another PCB, an edge connector is often used. The edge connector consists of small uncovered pads of copper located along one side of the PCB. These copper pads are actually part of the conductor pattern on the PCB. The edge connector on one PCB is inserted into a matching connector (often referred to as a slot) on the other PCB. In PC graphic cards, sound cards and other similar products are connected to the main board with the use of edge connectors. Edge connector (top) and slot (bottom) 2022-11-10 B1-04 Electronic Fundamentals Page 123 of 163 CASA Part 66 - Training Materials Only  PCB Types Single-Sided Boards Most basic boards have the components mounted on one side of the board and the conductor pattern on the opposite side. Since there only is a conductor pattern on one side, this type of PCB is called single-sided. It has severe limitations when it comes to routing the wires in the conductor pattern (since there is only one side no wires can cross and they have to be routed around each other), so it is only used in very primitive circuits. Single sided PCB top view (top) and bottom view (bottom) 2022-11-10 B1-04 Electronic Fundamentals Page 124 of 163 CASA Part 66 - Training Materials Only  Double-Sided Boards These types of boards have a conductor pattern on both sides of the board. Having two separate conductor patterns requires some kind of electrical connection between them. Such electrical connections are called vias. A via is simply a hole in the PCB that is filled or plated with metal and touches the conductor pattern on both sides. Since the surface available for the conductor pattern is twice as large compared to a single-sided board, and as wires can now cross (by routing them on opposite sides of the board), double-sided PCBs are much more suited for complex circuits than the single-sided ones. Double sided PCB top view (top) and bottom view (bottom) 2022-11-10 B1-04 Electronic Fundamentals Page 125 of 163 CASA Part 66 - Training Materials Only  Multi-Layer Boards A typical multi-layer PCB (which covers most of the PCBs used to implement a complex circuit) consists of a sandwich of conducting and insulating layers. It is onto this format that the PCB design must be produced. The sandwich of layers, or stack of layers, alternates between some sort of non-conducting dielectric layer and copper foil which has been patterned to form the circuit connections. Multi-layer PCB design 2022-11-10 B1-04 Electronic Fundamentals Page 126 of 163 CASA Part 66 - Training Materials Only