What Are the Components of a PCB
A printed circuit board is a versatile and integral component of electronic equipment. It provides mechanical and electrical support to various external components that the assembler has mounted on it. The printed circuit board also has internal components that provide it with the special characteristics necessary for it to function as intended in the equipment. Here, Rush PCB Inc provides some understanding of the external and internal components of a PCB.
External Components on PCB
There are several types of external electronic components and non-electronic components that can go onto a PCB. However, according to their method of mounting, it is possible to divide the external electronic components into two categories—Through-Hole Components (THC) and Surface Mount Components (SMC).
Whether THC or SMC, there are numerous electronic components that designers use on PCB for various electrical functionality to achieve their desired results. Depending on their functionality, it is possible to classify electronic components into passive and active types.
Passive Electronic Components
Passive electronic components usually impede the flow of electrical signals through them, but do not alter the signals in any way. Examples of such components are resistors, capacitors, inductors, connectors, and so on.
Active Electronic Components
Active electronic components alter the electrical signals that travel through them. They typically have a control that guides them to change the signal flow depending on specific conditions occurring around them. Examples of such components are diodes, transistors, integrated circuits, and so on.
Designers use a combination of active and passive electronic components to build up a schematic that functions as the designer intends. Based on the schematic, the designer prepares a layout that they transfer to the board as the printed circuit. When the assembler adds the necessary components to the board and solders them, the assembly functions as the designer intended the original schematic to function.
Both active and passive electronic components may be further classified according to their method of mounting.
Through-Hole Components (THC)
THCs usually have long leads that require mounting through holes drilled into the PCB. The design of the PCB is such that mounting of THCs is from the top, while the leads protrude through the board. Although there can be copper traces on both sides of the board, THCs occupy only the top side. At the bottom side, the leads terminate on pads, and a wave soldering process anchors them to the pads with solder.
As they have leads, THCs are often large components and therefore, do not offer high component density. Moreover, as they require drilled holes in the board, two additional processes, drilling and plating, are necessary in the manufacturing stage. The plating is necessary to produce barrels in the plated through holes or PTH that pass through the board from one side to the other.
Surface Mount Components (SMC)
SMCs offer much higher component density because these components have very small leads and they are physically much smaller. Mounting them does not require any holes in the PCB, and hence, it is possible to pack them much closer together on the PCB. For mounting SMCs, it is necessary to place them on pads on the same side of the board.
Mounting SMCs requires placing them on a layer of solder paste. Depositing this solder paste requires an accurate stencil. As the components are very small, using a pick-and-place machine is necessary to pick them up from their reels and place them accurately in their specific positions on the board.
Soldering SMCs requires a reflow soldering machine. Reflow soldering machines have successive zones heated with Infra-red lamps to raise the temperature up to the level where the solder in the solder paste can melt. Once melted, the solder anchors the component to the board, and the temperature cools off, allowing the solder to solidify.
Apart from electronic components, there may be other plastic or metal components on a PCB that function purely as mechanical support. These may be stiffeners, screws, connectors, shields, and other related components.
Internal Components of a Printed Circuit Board
For the circuit board to allow the external electronic components mounted on it to function as the designer intended, it must have a specific internal composition. The internal design of the board must allow it to perform in a specific way. This is possible only is the board has internal components like a core, prepreg, copper foils, solder mask, surface finish, and silk screen.
Core of a Printed Circuit Board
The core of a PCB gives it the necessary mechanical stiffness necessary to carry all the external components and a physical form. In general, the core constitutes woven glass cloth cured in epoxy. This is also known as copper clad, as the core has two copper foils bonded to it on each side.
Prepreg in a Printed Circuit Board
The core or copper clad forms only a double-sided PCB. However, most boards are more complicated with many layers. PCB fabricators use prepreg to build up the stack of a multi-layered board. Like the core, prepreg are also woven glass cloth immersed in epoxy, but not cured. The fabricator uses the prepreg in between a core and a copper foil, acting as an insulation. With the application of heat and pressure, the epoxy in the prepreg cures and bonds the two copper foils together. The fabricator adds more layers of prepreg and copper foils or copper clads and bonds them as necessary to make up a multi-layered board.
Before bonding, the fabricator must transfer the circuit on to the internal copper layer, etch it and remove the unwanted copper. They must also drill via holes and plate them to provide the plated through holes.
Copper Foils in a Printed Circuit Board
Copper foils are the most important part of a PCB, as these provide the electrical connections between the external components. Depending on the number of layers, there may be several copper foils in a PCB. The core of the PCB has two copper foils bonded to it. The fabricator may have to bond additional copper foils to the core using prepreg as insulator, depending on the number of layers the PCB must have. Before bonding, the fabricator must transfer the circuit for the layer on to the copper foil, drill and etch it to remove the unwanted copper.
If the copper traces on both external sides of a PCB remain exposed to the elements, they may tarnish and erode. This happens because the air may contain sulfur and other harmful chemicals. To protect the copper traces, the PCB fabricator covers them with an epoxy coating known as a solder mask.
The solder mask epoxy typically covers the entire copper trace, except for the pads where the component will be soldered. As most manufacturers use a green color for the solder mask, the entire PCB looks green. However, they may also use other colors for the solder mask other than green.
It is necessary to also protect the exposed copper pads from tarnishing until the assembler can mount components on them and solder them. For this, PCB fabricators use a covering of surface finish on the pads. The surface finish can be solder, silver, tin, gold and other metal or organic substances covering the pads. The surface finish apart from protecting the exposed pads also helps in proper soldering of the external components on the board.
The silk screen acts as an aid to the assemblers of external components, guiding them to their proper positioning, orientation, and polarity on the PCB. The fabricator applies the silk screen as a line drawing on the PCB. The material is an epoxy paint with a contrasting color to that of the solder mask to make it easy for assemblers to read. The silk screen also acts as a guide to mounting other mechanical components on the PCB.
According to Rush PCB Inc, all components of a PCB, whether internal or external, are important and necessary to allow it to function properly as the designer intended.