Following are the steps involved in building a PCB.
Designing the Circuit Schematic
Designing the circuit schematic is the first and most integral part of the process. PCB designers usually draw it on a graph paper or in a computer simulation software, mostly Eagle CAD or Multi Sim. But before they start designing, they create a net list that contains all the circuit nodes and component pins. They have to finalize the location of each device on the board and for that, they develop a grid of numbered columns and lettered rows to designate a place to each component. The devices can be manually placed at their designated locations or the software can automatically assign them to specific areas.
Once the device list is finalized, the software uses templates from a footprint library in order to form a footprint map for different devices. The map is actually the indication of each device’s pins with recommended drill-hole layouts and pads.
Once the logic diagram is complete, the designer prints it out and hands it over to an Electrical engineer for etching it on the board and then assembling the devices on it.
Etching the Design on the Board
Etching starts with putting a layer of copper on the substrate which is composed of a paper saturated with phenolic resin like Pertinax. The unnecessary copper is then removed by applying a temporary mask which is usually made up of ferric chloride. Ferric chloride at 46°C is poured in a tray and the board is then dipped in it for 20 minutes. Then it’s lifted using plastic tongs and gently tapped to remove the surplus copper, leaving desirable traces of the metal. This etching technique is referred to as ‘Silk Screening’. Other methods include ‘Photoengraving’ and ‘PCB Milling’.
Assembling the Components
After the engineer is done with etching, he moves onto assembling the devices and circuit components on the circuit board. Some boards are single-layered and some have trace layers in them due to which they are called multilayer PCBs. In order to place the devices, holes have to be drilled on the board which is done using small tungsten carbide drill bits. It’s an automated process and the placement of the board is controlled by a computer generated drill file called NCD file. The file also describes the size and location of each hole.The pads and components that are to be mounted are plated because bare copper cannot be soldered. This is called surface mount construction. Whereas, through hole construction requires the engineer to insert component leads in holes and fix them electrically to the board with a metal solder. The parts that are not to be soldered are concealed with a solder resist polymer layer.
The Printed Circuit board is now complete. It is then tested to ensure that it works accurately. Also, it is coated with silicone rubber or polyurethane to prevent corrosion and damage.
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