Printed Circuit Board Assembly
A printed circuit board assembly (PCBA) is a finished product that incorporates all of the electronic components required to function correctly. Various electronic devices, from small gadgets to complex machinery, rely on PCBAs. The fabrication process for a bare PCB can be quite complex and involves the creation of conductive copper layers with insulating material laminated between them. These layered conductors, called pads, are where the actual components will be mounted on the board using surface-mount technology or plated through-hole (PTH) technology.
A bare PCB may be single-sided, double-sided or multi-layer. The multi-layer PCB will usually have internal layers that serve as conductive planes or signal layers, while the single-sided and double-sided PCBs will contain surface traces that will connect the pads on each side of the board.
To create the conductive features on a printed circuit board assembly, the board must first be etched which removes the outer layer of the board’s material. The bare board is then coated with an electrically non-conductive material, such as epoxy resin. Next, a copper layer is electroplated over the insulating material. A pattern is then imprinted on the surface of the conductive copper layer and a non-conductive solder mask is printed over the pattern.
What Is a Printed Circuit Board Assembly?
The bare board is then drilled with holes in the appropriate locations for the components to be attached. The drill bits used in this process are coated with tungsten carbide instead of standard steel because the abrasive nature of the copper material would quickly dull regular metal bits and ruin the board. The holes are then made conductive by adding hollow, metal eyelets which can be used to attach the component leads or to connect the board layers. The holes are typically referred to as vias and they can be placed on either side of the board.
During the front-end engineering phase of the PCB design process, schematic capture is done with electronic design automation (EDA) software to translate the circuit diagrams into a physical layout that will guide the manufacturing processes and components needed for the final board. Lamination materials and stack-ups are selected to accommodate the requirement for signal layers, plane layers, and dedicated routing channels. Line impedance is determined for each of the signal layers based on the dielectric layer thickness, routing copper thickness and trace width.
Once the PCB has been drilled, plated and covered with solder mask, the assembler will begin to add the component parts. This can be done by hand for smaller assemblies, or by a pick-and-place machine for larger projects. The component’s connecting wires are then soldered onto the board. For THT (through-hole technology) components, the wires are inserted through the pads and into holes in the conductive layers of the board.
For SMT (surface mount technology) assembly, the component’s metal pins are placed on top of the pad and then soldered to the board using a wave or reflow soldering process. The soldered connections are then inspected to ensure all of the connections are secure and that the components are functioning properly.