What we usually call PCB assembly generally refers to the rigid PCB assembly, and the main steps are listed as below:
Solder paste needs to be added to the area that the components will sit in the final PCB before placing components. The solder paste usually appears gray and must be applied to the board in the correct position with the correct amount.
SMT places components on the board primarily with pick-and-place machines because of the high requirements for accuracy.
In this step, the actual connection takes place. After the components are put, the board is placed on a reflow furnace conveyor. The solder applied during the soldering process melts during reflow. Then the PCB passes a series of cooler heaters to allow the molten solder to cool and solidify in a controlled manner, which will connect the component to the board permanently.
If there are through-hole components or PTH components, waving soldering is needed. In this step, the board is placed on a mechanical conveyor driven system and passes through different areas of the PCB through the melt welding wave, which helps connect the PCB pads/vias, electronic components lead, and solder. Then the electrical connection is formed.
It is important to clean the PCB after assembly, which helps to clean all flux residues with the deionized water. This step also can be done after the inspection process.
Usually, movement during the reflow process results in poor connection quality or total connection loss. Short circuits are also a common side effect of this kind of movement because misplaced components sometimes connect parts of the circuit that should not be connected. So, the final step is testing the functionality of the PCB to identify any potential problems and fix them as soon as possible. Manual checks, automatic optical inspection, and X-ray inspection are the conventional inspection methods to check for these errors and misalignments.