The process of PCB assembly

PCB assembly refers to the process of soldering and assembling electronic components on a preform matting and manufacturing a printed circuit board (PCB). Usually using specialized production machinery, mass production, the printed circuit board assembly process is often referred to as PCBA.
So how is the PCB assembled? Let's take a look at the PCB assembly process:

1. Application of solder paste: First, apply solder paste (a small particle of solder paste mixed with flux) on the board. For this application, most PCB manufacturers use stencils (several sizes, shapes, and specs) that can only properly apply the correct amount of solder paste to certain parts of the board.

2. Placement of components: Unlike in the past, the PCB assembly process at this stage is now fully automated. The pick and place of parts, such as surface mount components, once done manually, is now performed by robotic pick and place machines. These machines precisely place components into pre-planned areas of the board.

3. Reflow: With the solder paste and all surface mount components in place, curing the solder paste to the correct specifications is critical to properly adhering the PCB components to it. This is this relevant part of the PCB assembly process - reflow soldering. To do this, the components with the solder paste and the components on it are passed through a conveyor belt that runs through an industrial-grade reflow oven. A heater in the oven melts the solder in the solder paste. Once the melting is complete, the components are moved through a conveyor belt again and exposed to a series of cooler heaters. The purpose of these coolers is to cool the molten solder and allow it to solidify.

4. Inspection: After the reflow process, the PCB should be inspected to check its functionality. This stage helps identify poor quality connections, misplaced components, and shorts due to the continuous movement of the board during reflow. PCB manufacturers employ multiple inspection steps, such as manual inspection, automated optical inspection, and X-ray inspection, to check the functionality of the board, identify lower-quality solder and pinpoint any potential pitfalls. After the inspection is complete, the assembly team will make a crucial decision. Boards with several functional errors are usually scrapped, on the other hand, if there are minor errors, the board is sent again for rework.

5. Through-hole component insertion: Certain types of PCBs require through-hole components to be inserted along with regular SMD components. This stage is dedicated to the insertion of such components. To do this, plated-through holes are created by which PCB components pass signals from one side of the board to the other. PCB through-hole insertion is usually achieved using manual or wave soldering.

6. Final Inspection: Now is the time for the second level inspection. Here, an assembled board is functionally tested, or a PCB is thoroughly inspected to monitor its electrical characteristics, including voltage, current, or signal output. Today's manufacturers utilize a variety of advanced test equipment to help determine the success or failure of finished boards.

7. Cleaning: Since the soldering process leaves a large amount of flux residue in the PCB, it is critical to thoroughly clean the components before delivering the final board to the customer. To do this, wash the PCBs in deionized water. After the cleaning process, use compressed air to dry the board thoroughly. The PCB assembly is now ready for customer inspection.