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PCB assembly is a long process involving several automated and manual steps. Each of these steps must be executed correctly with maximum attention to detail. Small errors in any step during the assembly process will cause the final assembly to fail.
In order to better understand the PCB assembly process, you must have a clear understanding of the most basic unit of the PCB-the base. The bottom of the PCB consists of several layers such as these.
1.Substrate: These are specialized materials that can conduct electricity to a minimum. Commonly used substrates used as the insulating layer between two conductive copper layers are fluorine series resins, PPO or PPE resins and modified epoxy resins, fluorine series dielectric substrates, PTFE, etc.
2.Copper: A thin layer of copper foil is added to improve the thermal resistance and current carrying capacity of the PCB.
3.Solder resist: usually a green solder resist used to insulate copper wires from other conductive materials.
4.Screen printing: The final layer of the PCB is screened, which helps to provide text indicators for the part. The silk screen layer helps to identify test points, part numbers, warning symbols, logos and manufacturer marks.
The above-mentioned basic layers are almost the same for all types of PCBs. The only difference on rigid, flexible, metal core, surface mount or through-hole PCBs is the use of substrates. The manufacturer chose the substrate after considering the application.
Start the actual 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 (there are several sizes, shapes and specifications are consistent with the specifications), these stencils can only correctly apply the correct amount of solder paste to certain parts of the board.
2.Component placement: Unlike the past, the PCB assembly process at this stage is now fully automated. The picking and placing of parts (such as surface mount components) was once done manually, and is now performed by pick and place machines. These machines accurately place components on the pre-planned area of the circuit board.
3. Reflow soldering: Now, the solder paste and all surface mount components are in place. What do you do next? Curing the solder paste to the correct specifications is essential for the proper adhesion of PCB components to it. This is the relevant part of the PCB assembly process-reflow soldering. To this end, the components with solder paste and the components on them are passed through a conveyor belt, and the conveyor belt is passed through an industrial-grade reflow oven. The heater in the oven melts the solder in the solder paste. Once melting is complete, the components will again move on the conveyor belt and be exposed to a series of cooler heaters. The purpose of these coolers is to cool the molten solder and solidify it.
4. Inspection: After the reflow process, the PCB should be inspected to check its function. This stage can help identify poor quality connections, misplaced components, and short circuits due to the continuous movement of the circuit board during the reflow process. PCB manufacturers have adopted multiple inspection steps, such as manual inspection, automatic optical inspection and X-ray inspection, to check the function of the circuit board, identify lower quality solder and find any potential hidden dangers. After the inspection is completed, the assembly team will make a crucial decision. Circuit boards with several functional errors are usually scrapped. On the other hand, if there is a minor error, send the board again for rework.
5. Through-hole component insertion: certain types of PCBs need to be inserted into through-hole components together with conventional SMD components. This stage is dedicated to the insertion of such components. To this end, which PCB components are used to transmit signals from one side of the board to the other to create plated through holes. PCB through-hole insertion usually uses manual or wave soldering to obtain results.
6. Final inspection: Now is the time for the second level inspection. Here, perform a functional test on the assembled board, or perform a thorough inspection on the PCB to monitor its electrical characteristics, including voltage, current, or signal output. Today's manufacturers use a variety of advanced testing 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 important to thoroughly clean the components before the final circuit board is delivered to the customer. For this, the PCBs are washed in deionized water. After the cleaning process, use compressed air to thoroughly dry the board. Now the PCB assembly can be prepared for customer inspection.
Although the PCB assembly process steps of all types of circuit boards are almost the same, there are still some changes in THT assembly, SMT assembly and hybrid technology. You can read our other articles to deepen your understanding of SMT。
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