First, we apply solder paste to the area of the PCB that fits the component. This is done by applying solder paste on the stainless steel template. The template and PCB are fixed together by a mechanical jig, and then the solder paste is evenly applied to all openings in the board by an applicator. The applicator applies solder paste evenly. Therefore, an appropriate amount of solder paste must be used in the applicator. When the applicator is removed, the paste will remain in the desired area of the PCB. Gray solder paste 96.5% is made of tin, contains 3% silver and 0.5% copper, lead-free. After heating in step 3, the solder paste will melt and produce a strong bond.
2. Automatic placement of components
The second step of assembling PCB is to automatically place SMT components on the PCB. This is done by using pick and place robots. At the design level, the designer creates a file and provides it to the automated robot. This file has pre-programmed X, Y coordinates for each component used in the PCB and identifies the location of all components. Using this information, the robot only needs to accurately place the SMD device on the board. Pick and place robots will pick up components from their vacuum fixtures and place them accurately on solder paste.
3. Reflow soldering
The third step after setting the components and applying solder paste is reflow soldering. Reflow soldering is the process of placing PCBs and components on a conveyor belt. This conveyor belt then moves the PCB and components into a large oven, which produces a temperature of 250°C. This temperature is sufficient to melt the solder. The molten solder then fixes the component on the PCB and forms a joint. After the PCB is processed at high temperature, it enters the cooler. These coolers then solidify the solder joints in a controlled manner. This will establish a permanent connection between the SMT component and the PCB. In the case of a double-sided PCB, as described above, the PCB side with fewer or smaller components will be processed first from steps 1 to 3 and then to the other side.
4. Quality inspection and inspection
After reflow soldering, it is possible that some components are misaligned in the PCB tray, and the components are misaligned, which may cause a short circuit or an open connection. These defects need to be identified. This identification process is called inspection. The inspection can be manual and automated.
(1) Manual inspection
Because the PCB has small SMT components, visual inspection of the circuit board for any misalignment or malfunctions can cause technician fatigue and eye fatigue. Therefore, due to inaccurate results, this method is not feasible for early SMT boards. However, this method is feasible for boards with THT components and lower component densities.
(2) Optical inspection
For large quantities of PCBs, this method is feasible. This method uses an automated machine with high-power and high-resolution cameras installed at various angles to view solder joints from all directions. Depending on the quality of the solder joint, the light will reflect the solder joint at different angles. This automatic optical inspection (AOI) machine is very fast, and it takes a short time to process large quantities of PCBs.
(3) cX-ray inspection
The X-ray machine allows the technician to browse the PCB to see the inner defects. This is not a common inspection method, only for complex and advanced PCBs. If used improperly, these inspection methods may lead to rework or scrapped PCB. Inspections need to be carried out regularly to avoid delays, labor and material costs.
5. THT component fixing and welding
Through-hole components are common on many PCB boards. These components are also known as plated through holes (PTH). The leads of these components will pass through holes in the PCB. These holes are connected to other holes and through holes through copper traces. When these THT components are inserted and soldered in these holes, they are electrically connected to other holes on the same PCB of the designed circuit. These PCBs may contain some THT components and many SMD components, so the soldering method described above is not applicable to THT components in the case of SMT components (such as reflow soldering).
(1) Manual welding
The manual welding method is very common, and usually requires more time than the automatic setting of SMT. Usually a technician is assigned to insert one component at a time, and the board is passed to other technicians who insert another component on the same board. Therefore, the circuit board will move around the assembly line to obtain the PTH component filled on it. This makes the process lengthy, and many PCB design and manufacturing companies avoid using PTH components in their circuit designs. But PTH components are still the favorite and most commonly used components by most circuit designers.
(2) Wave soldering
The automated version of manual welding is wave soldering. In this method, once the PTH component is placed on the PCB, the PCB is placed on a conveyor belt and moved to a dedicated oven. Here, the molten solder wave splashes to the bottom layer of the PCB, where component leads exist. This will immediately solder all pins. However, this method is only suitable for single-sided PCBs and not for double-sided PCBs, because the solder melted when soldering one side of the PCB will damage the components on the other side. After this, the PCB is moved for final inspection.
6. Final inspection and functional testing
The PCB is now ready for testing and inspection. This is a functional test in which electrical signals and power are given to the PCB at designated pins and the output is checked at designated test points or output connectors. This test requires common laboratory instruments such as oscilloscopes, digital multimeters, and function generators
This test is used to check the functional and electrical characteristics of the PCB and verify the current, voltage, analog and digital signals and circuit design described in the PCB requirements
If any of the PCB parameters show unacceptable results, discard or scrap the PCB according to company standard procedures. The test phase is very important because it determines the success or failure of the entire PCBA process.
7. Final cleaning, sorting and shipping
Now that the PCB has been tested in all aspects and declared normal, it is time to clean up unwanted residual flux, finger dirt and oil stains. Stainless steel-based high-pressure cleaning tools using deionized water are sufficient to clean all types of dirt. Deionized water will not damage the PCB circuit. After washing, the PCB is dried with compressed air. Now the final PCB is ready to be packaged and shipped.