The strategy of printed circuit board assembly is based on different steps and approaches. They are properly accomplished for the final product to work as designed. To make sure, printed circuit boards utilize controlled heating, screen templates, and cooling mechanisms. They are useful in handling the components and how they are applied and secured into place.
It is important to select the right technology when assembling printed circuit boards. Printed circuit board designs specify how all parts and pieces should be aligned in their designated spots. A slight deviation can have a big impact on the final board’s functionality.
Understand the Assembling Circuit Boards Technologies
PCB Assembly needs more than just the basic components or a handy board. The right technology is needed for this purpose. Moreover, printed circuit board assembly steps differ across the various technologies. Some basic steps and how they differ from technology to technology are discussed here:
Overview of Assembling Circuit Boards Process
- Solder Paste: First thing in traditional PCB assembly procedure is the application of solder paste. In contrast to THT, SMT requires to paste application or printing.
- Component Placement: The next step of the PCBA process is the placement of components on the board. You can do this manually or with the help of machinery (pick and place systems).
- Reflow: Next step of this process is reflow, which involves melting the solder and resolidifying it later on.
- Inspection: The next step of this process is a graphical assessment of the board, soldering, and components. Remember that this step already occurred in the THT and SMT procedures.
PCB Assembly (PCBA) Process
1. Applying Solder Paste
In the first step, we apply solder paste to the components’ mounting areas on printed circuit boards. We apply solder paste to the stainless steel stencil as well. The automated fixture holds the stencil and printed circuit board together. We use an applicator to apply solder paste properly to all slots in the board. When applying solder paste, we use the right quantity in an applicator. The paste will remain in the designated areas of the PCB when we remove the applicator. This lead-free gray-colored solder paste contains 96.5% of tin, 3% of silver, and 0.5% of copper. In step 3, when we heat, this paste will melt and create a strong connection.
2. Automated Components Placement
In the second step, we mechanically place the SMT components on a PCB. A pick-and-place robot does the entire activity. While processing, the designer creates a file for the automatic robot. This file contains the robotic X and Y coordinates of every component used in the PCB. It determines the placement of all components. The robot will simply place the surface-mounted devices on board precisely. The pick-and-place robots will pick each component from the vacuum grip and place them accurately onto the solder paste.
Prior to these automated machines, the machinists pick the components and place them carefully on a PCB. They use tweezers to avoid any jittering hands. That eventually caused fatigue and strained eyes in machinists and delayed the procedure of PCBA of SMT components. And this was also prone to high errors.
3. Reflow Soldering
After setting up the components and applying solder paste, the third step is reflow soldering. This process involves the placement of printed circuit boards along with components on the conveyor belt. The conveyor belt transports the printed circuit boards and components in an oven. A temperature of 250℃ is enough to melt the solder paste easily.
The melted solder will designate the components on the printed circuit board and form a connection. After heating the printed circuit boards at high temperatures, it then transported into coolers. The coolers will then thicken the solder connections hastily. This eventually makes a lasting connection between the SMT component and PCB. When dealing with two-sided printed circuit boards, from steps 1-3, the side which has smaller components will be treated first before treating the other side.
4. QC and Inspection
After soldering, due to some faulty movements in the holding tray, there is a possibility of misalignment of the components, which may result in a short circuit or open connection. In order to detect these flaws, the inspection process is employed. This either be done manually or through automation.
5. THT Component Fixation & Soldering
Many printed circuit boards normally have THT components. The components are also named Plated Through-Holes. These components have leads that directly pass through the holes of printed circuit boards. The holes connect to others by copper traces.
Through-Hole Technology components are electrically associated with one another in the same PCB as the circuit designed when mounted and soldered in these holes. These printed circuit boards may have some THT components and different SMD components. The soldering process discussed above in the case of SMT components like reflow soldering cannot be applied here.
Final Inspection & Functional Test
The printed circuit board is now ready for inspection and testing. This inspection involves the electric signals and power supply to the printed circuit board at designated pins. It also examines the output at specific test points or exits. Standard laboratory tools such as oscilloscopes, DMM, and function generators are used to examine the functionality and electrical components of the PCB. Also used to verify current, voltage, analog, and digital signals as mentioned in the prerequisites of PCB and circuit blueprint.
6. Cleaning & Shipment
The printed circuit board is now examined and proclaimed to be OK from each aspect. Now, it is time to remove unwanted flux, dirt, and oil stains. A stainless steel-based high-pressure washing instrument using deionized water is adequate to remove all dirt. This deionized water will not harm the printed circuit board. After washing, compressed air dries the printed circuit board. Finally, we have the printed circuit board, which is ready for packing up and freight.