Printed Circuit Boards Assembly (PCBA) Process

Printed Circuit Boards Assembly (PCBA) Process
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Printed Circuit Boards Assembly (PCBA) Process

The PCB assembly process includes several automated and manual steps. And that is simple. With every step of the process, board manufacturers have manual and automatic options for you to choose from. To help you better understand the PCBA process from start to finish, we explain each step in detail below.

Step 1: Solder Paste Stenciling

The first step in PCB assembly is to apply solder paste to the board. This process is like a screen printed shirt, with a thin stainless steel template placed on the PCB in addition to the mask. This allows the assembler to apply only solder paste to certain parts of the possible PCB. These components are the components in which the component is placed in the finished PCB.

Solder paste is mixed with flux, which is a chemical design that helps the solder melt and adhere to the surface. Solder paste is shown as a gray paste and must be applied to the board in the proper amount in the proper position.

In professional PCBA production lines, mechanical fixtures hold the PCB and the weld template in place. The applicator then places the solder paste on the desired area in precise amounts. The machine then spreads the paste onto the stencil and spreads evenly over each open area. After removing the template, the solder paste remains in the desired position.

Step 2: Pick and Place

After the solder paste is applied to the PCB, the PCBA process is moved to the pick and place machine, which places the surface mount component or SMD on the prepared PCB. Today, SMD accounts for most of the non-connector components on the PCB. These SMDs are then soldered to the surface of the board in the next step of the PCBA process.

The device initiates the pick and place process by picking up the PCB with the vacuum clamp and moving it to the pick and place station. The robot then orients the PCB at the workstation and begins applying SMT to the PCB surface. These components are placed on top of the solder paste in the preprogrammed position. 

Step 3: Reflow Soldering

Once the solder paste and surface mount components are in place, they need to be left there. This means that the solder paste needs to be cured to adhere the component to the board. PCB assembly accomplishes this through a process called "backflow."

After the pick and place process is completed, the PCB board is transferred to the conveyor. This conveyor belt passes through a large reflow oven, a bit like a commercial pizza oven. The oven consists of a series of heaters that gradually heat the plate to a temperature of approximately 250 degrees Celsius or 480 degrees Fahrenheit. This is hot enough to melt the solder in the solder paste.

After the solder melts, the PCB continues through the oven. It cools and solidifies the molten solder in a controlled manner through a series of cooler heaters. This creates a permanent solder joint that connects the SMD to the PCB.

Many PCBAs require special consideration during reflow, especially for double-sided PCB assembly. Double-sided PCB assembly requires separate stencil printing and reflow. First, the side with fewer and smaller parts is stamped, placed and reflowed, then the other side.

Step 4: Inspection and Quality Control

Once the surface mount component is soldered in place after the reflow process, this does not complete the PCBA and requires functional testing of the assembled board. Often, movement during reflow will result in poor or no connection at all. Shorts are also a common side effect of this type of movement, as the misaligned parts sometimes connect to portions of the circuit that should not be connected. 

Checking for these errors and misalignments can involve one of several different inspection methods. The most common inspection methods include:

• Manual inspection: Although automation and intelligent manufacturing are about to be developed, manual inspection is still required during PCB assembly. For smaller batches, the designer's on-site visual inspection is an effective way to ensure the quality of the PCB after reflow soldering. However, as the number of inspection boards increases, this approach becomes increasingly impractical and inaccurate. Observing these small parts for more than an hour can cause optical fatigue, resulting in inaccurate inspections.

• Automated optical inspection: For larger batches of PCBA, automated optical inspection is a more appropriate method of detection. Automated optical inspection machines, also known as AOI machines, use a series of high-power cameras to "see" the PCB. These cameras are arranged at different angles to view the solder joints. Different quality solder joints reflect light in different ways, enabling the AOI to identify low quality solder. AOI does this at a very high speed, enabling it to handle large amounts of PCB in a relatively short period of time.
• X-ray inspection: Another method of inspection involves X-rays. This is an uncommon detection method - it is most commonly used for more complex or layered PCBs. X-rays allow the viewer to see through the layers and visualize the underlying layers to identify any potential hidden problems.

The fate of the fault board depends on the standards of the PCBA company, and the PCBA company will be sent back for cleaning and reprocessing or scrapping.

Whether or not you find one of the errors, the next step in the process is to test the part to make sure it does what it should. This involves testing the quality of the PCB connection. Boards that require programming or calibration require more steps to test the correct function.

This check can be done periodically after the reflow process to identify any potential problems. These regular inspections ensure that errors are found and fixed as quickly as possible, which helps manufacturers and designers save time, labor and materials.

Step 5: Through-Hole Component Insertion

Depending on the type of board under the PCBA, the board may contain various components in addition to the conventional SMD. These include plated through-hole components or PTH components.

The plated through hole is a hole in the PCB that is always plated on the board. The PCB assembly uses these holes to pass signals from one side of the board to the other. In this case, the solder paste does not have any benefit because the solder paste will pass directly through the hole without the chance of sticking.

In future PCB assembly processes, PTH components require a more specialized soldering method than solder paste.

• Manual welding: Manual through hole insertion is a simple process. Typically, the task of a single person at a single site is to insert a component into the specified PTH. When complete, the board will move to the next workstation and the others are inserting another component. Every PTH that needs to be equipped will continue to cycle. This can be a lengthy process depending on how many PTH components need to be inserted during one cycle of PCBA. Most companies have specifically tried to avoid designing with PTH components, but PTH components are still common in PCB design.

• Wave soldering: Wave soldering is an automated version of manual soldering, but the process involved is very different. Once the PTH assembly is in place, place the board on another conveyor. This time, the conveyor cleaned the molten solder at the bottom of the oven through a special oven. This will immediately solder all the pins on the bottom of the board. For double-sided PCBs, this soldering is almost impossible because soldering the entire PCB side can make any delicate electronic components unusable.

After completing this soldering process, the PCB can continue with the final inspection, or if the PCB needs to add additional components or the other side of the component, the previous steps can be performed.

Step 6:Inspection and Functional Test

After the soldering step of the PCBA process is completed, the final inspection will test the functionality of the PCB. This type of inspection is called a "functional test." The test allows the PCB to complete its pace and simulate the normal operation of the PCB. In this test, the power and analog signals pass through the PCB while the tester monitors the electrical characteristics of the PCB.

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