PCB Assembly Basics: Processes, Techniques, Considerations

The rapid development of electronic technology has brought about significant changes in our daily lives. From high-end mobile phones to ubiquitous computers, to cutting-edge artificial intelligence products, these electronic devices are quietly transforming our lifestyles. The PCBA board plays an essential role in the internal core of these electronic devices.

As the foundation of modern electronics, PCB consists of substrates, wires, pads, and other basic components. However, it is important to note that bare PCB cannot function independently. They act as carriers, providing electrical connections for the components on the board. This means that the PCB can only fully serve its purpose after undergoing the assembly process. In this article, we will provide a detailed overview of the PCB assembly process steps, and also include an explanation of popular assembly technologies and important considerations to keep in mind during the assembly process. To expedite the tutorial process, we suggest watching a YouTube video to gain a better understanding of the PCB board assembly.

Complete PCB Assembly Steps

Step 1: Solder Paste Application

Solder Paste Printing in Circuit Board Assembly

To ensure proper component placement on the PCB, it is imperative to apply an appropriate quantity of solder paste onto the designated positions on the board. The formation of a firm solder joint, which ensures a lasting connection, is achieved upon heating the solder paste. This crucial step is facilitated by the use of a template. Typically, these templates are fabricated from stainless steel and are based on the PCB design drawings. The templates possess apertures that correspond to the assembly points on the PCB. During the printed circuit board assembly process, a robotic arm positions the board accurately onto the template, which is then aligned with the board. An appropriate quantity of solder paste is applied onto the template before a scraper mechanism accurately dispenses the solder paste onto the board, through the apertures, in a controlled manner.

Step 2: SPI Detection

SPI inspection must be carried out when assembling PCB

In a professional PCBA production line, the inspection process is essential, and one of the critical steps is the SPI inspection, which is used to examine the quality of the solder paste printing. It is undeniable that solder paste printing is the determining factor for the quality of PCBA, and statistics show that almost 80% of defects in the SMT PCBA process are caused by incorrect solder paste printing. During the solder paste application process, the SPI inspection machine uses a high-resolution camera and advanced image processing software to detect the coverage of the solder paste and takes images for technicians to analyze any faults. Furthermore, a laser sensor scans the surface of the PCB to measure the height of the solder paste and verify the accuracy of the amount applied.

Step 3: Placing Components

Complete electronic PCB assembly using a pick and place machine

After verifying the inspection results, the components can be mounted on the surface of the PCB to create a PCBA board. This task is accomplished with the use of a high-speed placement machine. Prior to starting the placement process, the designer must create a file containing the X and Y coordinate information of the components and input it into the placement machine. This ensures that the placement process is accurately executed. Conventional placement machines utilize vacuum nozzles to secure the components onto the arm before placing them onto the solder paste according to the coordinate information. In order to meet the demand for faster turnarounds, advanced pick and place machines with detection capabilities have been developed. These machines can immediately detect discrepancies between the design and measurement parameters of a component, and remove the component accordingly.

In the traditional process, the placement of components is done manually with the use of tweezers. However, this approach has drawbacks such as reliability and difficulties with mass assembly. The emergence of automatic picking robots has overcome these issues. Simply by entering the code, these robots can operate efficiently 24 hours a day.

Step 4: Reflow Soldering

Reflow soldering PCBA board

During the initial three stages of circuit board assembly, the electronic components and solder paste are positioned onto the circuit board. However, at this point, the solder paste is in a soft, mushy state and is unable to secure the components onto the board. To form permanent joints, the solder paste must be melted and cured, typically through the use of the reflow process in SMT PCB assembly.

After component placement, the PCBA board is transferred onto a high-temperature flow soldering furnace equipped with a connecting table. The furnace’s temperature ranges between 250°C and 260°C, which is necessary for heating and melting the solder. Upon entering the cooler, the solder joints are carefully cured to ensure a lasting connection. One way to assess the quality of a PCBA manufacturer is by checking the number of temperature zones within their reflow oven. The more zones present, the higher the soldering quality.

For PCB that require component assembly on both sides, the aforementioned steps 1-4 must be repeated until the assembly work on both sides is completed.

Step 5: QC and Inspection

Inspection of PCB assembly quality

After the components have been assembled, it is imperative to conduct quality testing to detect common PCB soldering defects or component misalignments. There are typically three testing methods involved in this process:

  • Visual inspection: This method is carried out by experienced technicians who examine the entire printed circuit assembly. Sampling inspections are conducted once the assembly is completed.
  • AOI: Involves the use of advanced optical inspection systems that scan the circuit boards to identify any assembly defects. AOI inspections are typically carried out after the reflow soldering process.
  • X-ray: This is a paid inspection service that manufacturers can provide as per your project requirements. X-ray inspection is particularly effective for high-density boards as it can detect subtle errors that may not be visible to the naked eye.

Step 6: Through Hole Assembly

Through hole assembly is a critical step in the electronic component assembly, alongside SMT. Unlike SMT assembly, through hole assembly components are DIP components that require their leads to be inserted into the holes of the PCB before soldering the other side. Two primary methods are used for this process: manual soldering and wave soldering.

While SMD components have become increasingly popular and now account for a larger proportion of PCB assembly, some expensive components still require through-hole assembly due to their superior reliability and stability. This process ensures a more robust mechanical and electrical connection between the component and the PCB, making it an essential technique for certain applications.

Step 7: Functional Testing and Cleaning

After the assembly PCB, it is essential to conduct functional testing on the PCBA boards to ensure they meet the required performance standards. In the event of any errors, the root cause should be identified and resolved. Once the boards are confirmed to be correct, it is crucial to thoroughly clean the board surface. This is particularly important as the use of flux, solder paste, and other chemicals during the manufacturing process can cause corrosion and short circuits if left on the board.

To clean the PCBA board, a cleaner such as isopropyl alcohol can be applied to the board surface using a nozzle or brush. The board should then be rinsed with deionized water or other cleaning solution to remove any residue. Finally, compressed air or an oven can be used to dry the board. Proper cleaning of the PCBA board is vital for the long-term storage and reliable operation of these circuit boards.

Precautions in the PCB assembly process

  • Design Specifications: Before beginning assembly, it is crucial to review the design specifications, including circuit layout, component orientation, pin spacing, and pad size, to ensure that the assembly process aligns with the design intent.
  • Component Selection: Choosing the right component suppliers is critical in ensuring that the components are of high quality and are functional on the board.
  • Solder Paste Management: Solder paste is a crucial element in the assembly process. Proper selection, use, and storage of solder paste can significantly improve the quality of PCBA.
  • Squeegee Pressure: It is important to set reasonable parameters before using the squeegee to avoid potential issues such as solder paste bridging and leakage.
  • Personal Safety Hazard: When soldering circuit boards, it is essential to take necessary protective measures to ensure personal safety.
  • Soldering Quality: The quality of solder joints is vital in PCBA manufacturing as it affects the reliability and performance of electronic boards. Control of soldering temperature and time is critical to achieve high-quality solder joints.
  • Anti-static Measures: Electronic components are sensitive to static electricity, which can cause electrical breakdowns. Therefore, it is necessary to implement measures such as grounding or wearing an electrostatic wristband.
  • Document Management: A robust document management system can aid in fault-finding by enabling traceability, quality control, and compliance with industry standards and regulations.

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