4 Common Inspection Methods for Printed Circuit Board(PCB) Assembly

In order to fulfill the requirements of printed circuit board testing, various testing equipment, and technologies have been produced accordingly. These detection technologies and equipment can detect problems in the assembly process of printed circuit boards. The following is a detailed introduction of these four detection methods.

The simplest form of PCB inspection is manual visual inspection (MVI). To perform such tests, workers can view the board with the naked eye or magnify it. They will compare the board with the design document to ensure that all specifications are met. They will also look for common default values. The type of defect they look for depends on the type of circuit board they are inspecting and the components on it.

The inspector inspects almost every aspect of the circuit board and looks for a variety of common defects in every aspect. A typical visual PCB inspection checklist may include the following

(1) Ensure that the thickness of the circuit board is correct, and check the surface roughness and warpage.

(2) Check whether the size of the component meets the specifications, and pay special attention to the size related to the electrical connector.

(3) Check the integrity and clarity of the conductive pattern, and check the solder bridges, open circuits, burrs, and voids.

(4) Check the surface quality, and then check whether there are dents, dents, scratches, pinholes, and other defects on the printed traces and pads.

(5) Confirm that all the through-holes are in the correct position. Make sure that there are no omissions or improper punches, the diameter matches the design specifications, and there are no gaps or knots.

(6) Check the firmness, roughness, and brightness of the backing plate, and check whether there are convex defects.

(7) Evaluate the quality of the coating. Check the color of the electroplating flux, and whether it is uniform, firm, and in the correct position.

Compared with other types of inspections, MVI has several advantages. Because of its simplicity, it is low-cost. Except for possible amplification, no special equipment is required. These checks can also be performed very quickly, and they can be easily added to the end of any process.

To perform such inspections, the only thing needed is to find professional staff. If you have the necessary expertise, this technique may help. However, it is essential that employees can use the design specifications and know what defects need to be paid attention to.

The functionality of this check method is limited. It cannot inspect components that are not in the worker's line of sight. For example, hidden solder joints cannot be inspected in this way. Employees may also miss some defects, especially small ones. Using this method to inspect complex circuit boards with many small components is particularly challenging.

You can also use a PCB inspection machine for visual inspection. This method is called automated optical inspection (AOI).

AOI systems use multiple light sources and one or more stationary or cameras for inspection. The light source illuminates the PCB board from all angles. The camera then takes a still image or video of the circuit board and compiles it to create a complete picture of the device. The system then compares its captured images with information about the appearance of the board obtained from design specifications or approved complete units.

Both 2D and 3D AOI equipment are available. The 2D AOI machine uses colored lights and side cameras from multiple angles to inspect components whose height is affected. 3D AOI equipment is relatively new and can measure component height quickly and accurately.

AOI can find many of the same defects as MVI, including nodules, scratches, open circuits, solder thinning, missing components, etc.

AOI is a mature and accurate technology that can detect any faults in PCBs. It is very useful in many stages of the PCB production process. It is also faster than MVI and eliminates the possibility of human error. Like MVI, it cannot be used to inspect components outside the line of sight, such as connections hidden under BGA and other types of packaging. This may not be effective for PCBs with high component concentrations, as some of the components may be hidden or obscured.

Another method of PCB inspection is automatic laser test (ALT) measurement. You can use ALT to measure the size of solder joints and solder joint deposits and the reflectivity of various components.

The ALT system uses lasers to scan and measure PCB components. When light reflects from the components of the board, the system uses the position of the light to determine its height. It also measures the intensity of the reflected beam to determine the reflectivity of the component. The system can then compare these measurements with design specifications, or with circuit boards that have been approved to accurately identify any defects.

Using the ALT system is ideal for determining the amount and location of solder paste deposits. It provides information about the alignment, viscosity, cleanliness, and other properties of solder paste printing. The ALT method provides detailed information and can be measured very quickly. These types of measurements are usually accurate but can be interfered with or shielded.

With the rise of surface mount technology, PCBs have become more and more complex. Now, the density of circuit boards is higher, the components are smaller, and include chip packages such as BGA and chip-scale packaging (CSP), through which hidden solder connections cannot be seen. These functions bring challenges to visual inspections such as MVI and AOI.

To overcome these challenges, X-ray inspection equipment can be used. The material absorbs X-rays according to its atomic weight. The heavier elements absorb more and the lighter elements absorb less, which can distinguish materials. Solder is made of heavy elements such as tin, silver, and lead, while most other components on the PCB are made of lighter elements such as aluminum, copper, carbon, and silicon. As a result, the solder is easy to see during X-ray inspection, while almost all other components (including substrates, leads, and silicon integrated circuits) are invisible.

X-rays are not reflected like light but pass through an object to form an image of the object. This process makes it possible to see through the chip package and other components to check the solder connections under them. X-ray inspection can also see the inside of solder joints to find bubbles that cannot be seen with AOI.

The X-ray system can also see the heel of the solder joint. During AOI, the solder joint will be covered by the lead. In addition, when using X-ray inspection, no shadows enter. Therefore, X-ray inspection works well for circuit boards with dense components. X-ray inspection equipment can be used for manual X-ray inspection, or an automatic X-ray system can be used for automatic X-ray inspection (AXI).

X-ray inspection is an ideal choice for more complex circuit boards and has certain functions that other inspection methods do not have, such as the ability to penetrate the chip package. It can also be used well to inspect densely packed PCBs and can perform more detailed inspections on solder joints. The technology is a bit newer, more complex, and potentially more expensive. Only when you have a large number of dense circuit boards with BGA, CSP, and other such packages, you need to invest in X-ray inspection equipment.

Even if the defects are minimized in the PCB assembly process, we still need to carry out the final inspection of the assembled printed circuit board, because it is the final unit of product and entire process evaluation. If you want to learn more about printed circuit boards after reviewing the above content, you can contact us for more professional solutions.

As a professional PCB product manufacturer, we have accumulated rich manufacturing experience in this field. We have a professional production team and strict quality inspection system, and we have formulated comprehensive and detailed solutions for the production and assembly of products. We can also continuously optimize and adjust the plan according to customer needs, and provide customers with thoughtful one-stop service. If you are interested in our printed circuit boards, please contact us immediately!