No matter what type of printed circuit board needs to be built or what type of equipment it is applied to, the PCB must work normally. It is the key to the performance of many products, and failures can cause serious consequences.

Testing the PCB during the design, manufacturing, and assembly process is essential to ensure that the product meets quality standards and performs as expected. Nowadays, PCB is very complicated. Although this complexity provides room for many new features, it also brings a greater risk of failure. With the development of PCB, inspection technology and technology used to ensure its quality are becoming more and more advanced.

According to the PCB type, production process and faults to be tested, it is important to select the correct testing techniques and develop a suitable inspection and test plan to ensure high-quality products.

Why do we need to test the PCB?

PCB testing is a key step in all PCB production processes. It can detect PCB defects in order to correct them and improve overall performance.

Inspection of the PCB can reveal any defects that may occur during the manufacturing or assembly process. Testing the PCB after each stage of the PCB manufacturing process can help engineers find defects before entering the next stage, thereby avoiding wasting more time and money to buy defective products. This process helps to ensure consistent quality between the circuit board and the final product.

In the worst case, if a defective PCB is used in products such as medical equipment or auto parts, it may cause injury or death. Such problems can lead to severe reputation loss and expensive litigation.

Testing the PCB can also help improve the entire PCB production process. If a defect is frequently found, measures can be taken in the process to correct the defect.

PCB/PCBA Testing Method

To ensure that the PCB can work as expected, the manufacturer must verify that all components are assembled correctly. This is accomplished through a range of techniques, from simple manual inspection to automated testing using advanced PCB inspection equipment.

Manual visual inspection is a good start. For relatively simple PCBs, you may only need this technique.

Manual Visual Inspection:

The simplest way of PCB inspection is manual visual inspection (MVI). Workers can view the board with naked eyes or magnify. They will compare the board with the design document to ensure that all specifications are met.

It is useful to perform MVI after almost every step of the PCB production process (including assembly). 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:

Make sure the thickness of the circuit board is correct, and check the surface roughness and warpage.

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

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

Check the surface quality, and then check for dents, dents, scratches, pinholes and other defects on printed traces and pads.

Confirm that all 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.

Check the firmness, roughness and brightness of the backing plate, and check for raised defects.

Assess 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.

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. As a result, employees will miss some defects, especially small defects. 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).

Automated Optical Inspection:

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 an 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 many 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.

Automatic Laser Testing:

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 or shielded.

X-ray Inspection:

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 different materials used in PCB absorb X-rays according to their 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 help us see the solder joints, and during AOI, the solder joints will be masked by the leads. In addition, when using X-ray inspection, there will be no shadows to interfere. 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 automatic X-ray system can be used for automatic X-ray inspection (AXI).

X-ray inspection is ideal for complicated circuit boards, and other tests having certain functions are not available, for example, have 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.

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