PCB X-Ray Inspection Comprehensive Introduction

Surface scratches can be easily perceived by the naked eye, even if they are very fine and can be easily detected with tools. However, when these scratches move to the interior, what should be done? With increasing demands for electronic device performance and functionality, more circuits are moving to the inner layers of PCB, making it impossible to identify defects through visual inspection. X-rays have become the optimal solution, penetrating PCB materials to obtain detailed information about the internal structure and solder connections, enabling comprehensive detection.

PCB X-Ray inspection visual diagram

Why X-Ray Inspection Is Becoming More Important

Observing electronic devices around you reveals an interesting phenomenon: while the size of devices is shrinking, their functionality is increasing. The functionality of electronic devices depends on the components inside, specifically the electronic elements on the PCB. These components are strategically planned at designated positions on the PCB, transmitting current through traces to activate and serve the device. In other words, to accommodate more components, a larger PCB size is inevitably required, contradicting the initial goal of miniaturization. An effective solution is to move circuit traces to the internal layers of the PCB, enhancing the utilization of PCB surface area, known as multilayer PCB.

The use of multilayer PCB significantly optimizes device performance and functionality. However, as the number of layers increases, so does the potential risk of faults. In the PCB production process, necessary measures such as AOI and electrical testing are usually taken to detect PCB. However, these methods are not sufficient for manufacturers. For example, AOI devices utilize optical cameras and image processing technology for visual inspection but are not suitable for internal circuit inspection. Electrical testing, performed post-production, detects production quality to avoid delivering faulty products but does not intervene in the production process.

Hence, the necessity of X-ray inspection is evident, not only overcoming the limitations of AOI in identifying internal defects but also addressing the shortcomings of electrical testing in the production process. Therefore, equipping X-ray equipment is considered essential for manufacturers on a larger scale.

Advantages of X-Ray Inspection PCB

Although acquiring X-ray equipment may involve additional investment, it is considered a worthwhile one-time cost. Firstly, X-ray inspection provides quality control during the production process, helping to avoid faults that may lead to expensive rework costs and time in the future. Secondly, X-ray inspection achieves automation, enhancing detection speed and reducing labor costs.

It is noteworthy that X-ray inspection is considered safe for both humans and PCB. On the human side, the radiation used by the equipment is minimal, significantly lower than medical X-rays, posing no harm to the human body. For PCB, X-ray equipment offers higher detection accuracy, stability, continuous monitoring, and ensures timely detection.

Working Principle of X-Ray PCB Inspection

The core principle of X-ray inspection for PCB involves the emission of high-energy and penetrating electromagnetic waves by the equipment. When the X-ray equipment is activated, the X-ray tube provides electrons from a hot cathode through an electric field, accelerating them to the anode. Under high voltage, electrons rapidly accelerate to a high-speed state, and their kinetic energy is then converted into the release of X-rays. When X-rays come into contact with a substance, depending on the material and density, the interaction between X-ray particles and the substance varies. Specifically:

  • When X-rays collide with atomic nuclei or electrons in the material, the direction of the particles is altered.
  • When X-rays penetrate the material, some energy is absorbed by the material.
  • If neither scattering nor absorption occurs, it is transmission, indicating that X-rays do not lose much energy.

In the end, the information from these interactions is captured by the equipment’s detector and cleverly converted into an image, revealing details of the internal structure of the circuit board. In an intuitive way to understand, if an area expected to scatter X-rays actually exhibits transmission characteristics, it may be a clue, hinting at the possibility of missing or different types of materials.

Types of X-Ray Equipment

Depending on the direction of discussion, X-ray equipment can be divided into fully automatic, semi-automatic, offline operation, or online operation. Essentially, they share the same nature. Here, TechSparks will compare 2D and 3D X-ray equipment:

2D X-Ray Equipment: The detection equipment and camera are placed beneath the PCB. X-rays are emitted from a point source, penetrating the PCB vertically to create a grayscale 2D image. This type of scan is mainly used to inspect the internal morphology of devices, including the shape of components, solder ball morphology, and void ratio calculation.

3D X-Ray Equipment: It employs tilted and rotated X-ray beams, illuminating the PCB at sharp angles to generate multiple 2D images for constructing a 3D image. In comparison, 3D X-ray equipment can slice through layers and display different depths of the object’s internal structure, thus completing the detection process in more detail.

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