X-ray inspection technology, also referred to as automated x-ray inspection, is a technology that can find hidden features within a target object. From the medical industry to aerospace manufacturing, x-ray inspection is a widely used method to identify manufacturing errors. It is especially prevalent in PCB inspection, as x-rays are an excellent way to test PCB quality and find hidden defects without risking damage to the board.

In this article, we’ll discuss what x-ray inspection is, how it works and what its benefits are, so you understand how to leverage PCB x-ray inspection for your company’s purposes.

Why X-Ray Machines Are Becoming Popular in Electronics Manufacturing

Electronics manufacturing has consistently become more sophisticated over the past several decades, with electronics in industrial, communication, military and aerospace industries advancing in ways that make inspection significantly more difficult. Some of the most significant trends that negatively affect inspection include:

Component placement: Equipment is becoming smaller and more layered to save space and maximize functionality. As a result, many solder joints and components are being moved to inner layers within electronics, either placed between PCB layers or hidden within the final product itself.

Component size: More circuit board components are trending toward miniaturization. With the need to create more dense boards, PCB components are expected to continue toward this trend.

SMT: Surface mount technology makes leads and packages smaller, which has resulted in PCBs having higher density with more components hidden between layers.

The increasing density, decreasing size and more complex placement of PCBs and their components have made it nearly impossible for traditional inspection methods to find faults thoroughly. Optical, ultrasonic and thermal imaging methods are relatively ineffective due to the density of modern PCBs.

However, x-ray inspection can collect detailed images where these other methods cannot. X-ray has the ability to penetrate layers of PCB to inspect inner layers and packaging, making it more useful for inspecting the solder joints of complex PCB assemblies. For this reason, PCB inspection with x-ray technology is becoming increasingly popular.

How X-Ray Inspection Works

To understand what x-ray PCB inspection is, you first need to know how x-ray inspection works. All x-ray inspection devices are made up of three basic features:

X-ray tube: This tube has the ability to generate x-ray photons.

Platform: The operation platform moves the sample in different directions, allowing it to be inspected from various angles and magnitudes.

Detector: The detector on the other side of the sample catches and collects x-ray photons, converting them into an image.

X-ray imaging works using x-ray photons, which are passed through a material and collected on the other side. The x-ray photons pass through materials differently based on their physical properties, including atomic weight, density and thickness.

Generally, heavier elements absorb more x-ray photons, making them easier to image, while lighter elements are more transparent. Because different materials behave uniquely, differing amounts of photons are collected on the other side of the object, resulting in an image. Components in a PCB typically consist of heavier elements, making them easy to see on an x-ray image.

X-Ray Machine Classifications

PCB x-ray inspection systems can be divided into two sets of categories. Systems may be 2D or 3D and may be operated offline or online. These are explained in more detail below:

2D system: A 2D x-ray system displays the 2D images from both sides of the PCB simultaneously, creating a picture of the board’s components. This is similar to the traditional concept of an x-ray being used to look at bone fractures. 2D Systems can be operated either online or offline.

3D system: A 3D x-ray system has the ability to create 3D images of a PCB by building up a series of 2D cross-sections. This is similar to how a CT scan works in the medical industry. 3D x-rays also function using the laminography method, which combines cross-sections to build up a particular part or area. The CT method can only be accomplished offline due to the complex algorithms used, while the laminography method can be used both online and offline.

Online operation: Some x-ray devices are used online for data collection and comparison purposes. This makes it easier to process large quantities of complex PCBs. However, online capability makes the x-ray machine slower, which increases costs while decreasing capacity.

Offline operation: All x-ray devices can be operated offline to accomplish panel inspection and sampling inspection. Offline tools are generally quick to run but become less cost-effective with higher quantities of product.

Selecting the best system for your operation must be carefully done — choosing a 2D or 3D system with online or offline capabilities will depend on the quantity of product inspected, the desired inspection quality and the amount of time available to complete the inspection process.

Benefits for Quality Control and Inspection

In x-ray images of PCBs, the metal is dark while other materials like glass, plastic and ceramics are transparent. This makes it easy to see metallic components. Combined with magnification capabilities, this allows inspectors to see various metallic parts on a board during PCB inspection with x-ray equipment. The x-ray will enable inspectors to see multiple qualities of a printed circuit and identify a range of potential problems, including:

Solder bridges: Solder bridges are a common problem on circuit boards. This occurs when solder joints are too close or create a connection where there should be none. These solder problems are typically hidden by other components, making them difficult to spot with optical inspections, but x-rays identify them easily.
Solder voids: Solder voids are created when gas or flux is entrapped during soldering, often as a result of water contamination of solder paste. These voids decrease heat conductivity at the solder joint and can cause physical defects and reduced reliability. X-ray is one of the only ways to identify solder voids, so it is essential to use regularly to ensure product and process quality.
Pin-hole fills: Inadequate pin-hole fill for through-hole connectors is a prevalent issue in circuit boards. X-ray machines can be used to identify these errors and even quantify the amount of fill missing using coordinate measurements.

One specific example of how x-ray is used is in a ball grid array (BGA) inspection. A BGA is a surface mount device that uses an array of metal spheres made of solder to make connections. This type of device is popular, as it enables high-density connections without complicating the PCB layouts. The difficulty with BGAs, however, is that the connections are made on the underside of the BGA chip. This feature, as well as the density of the solder balls, makes it difficult to solder and de-solder and increases the likelihood of forming solder bridges. The location also makes BGA chip solder joints particularly difficult to inspect. X-ray simplifies the inspection process by looking through the device to see the solder joints underneath, easily identifying bridges.

With magnification and the capability to change the viewing angle, x-ray machines can be used to find these problems and more. Even better, all of these problems may be identified during a single inspection because, unlike medical imaging, operators do not need to worry about radiation dose during operation and can use the equipment for as long as they need.