PCB assembly technology
X-ray Inspection
X-ray Inspection
X-ray evaluation is a non destructive method to visualise parts that are typically hidden from view.
The system provides a real-time live 2D image, and has the possibility to construct 3D images as well.
The whole process is controlled by an internal software. With this tool we can inspect all types of SMD components and their solder joints. In practice we check the position and solder joints on BGA, micro BGA, LGA and QFN components.
X-ray inspection is a time consuming process, not all boards are inspected.
We inspect the first boards and other assembled boards on a sample basis. The X-ray inspection is part of the process control in our quality management system.
The machine can create a real picture of the inner structure of the solder alloy, showing how many voids are there in a solder joint.
The images can help us also to analyse root causes of detective boards, such as excessive solder, lack of solder, mechanical failures of components, cracks, but also to detect hidden problems inside the PCB, such as bad metallisation in the holes or via cracks.
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:
1.X-ray tube: This tube has the ability to generate x-ray photons.
2.Platform: The operation platform moves the sample in different directions, allowing it to be inspected from various angles and magnitudes.
3.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.
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.
Related post:
GWT’s PCB assembly board Inspection
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