Assembled Circuit Boards
X-ray inspection of assembled circuit boards is an important and often overlooked part of the PCB manufacturing process. It helps detect hidden defects and errors that may not be visible to the naked eye, such as solder bridges, missing vias and solder voids. It also allows for an in-depth examination of components, such as decoupling capacitors. Taking X-ray inspection into account during PCB design can help optimize inspectionability and improve quality.
During X-ray inspection, an X-ray source passes through the assembled circuit board to be inspected and is detected by a detector. The X-rays are then absorbed by the materials in the PCB and analyzed by a computer, which converts the data into an image. X-ray inspection systems are available with 2D and 3D imaging capabilities. They can be operated online or offline, with or without magnification.
X-Ray source – This generates X-rays using high voltage across vacuum diodes or radioisotopes. A control collimator shapes the X-ray beam for the application. PCB manipulation system – This moves the board through the X-ray beam for inspection. This could be either a high precision robotic stage or an operation platform that can position and move the component from all directions and angles. Detectors – These are sensor arrays that capture the X-rays and convert them to electrical signals. They are then processed by a computer to reconstruct and enhance the image. High-resolution monitors display the results.
X-Ray Inspection of Assembled Circuit Boards
The sensitivity of X-rays to different elements varies according to their atomic weight, allowing for the distinction between materials. Heavier components like solder absorb more X-rays and can be seen easily, while lighter ones such as the substrate and leads are less visible. This makes X-ray inspection an essential tool for inspecting BGAs, CSPs and other surface mount devices.
Some of the most common issues that X-ray inspection can identify include solder bridges, pin-hole fills and copper oxidation. Solder bridges are the result of two or more solder joints connecting together where they shouldn’t, which can cause physical damage to the assembly and reduced reliability. X-ray inspection is a highly effective method of detecting solder bridges, as it can clearly show the extra connections that have been formed.
Insufficient pin-hole fills for through-hole connectors are another problem that X-ray inspection can easily detect. These issues can lead to short circuits and other malfunctions, but they’re easy for X-ray inspection to spot by comparing the pin-hole fill to the original CAD model.
Other defects that X-ray inspection can identify include leaking capacitors and incorrectly placed decoupling capacitors. These capacitors are important for reducing transients and oscillation, as well as ensuring power supply stability. However, engineers must carefully place them to prevent the capacitors from getting too close to the power supply and causing overheating or other problems. X-rays can help to determine the correct positioning of these capacitors to avoid any issues during production.