Advanced X‑ray Inspection for High-Precision Circuit Boards

In the circuit board industry, an increasing number of parts and boards are proving to be difficult to inspect with automated optical inspection (AOI) because the solder is invisible. Furthermore, high-quality requirements such as bonding strength of the automobile industry and full surface inspection of the solder are increasing. To address these needs, Omron has introduced new technology for accomplishing inspections within the required inline take time (the rate at which a product must be completed to meet customer demand). This has been one of the most challenging requirements for computed tomography (CT) X-ray automatic inspection equipment. For continuous imaging technology, highly accurate positioning control and high-speed image sensing are required.

The Case for a New Inspection Method

In recent years, remarkable technological advancements have been made in electric vehicles (EVs), advanced driver-assistance systems (ADASs) and even automated driving. For the world of circuit board mounting, this means a move towards further densification, while an increasing number of parts and PCBs have visually inaccessible soldered joints that cause difficulties in visual inspection. Typical examples of these include fillet-less chips and ball grid arrays (BGAs) with solder joints arranged on the underside of the package.

The automotive industry imposes particularly stringent quality assurance requirements to protect consumers, and suppliers are often required to perform inline full-surface circuit board inspections (rather than sampling inspections) and measure solder shapes and inspect down to bonding strength. Compounding this is the problem of line worker shortages, which is partly responsible for the current rapid increase in demand for high-precision, high-quality automated inspections.

Hence, events in the mounting industry like circuit board quality issues and production line stoppages can pose serious risks to customers. An outflow of defective circuit boards would immediately lead to a crisis that could threaten the safety of people and society. For this reason, it is more important than ever to provide a mechanism that prevents any outflow of defective circuit boards to the market.

In response to these trends, Omron developed its AXI (automated X-ray inspection) system, which has become widely used in surface mount technology (SMT) production lines thanks to its ability to inspect visually inaccessible items like solder joints on the underside of parts. Because of the problem with takt time, however, a conventional model has been used mainly for offline sampling inspections or for inline inspections of key parts only.

This article presents an outline of the technologies employed for the VT-X750 Series automated inline CT X-ray inspection system ( Figure 1) to improve this problem and achieve speeds sufficient for inline use in automotive circuit board mounting processes, thereby allowing quality assurance of circuit boards in large quantities.

Achieving High Image Quality with CT-Based AXI

The major types of X-ray-based diagnostic imaging methods include two-dimensional (2D) X-ray, tomosynthesis, and computed tomography. The 2D X-ray method is used to obtain one image per shot with an X-ray source, a workpiece, and an X-ray camera arranged vertically ( Figure 2). The image projected by this method is recorded as two-dimensional data. While capable of image acquisition in a shorter time, this method is inferior to the other methods in terms of image quality because the amount of data that it handles is small.

The tomosynthesis method is used to obtain a certain number of images of a workpiece in a relative position to an X-ray source or X-ray camera within a limited angular range. This method allows for the acquisition of tomographic images with the desired heights highlighted ( Figure 3). Although it is more time-consuming than the 2D X-ray method, tomosynthesis enables faster image acquisition than the CT method and is superior to the 2D X-ray method in terms of image quality. It must be noted that if tomographic images are captured at a far enough distance from the focus position of the X-ray source or camera, they tend to be blurrier than CT images.

The CT method is used to obtain a number of images of a workpiece in a relative position to an X-ray source or camera during a 360-degree rotation and reconstruct them into three-dimensional (3D) data. This method handles a larger volume of data than the other methods and therefore provides the best image quality. Its strength is that it enables the extraction and use of not only horizontal planar direction data, but also height direction data from the restructured 3D data. Even when captured far from the focus position of the X-ray source or the X-ray camera, a tomographic image using this method will have a clear, low-blur image quality. On the other hand, this method takes more time for image acquisition and usually delivers a higher dose to the workpiece.

The AXI Solution

Omron adopted a new inspection method that can identify the desired points in 3D data and perform image-based diagnosis to accurately inspect the shape of each solder joint surface. The Omron AXI solution takes advantage of the CT method and enables high-precision inspections that are free from circuit board underside restrictions. Its major technical components consist of hardware that is capable of safe, high-precision sensing, along with software that allows for high-speed control with excellent responsiveness.

The hardware consists largely of mechanical, electrical, and imaging components. Therefore, the design parameters — such as electromechanical safety, shielding, axis motion accuracy, control responsiveness, image quality, and imaging rate — play an important role in ensuring system performance. The software part of the system consists of assembly optimizer for machine difference corrections, a main application for inspection program development, a reconstruction process for turning captured images into 3D data, and an algorithm used to perform the inspections of the obtained 3D data. These technical components are related to each other in a complex manner and must work together seamlessly within each function module for high-precision, high-speed inspections. This is particularly important for high-quality CT image acquisition, which is the core of this technology and provides the basic performance of the imaging devices, high-precision geometry design and control, and robust correction processing and inspection algorithms.

The following sections take a look at each of these capabilities.

1. Basic Performance of Imaging Devices (FPD and X-ray .