Seeing through the problems - real time microfocus X-ray inspection
Jannette Walker is a technician in the Microtechnology and Reliability Centre of TWI's Advanced Materials and Processes Department. She has been working on non-destructive testing using real time microfocus X-ray inspection systems for the last four years.
The requirement for rapid turnaround for trouble shooting and non-destructive examination of components has led to development of inspection techniques such as real time microfocus X-ray. Jannette Walker explains the principles of this technique and introduces some recent commercial applications.
Advances in imaging capabilities for real time X-ray inspection systems have, in recent years, allowed this type of examination to become a useful tool for a more diverse range of applications. The use of these systems for trouble shooting and quality control is growing rapidly and enhanced imaging capabilities incorporated within the software allows more detail to become visible. There are many such systems available but the equipment in use at TWI is an X-TEK VTX high performance real time inspection system with IXS™ software. The system can be used for batch processing or single item multi-axis examination.
When using conventional X-ray a burst of radiation is emitted over a wide area, the high-density materials absorb the X-rays to various degrees, and the image is imprinted onto thin film ( Fig.1). These systems are often housed in special areas and operators may require training in radiation safety techniques. In comparison, modern microfocus X-ray systems are commercially available packages, limited training is required for operators and as stand alone systems, locational problems are reduced.
In operation they project a continuous beam through a small window. The beam, allowing for equipment variations, is approximately 2-10 microns in size. The absorption pattern of the materials is collected by an image intensifier and passed to a video camera. The image is then displayed on a monitor attached to a PC ( Fig.2). The real time aspect and manipulation capabilities of these systems allow the specimen to be examined in a range of angles and at various degrees of magnification.
Fig.1 Conventional X-ray system.
Fig.2 A Microfocus X-ray system.
The adaption of this technology provides a fast and efficient way of detecting defects such as cracks, poor joint wetting, misalignment and deformities within structures. Whilst the equipment can be expensive, it does provide an inspection facility, readily applicable to a wide range of industries and components. Some examples of these are described below.
Case studies
Die attach
The setting of process parameters is a vital aspect of any product in the electronics industry, including dies which are attached to substrates by techniques such as soldering, (
Fig.3). Voids within the die attach medium can affect the thermal or electrical performance, or weaken the structure of the finished component. Microfocus X-ray inspection systems allow fast and easy detection of voids, calculations can also be made on the percentage of voids present. This allows fine tuning of the parameters before large scale production begins.
Solder joints
Solder joints can be found in many applications including electronics, automotive and white goods, and are particularly amenable to X-ray examination, since defects such as voids, misaligned leads and components, cracks and general joint quality can be readily inspected. Used in a trouble shooting capacity it is possible to assist companies rapidly with production problems.
Figure 4 illustrates a case where badly cropped leads were causing strands of material to stray across the solder joint and hence short the component. With quick identification of the problem, it is possible to resolve the defect.
Fig.3 Large die soldered onto substrate showing areas of non-wetting.
Fig.4 Shorts across solder joints.
Fig.5 Bridging in a ball grid array. Approximate ball size 1.27mm.
Ball grid arrays (BGAs)
The need to allow higher density population of boards has, in recent years, led to the increased use of BGAs. For this type of assembly, a series of solder balls are positioned in close proximity on the underside of the component. These are then used as the points of contact; however, with the interconnections located beneath the component inspection difficulties increase and X-ray is one of the few techniques available to BGA users. Figure 5 illustrates solder bridging of a double-sided board populated with BGAs. Missing solder balls, alignment voiding, general ball shape, size and quality can also be examined.
Wirebonds
Wirebonding is used extensively in the electronics industry to interconnect chips and dies to the main substrate, ( Fig.6). For optimum performance it is vital that the bonds are properly formed. X-ray allows missing, misaligned or deformed Au wire bonds to be identified quickly. By rotating the specimen, lifted bonds can also be exposed, a technique not available through conventional X-ray. The software used also allows the facilities for measurements of wirebond sweep (when the wirebond loops have been swept to one side due to a subsequent encapsulation process), bond diameters, wire size and pad sizes. For companies purchasing components this can also be a useful quality check.
Braze joints
In Fig.7, two silicon carbide based ceramic tubes have been joined using a metallic braze for a heat exchanger application. X-ray inspection has allowed examination of the quality of joint produced and in this example the areas of non-wetting (light coloured) within the brazed area are clearly visible. From this non-destructive examination, it is then possible to repair the fault by adding further braze into the joint area and then reflow it. This type of joint inspection has been regularly used as part of the design process, allowing the processing parameters to be optimised prior to full-scale production.
Structural defects
Internal failure of encapsulated samples such as sensor devices can also be detected. By applying this non-destructive testing technique, less obvious defects such as cracks in the structure have been identified. Figure 8 illustrates cracks in a ceramic tube which was encased in a plastic encapsulant material as a sensor. When the system failed it was impossible to examine the interior structure of the device with normal visual techniques; however, the use of microfocus X-ray enabled the component to be kept intact and the defect to be located and identified.
Fig.7 Brazed joint in a 35mm diameter tube.
Conclusions
Microfocus X-ray is now a well established procedure for the examination of miniature and hidden features. Improvements to the software incorporated within these systems will continue to allow increased clarity of small details, and a wider range of materials to be examined. Equipment is also in the experimental stage that may be able to detect the boundaries of non-conductive materials. In the future, this may allow more materials such as glasses, plastics and non-conductive adhesives to be inspected.
