Non-Destructive Failure Analysis and Measurement for Molded Devices and Complex Assemblies with X-ray CT and 3D Image Processing Techniques

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1 SINCE2013 Singapore International NDT Conference & Exhibition 2013, July 2013 Non-Destructive Failure Analysis and Measurement for Molded Devices and Complex Assemblies with X-ray CT and 3D Image Processing Techniques Xiaoming YIN 1, Seaw Jia LIEW 1, Ting Ying JIANG 1, Jian XU 1, Ramakrishna KAKARALA 2 1 Singapore Institute of Manufacturing Technology; Singapore Phone: , Fax: ; xmyin@simtech.a-star.edu.sg 2 Nanyan Technology University; Singapore; RAMAKRISHNA@ntu.edu.sg Abstract In both automotive and healthcare sectors, reliable failure analysis and accurate measurement of molded devices and complex assemblies are important. Current methods of failure analysis and measurement require these molded parts to be cross-sectioned so that internal features or dimensions can be accessible. As a result, the parts are deemed unusable and additional failure introduced by sectioning may cause misinterpretation of the results. X-ray CT and 3D image processing techniques provide a new nondestructive solution for failure analysis and measurement of molded devices and complex assemblies. These techniques simplify failure analysis and measurement of molded devices and assemblies, and improve the productivity of molding manufacturing significantly. Keywords: Failure analysis, molded devices, X-ray CT, 3D visualization, image processing 1. Introduction The need for three-dimensional analysis of failure during both process development and volume production has experienced a dramatic increase as new technologies, materials and processes are introduced to molding manufacturing. Cross-sectioning is a conventional failure analysis technique which mechanically exposes a plane of interest in a device or assembly for further analysis or inspection. It usually consists of sawing, grinding, polishing, and staining the specimen until the plane of interest is ready for optical or electron microscopy [1]. The conventional method of cross-sectioning requires the encapsulation of the specimen in plastic to give it stability, support, and protection. The positioning of the specimen in the mold during encapsulation is critical. It must be chosen well to minimize the sawing and grinding needed to expose the plane of interest. Cross-sectioning methods used to evaluate molded devices and complex assemblies are limited, time consuming and destructive. Occasionally, the results or measurements attained may not be accurate or misinterpreted due to loss of material or deformation of the component by sectioning. To improve and raise the inspection quality, the X-ray 1

2 imaging technique is proposed to provide an alternative and simple method for failure analysis and measurement non-destructively. X-ray imaging technique is a non-destructive technique to visualize the internal features within an object based on the difference in absorption characteristics [2]. The degree of absorption is dependent on the nature of material and the characteristics of the incident x- ray. Incident x-ray are influenced by the target material, the energy distribution and number of incident electrons. Varying the tube voltage changes the x-ray energy distribution and therefore the penetrating power. The applied current primarily influences the number of electrons and consequently the x-ray flux. Both current and voltage are the easiest variables to modify and are used to optimize the system resolution. This valuable tool can be applied in a 2D or 3D manner. In 3D or X-ray computed tomography (CT), the transmitted intensity of x-rays passing through an object from all angles in a plane through the object is measured and mapped relative to the X-ray linear attenuation coefficient. The mapped x-ray linear attenuation coefficient allows the 3D volume reconstruction of the object. Then the reconstructed 3D image of the object can be visualized and manipulated conveniently with 3D image processing software. With the introduction of X-ray CT and 3D image processing techniques, the inspection and measurement of molded devices and assemblies can be simplified greatly. As the X- ray technique is a non-destructive method, sectioning of the devices and assemblies becomes unnecessary and immediate information for possible internal failure can be attained easily. With the 3D image processing software, reconstructed 3D images of the devices and assemblies can be sliced virtually at various locations to observe their internal features. This tool is very useful for failure analysis of complex assemblies as the actual physical internal failed structure can be observed immediately. Measurement of critical dimensions and internal features of interest can also be carried out on the screen directly. We have developed 3D image visualization and processing software named 3DiPam, which can provide powerful 3D visualization and image processing functions. In the paper, we will demonstrate how the software can be used for failure analysis and measurement of molded devices and assemblies. 2. Failure Analysis and Measurement by 3D Image Processing Software 2.1 Visual Analysis Our software 3DiPam provides powerful functions to visualize and manipulate the volume data in 2D&3D format. The slice image can be displayed in different layouts in 2D&3D spaces and explored slice by slice, as shown in Fig 1. It can be rotated, paned 2

3 and zoomed in 3D space by the mouse control. It also can be translated, and zoomed in 2D space. The intensity and contrast of the slice image can be changed by slide controls on the control panel of the software. The volume data can be sliced at various angles and locations. By slicing the devices and assemblies virtually at different angles and locations, the devices and assemblies can be inspected thoroughly using only one sample. There is no need to prepare multiple samples for cross-sectioning at the different locations, which could be unfeasible sometimes because some defects are difficult to be duplicated. Fig 1 Display slice images in 3D&2D spaces 2.2 Volume Rendering 3DiPam provides several techniques for volume rendering of the 3D data set, as shown in Fig 2. The software also uses GPU technology to accelerate volume rendering. More, there is no need for any specific hardware configuration, the software takes advantage of the GPU capability of the NVIDIA graphic card directly, and accelerates volume rendering automatically. The rendered volume image can be rotated, paned and zoomed by the mouse control. It can also be cropped by using a rectangular Region of Interest (ROI). Pseudo-color can be mapped to the rendered volume image so that the internal structure of the device becomes more obvious, as shown in Fig 3. 3

4 With the CT scanning and volume rendering techniques, the internal structures of the devices and assemblies can be visualized and observed from all kind of positions and angles, there is no need to destructively cross-section the devices and assemblies any more. Fig 2. Volume rendering 2.3 Alignment (a) (b) Fig 3. (a) Crop the volume by a ROI (b) Map pseudo-color to the voulme When slicing and measuring the device, the orientation of the device is critical. Normally, it is required to be perpendicular to the datum plane. In the conventional cross-sectioning method, the orientation of the device is difficult to be guaranteed during encapsulation. In 4

5 CT scanning, it is also difficult to make sure the object is at the best orientation. However, with 3D image processing software, the orientation of the object can be aligned easily. 3DiPam provides 3 methods to do object alignment. Firstly, a planar surface is used as the datum plane. Some fiducial points are selected manually on this planar surface as shown in Fig 4. The software uses the selected points to fit a planar surface, calculate the transformation matrix, and aligns the object to the orientation perpendicular to the datum plane, as shown in Fig 5. Secondly, if the object has a cylindrical surface, the cylinder axis can be used as the datum line. Some fiducial points are selected manually on the cylindrical surface by clicking mouse on the screen directly, as shown in Fig 6. The software will use the selected points to fit a cylinder surface, calculate the transformation matrix, and aligns the cylinder axis of the object to the vertical orientation, as shown in Fig 7. If the object is not a cylinder, but is something like a cone, surface fitting cannot be done directly. In such a case, the third method based on the center axis of the object can be used for object alignment. In this method, some fiducial points are selected manually along the circle on one slice image, then the software uses the selected points to fit a circle and determine the center of this circle, as shown in Fig 8. The same procedure is applied to another slice image, and another circle center is determined. After 2 or more circle centers are determined, the software uses these circle centers to fit a line as the central axis of the object, and align the center axis of the object to the vertical orientation, as shown in Fig Surface Model Generation 3DiPam can generate the surface model of the object. Before surface model generation, the object needs to be segmented to create a label map. In the label map, each connected region in the volume image is set to a unique label value. 3DiPam provides manual and automatic methods for object segmentation. In the manual method, the user can label each voxel by dragging or clicking mouse on the slice images. In the automatic method, the software uses some image processing methods, such as ostu automatic segmentation algorithm, to segment the object automatically. After segmentation, the label map can be used to generate surface models of the object, as shown in Fig 10. For each label value in the label map, one surface model is generated to represent a separate part of the device. Several models can also be combined into one model. The software provides powerful functions to manipulate the surface model, such as rotation, zooming, panning, clipping, back face culling etc. The opacity of the model 5

6 Fig 4 Select fiducial points on a planar surface for object alignment Fig 5 Align the object by fitting a planar surface 6

7 Fig 6 Select fiducial points on a cylindrical surface for object alignment Fig 7 Align the object by fitting a cylindrical surface 7

8 Fig 8 Find a circle center for object alignment Fig 9 Align the object by fitting the central axis using circle centers can also be adjusted from 0 to 100%, so that the internal structure of the device can be observed more clearly. 8

9 In the future, the surface model could allow for a comparison of the device to CAD data. The deviations for both external and internal geometries can be shown on the surface color map chromatically on the 3D representation. 2.5 Dimension Measurement Fig 10 Generate the surface model of the object With the 3D image processing software, measurement of critical dimensions and internal features of devices and assemblies can be carried out on the screen directly. 3DiPam provides measurement functions for length, angle and radius. The measurement of length can be carried easily by manually dragging the end points of the length ruler to the points of interest in the slice image or on the surface model. The motion of the ruler end points can be constrained on the surface model or slice plane. Then the measurement value will be shown on the screen directly. This value is calculated based on the preset volume resolution as shown in Fig 11. In order to get the real dimension of the device, the X-ray CT machine needs to be calibrated to get the actual mapping between the voxel size and the physical dimension in advance. The measurement of angle is similar to that of length, dragging the end points of the angle ruler to the points of interest on the slice image or surface model, and the measurement value will be shown on the screen directly, as shown in Fig 12. 9

10 The measurement of radius can be applied to slice planes only. Some fiducial points are selected manually along the circle on one slice image, then the software uses the selected points to fit a circle and calculates the radius of the circle. After that, the software can also detect exact edge points of the circle using image processing method based on the previous result, and refine circle fitting by using detected edge points. Fig 11 length measurement Fig 12 Angle measurement Fig 13 Radius measurement 10

11 2.6 Defect Detection The segmentation method provided by 3DiPam can also be used for defect detection. By setting threshold manually or automatically, some internal defects of the device, such as inclusions and voids can be detected easily. The surface models of detects can also be generated and visualized in the 3D representation, so that the shape, size and distribution of the defects can be studied intuitively, as shown in Fig 14. Fig 14 Detect internal defects such as voids by image segmentation 3. Conclusion X-ray CT and 3D image processing techniques offer a new non-destructive solution for inspecting molded devices and complex assemblies with the following benefits: 1. Reliability of the molded devices and complex assemblies can be accessed without performing further destructive tests. 2. Immediate information for possible internal failure in the molded devices and complex assemblies. 3. Measurement of internal features of interest. 4. Improvement in the inspection quality. 5. Reduce inspection time while increasing productivity With the introduction of X-ray CT and 3D image processing techniques, the inspection and measurement of the molded devices and assemblies can be simplified greatly, and the productivity of molding manufacturing can be improved significantly. 11

12 Reference: 1. Cross-sectioning, 2. G. L. Clark, The Encyclopedia of X-rays and Gamma rays, Reinhold Publishing House, New York, (1963),