Whatever the nature of a failure analysis task, optical examination linked to a digital camera is almost always required. This can be the simple matter of recording an image to show the initial state of the object or product before more destructive processes are carried out, or part of a more detailed examination of specific parts at a much smaller scale. Having a wide choice of imaging methods is essential if the demands of the wide range of failure analysis subjects are to be met.

Macro lens camera
Tripod mounted macro lenses are an invaluable starting point for recording both colour and topographic detail of subjects with macro zoom lenses allowing rapid changes of scale from several hundred millimetres to less than 1mm. In this setup changes of lighting angle can be helpful in highlighting changes in surface roughness. The use UV lighting can be helpful in some situations.

Figure 1 shows an image of a printed circuit board which is a typical failure analysis subject. This is a whole circuit board in top-down view. Such images are essential as a point of reference for further examination. Annotated images of this type, particularly of complex objects, can make report references much less ambiguous; indeed in this context such images can be worth a thousand words.

Figure 1 Typical survey image using tripod mounted macro lens camera

Extended field camera
Not all subjects are flat however and maintaining sharp focus over a significant depth of field can be a problem for larger objects. For such subjects, the use of extended field cameras can be helpful. Figure 2 shows the same PCB viewed from one edge using and extended field camera and, despite the severe angle, components at both sides of the board remain in reasonable focus.

Endoscopic Camera
Focus is not the only problem with complex shapes as imaging in awkward locations such as within a small recess can also be difficult. In such situations the use of an endoscope-based camera is a useful option. These are small enough to fit into fine holes and can deliver sufficient light to record useful images. The utility of such a camera can be shown again using the same PCB. Figure 3 shows a detailed view inside the orange connector block seen on the right in Figure 2 and delineated by the red box in Figure 1.

In this case the image shows no evidence of flux ingress at the base of the pin or other contamination on the pin surface. Such a camera can be inserted into hole with diameters greater than 3mm but can give useful images into spaces that are even smaller. The use of an angled mirror adapter can be used to obtain profile images of devices in congested situations. Options that are particularly suited to the non-destructive assessment of assembly quality, examining solder wetting and fillet shape.

Figure 3 View of pin connector in pcb

Stereomicroscope camera
Beyond the range of macro lens, cameras mounted on stereomicroscopes come into their own. providing images of object from a few centimetres across to less than 0.1mm bridging the gap between the macro and micro world. Ring lighting mounted just below the object lens gives the best all rounds illumination and colour fidelity.

Figure 4 and 5 show typical higher magnification inspection images of the PCB using a stereomicroscope - the first a single device with excess solder at one end, and the second showing evidence of residues between components.

Figure 4 Surface mount component on PCB showing excessive solder at one end

Figure 5 PCB showing evidence on residual contaminants on the board

Reflected light microscope camera
Higher magnification is of course possible using digital cameras mounted on metallurgical microscopes but with magnifications between 50 to 1000 times the depth of field is now extremely small and usually only applicable to specially prepared parts or sections. Figure 6 show a typical image of a solder joint taken directly from a PCB using a metallurgical microscope. This is a compromise mid-field image showing the disturbed surface of the solder but the lead and pad surfaces are not in focus. However the use of appropriate image software can overcome focus difficulties and provide extended depth of field at this smaller scale.

Figure 7 shows the impact of extended field software with all parts of the joint in focus. This was obtained by taking several images at different focal heights and using the software to combine these into a single images with optimised focus throughout.

Figure 6 Image of solder joint from reflected light microscope - at this magnification the limited depth of field makes it impossible to get the whole image in focus

Figure 7 Image of the same joint as in figure 6 using software to combine multiple image “slices” to form an image which is in-focus throughout

This technique can be beneficial when examining curved surfaces, via holes or fracture features, in fact any subject whose sample height exceeds the normal depth of view.

An alternative imaging technique
Images with a depth of field 300x that of an optical microscope and surface topography unaffected by reflected light can be obtained directly using a scanning electron microscope.

Conclusion
In the real world of course there is no one right camera as most failure analysis subjects will be examined several times for several reasons and at a range of different magnifications. At ERA we regard it as important to be able to deploy all these imaging systems as appropriate in order to maximise the information recovered during the wide ranging failure analysis problems we undertake. To find out more call us on +44 (0)1372 367444 or email .