Is Fully Automatic Bond Testing Possible? » Machine Vision

The last three of the requirements for full automation,

• Measurement of gross sample position (Fiducial camera alignment)
• Accurate automatic alignment for variations in bond position
• Automatic failure mode analysis

Fig 10. Deep Access. Parts of the sample restrict tool and fiducial camera access

are solved by a machine vision approach. They all require high resolution imaging of the sample with flexible illumination to assist a robust image analysis. Before we discuss each of the requirements in detail it helps to set out what is possible from state of the art machine vision.

An important issue that affects all bond testing vision requirements is that known as “Deep Access”. In many applications there are parts of the sample above the bond to be tested that obstruct access for both the test tool, camera optics and illumination (Fig10).

In a lot of bond testing, the bonds and associated features are small. These small features require microscopic imaging methods with a resolution in the order of 1µm. The deep access issue requires that imaging systems are a significant distance above the sample. The distance between the imaging optics and its target is known as the working distance. Some deep access requirements can be up to 45mm. Achieving a 1µm resolution with a working distance of 45mm is possible, but due to the laws of physics alone, the image quality can never compete with a small working distance microscope.

In addition to the camera optics, illumination plays an extremely important role in image analysis. In fact, camera optics typically set the image field of view and resolution that define useful magnification. The illumination affects contrast which is a very significant factor in image analysis. Two basic forms of illumination are possible with bond testing, Diffused and Coaxial, known as Bright Field and Dark Field respectively in the world of microscope systems (fig 11). The goal of diffused lighting is to illuminate the sample equally from all directions, coaxial along the axis of the camera only. Deep access prevents the lower angles of diffused lighting but does not affect coaxial.

Fig 11a Deep access sample

Diffused Intensity reduces radially in all directions and so is affected by both its vertical and horizontal position from the source.

Fig 11b None deep access sample

The illustrations in Fig 11 demonstrate the limitations of a diffused light source to illuminate a sample equally from all directions. Light intensity decreases from the center of its illumination and deep access restricts illumination from the lower angles. The ideal source would be very large (wider in the horizontal axis of this paper in the illustration but also normal to it in the 3 dimensional real world) and very close to the sample. In practice, its size is limited by other requirements important to the bond tester design and its height by the sample itself. In order to obtain illumination from the lowest angles possible, the diffused light source should be adjustable enabling it to be positioned as close to the sample as is possible depending on the deep access requirement (Fig 11b). Low angles of diffused illumination are required for the optimum contrast of rounded features.

The advantages of these two types of illumination are evident in the examples of the same sample, one taken with diffused lighting (Fig 12), the other coaxial (Fig 13).

Fig 12. Diffused illumination, focused at the height where the wires might be tested

Fig 13. Coaxial illumination, focused on the fiducial mark and pads. This is the same sample as Fig 12.

Diffused lighting, coming from many directions, highlights by direct reflection rounded objects like the wires in Fig 12. Coaxial, being on axis, highlights again by direct reflection, surfaces normal to the axis of the camera system. In both cases the highlighting effect is most pronounced when the surfaces are reflective. It should be possible to combine both types of illumination with independently variable intensities. The different illumination properties can then be selected or mixed to assist image analysis by increasing the contrast between the feature of interest and the other areas of the sample.

Continue to read:

1. Introduction
2. What is Required for Automation?
3. What is Possible with Modern Automation?
   
   • Multiple test heads
   • Accurate tool positioning
   • Automatic load and unload
   • Machine Vision
     
     • Measurement of gross sample position (fiducial camera alignment)
     • Accurate automatic alignment for variations in bond position
     • Automatic failure mode analysis (auto grading)
   • Test debris management
4. Non-Destruct Testing
5. Conclusion

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