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Indentations in Microelectronic Bond Pads |
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Introduction
From the viewpoint of cost and reliability, the bonding of leads from the contact pads on a microelectronic device to the terminals in its package is one of the most critical steps. A common method of attachment utilises an ultrasonic transducer. In this method, the wire is brought into contact with the bond pad and then vibrated under pressure; this breaks through any surface oxide or contamination and effects a cold weld between the freshly exposed metallic surfaces. The ultrasonic vibration in the plane of the bond imposes a varying horizontal stress level with the maximum stress near the outer part of the contact. |
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| As device sizes continue to decrease and the number of connections (i.e., the pin count) to increase, in future devices it will be necessary to reduce both the sizes and spacing of bond pads. By the same token, wire bonding will require improvements in process speed. It is clear, therefore, that the bond pad structure should exhibit consistent and optimised mechanical properties. | |||||||||||||||||||||||||||||||||
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Results
Two specimens were investigated, each with multiple-layer bond pads of area 150 µm x 150 µm. Two indentations were produced in each specimen using the measurement parameters given below. The indentations were positioned using the NanoTest high resolution optical microscope, and were at least 30 µm away from the edge of the bond pad.
It is clear from the figures that whereas a well-behaved indentation response was obtained from the first specimen, the loading curves for the second specimen contained anomalous and reproducible discontinuities due to either adhesion failure or fracturing phenomena. |
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