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Development of a microjoining technique for interconnecting semi-conductor devices with non precious wires


1. Ball wedge bonding of Cu wires is possible in a comfortable process window. Bond strengths equal or surpass the values to Au wires. Reliability tests showed no failures.

2. Functional specifications with respect to the microstructure and surface condition of the bonding wires have been derived. A 6N starting material with specific dopants added to control the grain size is preferred.

3. Oxidation of copper wires has to be avoided, especially the high temperature form. Storage conditions of the wire spools have to be further defined, as well as functional requirements for the wire with respect to storage.
Voltammetry seems to be a simple tool to characterise the surface conditions of copper bonding wires. Calibration has to be done.

4. The results for Al ball wedge bonding are very promising. A 10.000 bonds experiment was done without machine stops. Even after 10.000 bondings, pull strengths are acceptable (mean values ranging between 10.4g and 11.2g) although always some necking above the sparked ball was observed.

5. A Ni-plated lead frame is advised as wedge bonding of Al on bare Cu was impossible and the interface Al/Ag is not stable.

6. In situ-resistance measurements of ball bonds to different metallisations show that CU-additions to the metallisation retards the formation of Au/Al intermetallics as well as void nucleation. Replacing Au by Cu results into a much more stable and reliable interconnect. Diffusion phenomena are shifted to higher temperatures with 50 to 100 C compared to Au ball bonding.


Philips Gloeilampen Fabrieken NV

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