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January 1997

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Mr. Stewart,

In my experience, it is very difficult to consistently attach gold  bonds
to a gold film 0.07 um (3 microinches) thick.  Nickel tends to diffuse
through a gold layer this thin, oxidizing at the surface.  Aluminum wedge
bonding seems to be more forgiving.

I believe that 0.5 um (20 microinches) pure (soft) gold is near the minimum
thickness that I would use for thermosonic gold bonding.

I think that the 5 um (200 microinches) of nickel is what the U.S. military
specified as a diffusion barrier for reliability purposes.  I am not sure
about the reliability issues, but one can gold wire bond to 2.5 um (100
microinches) or less.

A reference text would be:

Reliability and Yield Problems of Wire Bonding in Microelectronics by
George Harman.

Dave Anderson

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