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Thermally Driven Diffusion in Copper-Wirebonded Packages
Keywords: packaging, reliability, copper diffusion
The desire for lower-cost packages have led to the consideration of copper wire as an alternative to the widespread use of increasingly costly gold wire in flash memory packages. However, as has been discussed extensively in the literature, copper has a tendency towards electromigration under applied voltages. Therefore, the possibility of wire shorts is a significant concern of the copper wirebonding process. This paper investigates the role of package temperature in driving such diffusive shorts. The paper begins with a review of prior work in the literature on electromigration in general, and copper-related diffusion in particular. We propose simplistic diffusion models for predicting the rate of copper migration, and find that the migration tendencies are significantly lower relative to that of gold wire. The role of temperature in these diffusive models is then isolated, and the predictions from the model are compared to experimentally induced failures with good agreement. The guidelines thus established by the model are then utilized in conjunction with simple thermal resistance models to estimate the maximum allowable power dissipation within the package. Thus, the paper provides an indication of the limitations of passive cooling for copper-wirebonded flash memory packages, and suggests a power limit beyond which some sort of active cooling may be necessary to prevent thermally induced diffusive failure in the package.
Amip Shah, Research Scientist
Hewlett Packard
Palo Alto, CA

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