The fluid mechanics of bonding with yield stress epoxies

Kenneth John Zwick, University of Pennsylvania

Abstract

An important component of most microelectronic packages is the silver filled, thermally conductive epoxy, which forms the bond between the microelectronic chip and the metallic substrate called the lead frame. As microelectronic chips become larger, more epoxy is needed to form this bond. This presents two fundamental heat transfer and fluid mechanics problems. (i) The time necessary to form the bond becomes very long even for modest increases in chip size. (ii) Curing the larger volume of epoxy leads to higher concentrations of epoxy volatiles, which can contaminate sensitive chip surfaces. The flow associated with the formation of the epoxy bond has been investigated both analytically and experimentally. Squeezing flow experiments have shown that the epoxy behaves as a yield stress fluid. A variational analysis of the flow reveals that the yield stress nature of the epoxy makes the quick formation of thin bonds difficult. However, further experiments have shown that oscillating the magnitude of the bonding force reduces the apparent yield stress by breaking down the fluid microstructure, and dramatically reduces bonding time. A detailed numerical and experimental analysis of the flow, heat transfer, and mass transfer in an oven used to cure the epoxy is also presented. A control volume numerical analysis shows that condensation and contamination may be best avoided by routing an evacuation flow through the oven so as to maintain the partial pressure of the epoxy vapor below its saturation pressure. The numerical results were validated by a flow visualization study on a 10$\times$ upscaled model of a typical curing oven.

Subject Area

Mechanical engineering|Fluid dynamics|Gases|Electrical engineering

Recommended Citation

Zwick, Kenneth John, "The fluid mechanics of bonding with yield stress epoxies" (1996). Dissertations available from ProQuest. AAI9628038.
https://repository.upenn.edu/dissertations/AAI9628038

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