Date of Award


Document Type

Master Thesis

Degree Name

Master of Engineering (Research)


Mechanical Engineering

First Advisor

Dr. Gerard Kelly


This thesis is concerned with the thermo-mechanical design, thermal modelling and temperature profile measurement of a 100 GHz flip chip bump bonded MMIC medium power amplifier.

Flip chip mounting offers superior RF performance in terms of output power, gain, noise figure and bandwidth. The thermal challenge was to dissipate approximately two watts of DC power from the GaAs MMIC (monolithic microwave integrated circuit) through Au (gold) stud bumps which equates to less then 7% of the total surface area of the die. Infrared thermographic images confirmed that the flip chip MMIC amplifier was functional with all eight FET stages on the die clearly visible and recording the highest temperatures. The primary circuit features of the MMIC were clearly visible on the thermal image which implies that GaAs is transparent to the infrared (IR) radiation detected by the IR camera. Infrared thermography is a potential method of characterization of flip chip GaAs devices (not seen before).

Various substrate material combinations were modelled using Finite Element Analysis tools in order to predict thermal gradients and heat flow paths through the structure. Junction temperatures between 14°C and 49°C were predicted with substrates made from BeO, AIN and AI2O3. Subsequent detailed geometric modelling of the structure using optimised Au stud bumps predicted a junction temperature increase of 21°C for the alumina substrate.

Infrared thermographic images of the flipped 100 GHz MPA reported an average FET temperature increase of 11°C. These infrared micrographs were taken looking ‘down through’ the 0.63mm thickness of GaAs die. When transmittance losses through the GaAs were taken into account by using a correction factor to modify the emissivity value on the IR camera, a corrected temperature increase of 17°C was recorded.

After extensive RF testing of the x4 multiplier amplifier module, the 100 GHz MPA MMIC was detached from the alumina substrate. Visual inspection under a high power optical microscope revealed no evidence of any thermal damage on the MMIC surface. It is clear that the thermal bumps located close to the FET’s extracted a significant portion of the heat from the chip reliably.

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