Date of Graduation

12-2014

Document Type

Thesis

Degree Name

Master of Science in Electrical Engineering (MSEE)

Degree Level

Graduate

Department

Electrical Engineering

Advisor/Mentor

Ang, Simon S.

Committee Member

Balda, Juan C.

Second Committee Member

McCann, Roy A.

Keywords

Electronic Packaging; Parasitic Extraction; Power Module Thermal Analysis; Wirebond Reliability Assessment

Abstract

This thesis research investigates thermal performance, parasitic extraction and wirebond/encapsulation reliability of power electronic modules. Thermal performance is critical to the power electronic modules. As such, thermal analysis is an important part of the power electronic module design process. Several cases are studied on generalized power modules with a full bridge layout. A database is built based on these results. The studies are performed using SolidWorks thermal simulation tool. The database involves several parameters such as power dissipation, maximum junction temperature, ambient temperature, convection coefficient required to cool the module, size of the baseplate, heat-sink size, substrate size, spacing between dies, and different materials that can be used for the power electronic module. Using this database, procedures to select appropriate parameters in a thermally efficient layout for the power electronic modules are illustrated using examples. It was found that, for optimum performance, ∆T (maximum junction temperature - ambient temperature) should be greater than 125ᴼC for power modules with medium and high power dissipation. Also, for a low ∆T and high power dissipation, baseplate acts more like a thermal resistance than a heat-spreader. Hence, it is ineffective to employ base-plates for these cases. Increasing substrate size to bring down maximum junction temperature is more effective in higher power dissipation cases than those for medium or low power dissipation.

Parasitic extraction for an electronic power module using a time domain reflectometry (TDR) method in the form of differential inductance waveforms was developed. These measured parasitic inductance and parasitic capacitance are compared with the parasitic parameters extracted using a Q3D extractor software. The accuracy of the measurement results from these two different approaches is studied in detail.

Reliability of wirebonds in the case of encapsulated and un-encapsulated power modules were investigated by subjecting them temperature cycling from -55ᴼC to 250ᴼC. It was found that the solder flux affected the reliability of the wirebonds. As such, it is recommended that the power connectors on the power substrate should be free of flux, as the residual solder flux can affect the nearby wirebonds in the power modules. As expected, the differences in the coefficients of thermal expansion between the power substrate bond wires and encapsulation affect the reliability of the wirebonds. Large diameter wirebonds tend to be stronger and can withstand the stress and strain created by the different material systems in the power electronic modules.

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