Thermal Stress Analysis of Contactless Energy Transfer Micro Module

The technology of contactless energy transfer is emerging for application in implantable medical device. High transfer efficiency and high quality factor are specific design requirements for micro-inductors which are key component of contactless energy transfer micro-modules. Thermal stress analysis and electrical analysis of an inductor are sequentially performed to elucidate the effect of thermal strain on the electrical properties of a micro-inductor. Three dimensional finite element models of an inductor were generated by the commercial stress analysis software, ANSYS, to calculate the thermal stress and thermal strain. The deformed model of the inductor was then imported to the commercial electrical property analysis software, ANSOFT, to calculate changes of the electrical properties, such as the inductance, the quality factor and the electrical resistance. The investigation shows that the thermal stress is concentrated at the four corners of the inductor with SU8 support, and that the inductor without SU8 support is twisted without substrate warpage. The electrical properties of inductors do not change dramatically under thermal loading caused by a temperature change from −40°C to 125°C.

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Optimum Design of Inductors Used for Wireless Power Transmission Micro-Module

Design Engineering ◽

10.1115/imece2004-59934 ◽

2004 ◽

Author(s):

Chao-Liang Chang ◽

Uei-Ming Jow ◽

Chao-Ta Huang ◽

Hsiang-Chi Liu ◽

Jr-Yuan Jeng ◽

...

Keyword(s):

Thermal Stress ◽

Electrical Properties ◽

Optimum Design ◽

Thermal Loading ◽

Power Transmission ◽

Thermal Strain ◽

Electrical Performance ◽

Wireless Power ◽

Wireless Power Transmission ◽

Analysis Software

The micro-inductor is a key component in wireless power transmission micro modules. In this paper, an optimum design for the micro-inductor was studied and related MEMS fabrication techniques were also developed. Commercial electromagnetic property analysis software, ANSOFT, was used to screen the main design factors of the micro-inductor. It was found that the high inductance and high quality factors of the micro-inductor implied high power transmission efficiency for the micro-module’s wireless power transmission. The electrical performance of the micro-inductor was affected by the thermal stress and thermal strain induced in the operational environment of the wireless power transmission micro-module. In order to investigate the reliability of the micro-inductor, commercial stress analysis software, ANSYS, was used to calculate thermal stress and thermal strain. The deformed model of the micro-inductor was then imported into ANSOFT in order to calculate its electrical properties. Glass substrate Pyrex 7740 was used to reduce the substrate loss of the magnetic flux of the micro-inductor. The surface micromachining technique, a kind of MEMS processing, was chosen to fabricate the micro-inductor; the coil of the micro-inductor was electroplated with copper to reduce the series resistance. The minimum line width and line space of the coil were 20 μm and 20 μm respectively. Polyimide (PI) was used for supporting the structure of micro-inductors. The maximum shear stress was 74.09MPa and the maximum warpage was 2.197 μm at a thermal loading of 125°C. For the simulated data, the most suitable areas for 31-turn and 48-turn coils were at an area ratio of 1.27 and 2, respectively. The electrical properties of the inductors changed slightly under thermal loading.

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