Power densification and rising module heat losses cannot be managed by traditional “external-to-case” cooling solutions. This is especially pronounced in high voltage systems, where intervening layers of insulating material between the power devices and cooling solution need to be sufficiently thick to provide adequate voltage isolation. As operating voltages increase, the required thicknesses for these insulating layers become so large that they limit the ability to extract the heat. A direct cooling approach that addresses voltage separation issues represents a unique opportunity to deliver coolant to the hottest regions, while opening up the opportunity for increased scaling of power electronics modules. However technical concerns about long-term performance of coolants and their voltage isolation characteristics coupled with integration challenges impede adoption.&nbsp; In this study, we explore the durability and reliability of direct cooling to enable improved thermal management of high-voltage, high-power modules. Our approach focuses on a combination of dielectric fluids and dielectric surface coating for direct cooling, while providing adequate voltage isolation.The efforts on dielectric fluid robustness validation focus on fluorinated compounds including perfluorocarbons such as FC-72 and hydrofluoroethers such as HFE7500, which are non-toxic and non-combustible. Unlike hydrocarbons of paraffinic and iso-paraffinic type (including mineral oils), which are often used in cooling applications for systems such as transformers, these fluorinated fluids also exhibit low viscosities even at low temperatures, reducing pumping requirements. However, while these fluids exhibit high nominal breakdown voltage, discharge through the fluids is also dependent on flowrate and temperature, and understanding these behaviors is key to designing direct-cooled packages with adequate built-in safety factor in terms of thicknesses of the layout of power devices, traces, interconnects and terminals. As part of this effort, the voltage blocking characteristics of these fluids are characterized through relevant temperatures (22 oC – 105 oC), flow conditions (up to 4 LPM), and electric fields. The voltage shielding capacity of the fluids under these flow conditions is measured by monitoring the leakage current between a pair of electrodes, whose spacing is controlled to achieve a desired E-field intensity. The results of the characterization efforts will be used to develop design rules for direct integrated cooling schemes.As a measure to further reduce the risk of electrical discharge under direct cooling with dielectric fluids, this study also investigates conformal dielectric surface coatings. These coatings, which are applied via advanced deposition methods such as Chemical Vapor Deposition (CVD) are thin enough to not add significant thermal resistance, yet sufficiently thick to enable high voltage blocking potential. Tests considering the combined voltage blocking capability of the above-mentioned fluids and thin layers of Parylene C will be undertaken, with future aims to explore more thermally conductive ceramic-based dielectric coatings made of materials such as silicon dioxide or silicon nitride.

Performance and Durability Validation of Voltage Blocking Technologies to Enable Direct Cooled High-Voltage, High-Power Modules

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2021/volume/85505

ASME 2021 International Electronic Packaging Technical Conference and Exhibition (InterPACK2021)

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