# Heat Transfer Optimization of an Electronic Control Unit Immersed in Forced Liquid Coolant
## 2 Experimental Validation  
### Electronic Module Description  
The reference electronic module used in the experiments, as shown in Figure 1a, consists of a PCB placed in aluminum housing and covered by steel upper housing. For monitoring junction temperatures, there are five temperature-measuring points: a PCB, a microcontroller (\(\mu\)C), two buck converter integrated circuits (ICs), and a two-phase DC-to-DC boost converter IC with serial peripheral interface (SPI), all of which have embedded thermal sensors. The microcontroller's roles include managing all LED functionalities, reading the temperature sensor on the PCB, detecting under- and overvoltage supply conditions, and managing LED drivers over the SPI bus. While the boost converter maintains a steady power supply for the LED drivers using the vehicle's battery supply voltage, the buck converter controls the LED current. Thermal Interface Material (TIM) is used under the PCB in the reference module, where components Boost, Buck1, and Buck2 are located. The compound between the PCB and the heatsink bridges the air gap, which otherwise prevents heat from moving efficiently. The reference module is cooled only by thermal paste, which is the most used cooling solution in automotive electronic modules. In this reference measurement, the main heat transfer path from the hot spot to the housing is via thermal paste only, as illustrated in Figure 1b. In the proposed experiment for this study presented in Figure 2, the main heat transfer path is via forced convection and molecular conduction of the fluid. When the surrounding fluid is a liquid, radiation from the heat-producing components is nonexistent. In this experiment, two PCBs were used for this system, as shown in Figure 2, which were immersed in a plastic housing. The electronic components are fully submerged in the dielectric fluid, which will act as a heat transfer medium between the electronic components and the surrounding air. The coolant liquid is circulated throughout the system by a low-pressure pump. The liquid is then cooled by passing it through a copper heat sink, over which a fan is mounted. Figure 1: Schematic of (**a**) reference electronic module exploded view and (**b**) heat transfer of reference module. The Design for Six Sigma (DFSS) methods were used to identify the most critical design parameters for the heat dissipation of the system. Requirements were identified, and QFD (Quality Function Development) assisted in translating them into product specifications. Thermal resistance from component to ambient (total thermal resistances), hot spot temperature, and pump flow rate are critical engineering parameters to meet the product criteria, according to House of Quality 1 (HoQ1). These three CtQs were carried over into House of Quality 2 (HoQ2) in order to determine critical design characteristics. Moreover, the development of hierarchical decomposition revealed that the system's principal purpose is to regulate inside-to-outside heat transfer, with a focus on forced convection and system optimization between conduction and convection. Additionally, the functional decomposition method was utilized to pinpoint how important design parameters affect fundamental function in the area of study. The design parameters identified with these methods are pump flow rate, fan airflow, and fluid volume from the housing, which are controllable in the investigation to analyze their influence on the hotspots. The values of the design parameters were fixed based on the equipment capabilities of these solutions (pump, heatsink, and fan) to resist a high ambient temperature. But the main goal of this study is to examine how these three design parameters interact with one another and to utilize the transfer function obtained from DoE to predict the hotspot temperatures using alternative design parameter values based on statistics. The fan has variable speed.