Experimental and numerical study on thermal behavior of space equipment by using two resistor model

Abstract

In this study, the qualification level Thermal Balance Test (TBT) performed on the On-Board Computer (OBC) developed within the framework of the satellite programs of the Turkish Aerospace is explained. The numerical solution developed using Simcenter 3D is explained. Test and analysis correlations are made, results and recommendations are explained. The IPS module, the power module of the OBC, has been tested in a vacuum environment. The main reason for choosing the IPS module is to evaluate heat generation and heat dissipation and thermal design. A IPS module (carrier and PCB), cover and baseplate are included in this work. It takes a lot of time to include every component on the PCB in the thermal analysis. For this reason, only components that emit heat above 50 mW are included in the analysis. The aim of this study is to validate the thermal analysis. This study does not contain electrical information. IPS module connections were chosen as AISI 310 stainless steel. According to the data sheets, thermal pads are placed under the components that require thermal pad application. In addition, thermal pad was applied between the carrier and the baseplate. Aluminum bulk structures had to be placed under 2 components due to supply problems. The DC-DC converters, which dissipate a lot of heat, are located in the lower part of the PCB. In order to conduct heat, the upper surface of the PCB under these components is left as copper. The layout of the other components is placed in such a way that they do not heat each other thermally according to the electrical constraints. The carrier design has been meticulously made with ECSS requirements in mind, to ensure mechanical strength and transfer heat to the baseplate by conduction. The baseplate is designed to hold the backplane and supporting all boards, dissipating the equipment heat to the satellite panel. It is also used to support the baseplate cover and carriers, hold them together and provide thermal conduction. There are 8 mounting holes in total for OBC equipment. The equipment is designed with these interfaces to be mounted on the satellite panel and the thermal plate in the thermal vacuum chamber. For this equipment to be used in the satellite system, the space environment is reflected in the thermal vacuum chamber. The qualification temperature range of the equipment is [-30°C, +60°C]. Hot case of qualification level was tested within the scope of this thesis. The thermal balance test was performed for the hot case, because in previous thermal camera measurements and analysis showed that the components were overheated. The thermal balance test was carried out at the Assembly, Integration and Test Center (AIT) of the Turkish Aerospace company. The HVT400 thermal vacuum chamber inside the clean room at AIT is used for equipment testing. Thermocouple placement was done before the equipment was put into the TVAC. Thermocouples are placed on components that emit heat above 50mW, on the PCB 1 mm away from the components, near the bolt connections, on the cover and on the baseplate. A total of 33 thermocouples are placed on the equipment. The IPS module is mounted on the plate in the thermal vacuum chamber using appropriate interfaces. After the necessary checks and connections are made, the door is closed. First, the pressurization process was carried out. The pressure in the chamber was reduced to 10-5 mbar. Tests were performed for 60°C with certain power configurations. Thermal stabilization criteria was determined as 0,5°C/hour for 60°C. The thermocouples data were read, the room temperature and pressure were normalized, and the door was opened. Analysis was performed using Simcenter 3D software. The Thermal/Flow solver was chosen for the solution of the thermal analysis. The Space Systems Thermal Solver ignores convection and creates a suitable solution environment for the space environment. One of the advantages of Simcenter 3D software for electronic equipment is that PCB material properties can be calculated. PCB layers, percentage of dielectric and copper material and layout can be seen, therefore the thermal conductivity is calculated by the software. Thanks to its automatic meshing feature, PCB and components are created automatically in 2D. As the analysis method, two resistor thermal modeling method was chosen among the compact modeling methods. The purpose of choosing this method is that it is simple and the data can be easily accessed. Simcenter 3D offers 2 different methods for modeling with two resistors. First; resistors from junction to case and from junction to board; the second: the resistors from junction to case and from case to board. These resistance values are calculated according to JEDEC standards. These resistors are supplied from component data sheets or manufacturers. In this study, an analysis model was created by using the resistors from junction to case and case to board. These resistance values were obtained from datasheets and manufacturers. Various assumptions were made for the values that could not be found. Other parameters required for modeling are maximum junction temperature, maximum case temperature, and heat dissipation of the components. The maximum junction temperature value and the maximum case temperature value are usually obtained from the datasheet. The heat dissipation values were obtained from the hardware design team working in the Turkish Aerospace company. The heat dissipation value calculation is not included in this thesis. Afterwards, boundary conditions were entered into the analysis. Ambient temperature, thermal plate temperature and radiation are given as boundary conditions. In order to calculate the radiation, the emissivity values of the components, aluminum parts and PCB were included in the analysis. View factors were calculated with the help of enclosure radiation property of Simcenter 3D. Three separate view factor values were assigned for the open sides and the closed side of the equipment. The conductivity value between materials depends on the material to fill the gap, surface roughness, material, surface finishing method and pressure. These values are usually obtained experimentally. As a result of the researches, many conductivity values were found for steel, aluminum and copper. Optimized results were obtained by performing several iterations. Finally, the analysis was reached to an acceptable level as a result of various iterations. The difference between test and analysis results is determined as 10% for acceptable level. This is well accepted value among the companies which conduct equipment thermal analysis. As a result, the correlation of analysis and test results has been completed.