# Study on Thermoelectric Conversion and Conjugate Heat Transfer for PCBA by Finite Element Analysis
## 2 Literature Review  
The parameters for the design of LED lights include luminous flux, chip temperature, input power and cooling mechanism, etc. An excessively high operating temperature will decrease the performance of LEDs. Generally, the higher the luminous flux is, the higher the operational temperature is. Precise measurements are thus very important for the verification of LED lights' performance. It was pointed out that the factors such as heat flux intensity, interference by heat flux, forward voltage, must be carefully handled for an accurate measurement of LED lights [1]. In 2017, [PERSON] analyzed the temperature distribution for different arrays of AlInGaN LEDs. With the physical model developed for the LED array, [PERSON] built an expression for the thermal resistance. Using optical microscopy and a thermographic camera, the relationship between thermal resistance and current was established by [PERSON], and the actual temperature distribution was then obtained for the LED array [2]. Studies showed that the current density on the surface of the chip is critical for the performance of LEDs. According to a study by [PERSON], when the current density is controlled to be uniform, an extra 8% of luminous flux can be achieved [3]. As the size of the chips is gradually miniaturized, the heat density of the LEDs is accordingly increased. Thus, it is important for the trend of miniaturization for LEDs that the reliability and service life can be accurately predicted by the corresponding measurement and/or analysis. The topics of studies about LED lights include LED long-term reliability analysis, real-time temperature monitoring, measurement of their mechanical properties at high temperatures, and performance estimation of LEDs through TIMs temperature monitoring [4; 5]. Nowadays, the heat dissipation methods for the electronic components and LEDs in the industry are divided into two major parts: active heat dissipation and passive heat dissipation. Excellent heat dissipation means the high-quality performance of LEDs. However, the operation for measuring the performance of LEDs is complex and very time consuming. Instead of practical measurements, many studies have approached the study of LEDs by simulation. For example, [PERSON] used ANSYS-13 TMFFM to investigate chip-on-plate LEDs for solid state lighting and backlight displays. The results showed that a rectangular substrate has an effective thermal diffusion and uniform temperature distribution as compared to the circular and irregular substrates [6]. Sealed in a close and compact housing, the PCBAs of the modern electronic devices often suffer from overheating problems. In the housing, high heat conduction efficiency is necessary for producing a uniform temperature distribution over the PCBA. However, the little space between the PCBA and the shielding case will hinder the heat dissipation due to the poor heat convection capability [7; 8; 9]. New TIMs have been developed for solving the overheating problem, and are used for the up-to-date electronic devices with compact size. TIMs tuned with carbon nanotubes (CNTs) or graphene are proposed for heat dissipation. A study by [PERSON] demonstrated that the carbon nanotube-thermalinterface material (CNT-TIM) reduced the thermal interfacial resistance significantly compared with the state-of-art commercial TIM. The optimized thermal resistance of the CNT arrays is as low as 7 mm\({}^{2}\) K W\({}^{-1}\). The results indicated that the light output power was greatly improved with the use of the CNT-TIM [10]. In 2004, [PERSON] and [PERSON] at the University of Manchester used mechanical exfoliation to separate graphene from layered graphite. It was found that graphene has good mechanical properties which supported its prospective applications in academia and industry [11]. Single-layer graphene is often discussed in a theoretical framework, but in practical applications, multi-layer graphene is mainly used. Multi-layer graphene has about three to 10 layers, and the thickness of each layer is about 0.33 nm.