Toshio Tomimura

Evaluation of Effective Thermal Conductivity of Multilayer Printed Circuit Board

To perform a rational thermal design of a printed circuit board (PCB) with highly anisotropic heat transfer nature in its initial stage, effective thermal conductivities in thickness direction and in in-plane direction must be given depending on the electric circuit of the board. However, a simple evaluation method for the effective thermal conductivities of such PCB has not been developed yet. In this study, as the first step to propose a simple evaluation method, the heat transfer coefficient by natural convection around a horizontal disk, which is indispensable for measuring the effective thermal conductivity, has been evaluated. Furthermore, the thermal conductivity of the glass epoxy resin in in-plane direction has been evaluated by applying the evaluated heat transfer coefficient, and then, the validity of the proposed thermal conductivity measurements of the anisotropic PCB has been confirmed.Copyright

Investigation of effect of wiring patterns on in-plane thermal conductivity of printed circuit boards

This paper describes a measurement method for the in-plane thermal conductivity of Printed Circuit Boards (PCBs). We designed two types of PCBs with several wiring patterns on their surfaces. This means copper amount on the PCBs is different. We measured their effective thermal conductivity in thickness direction to investigate the effects of the wiring patterns on the in-plane thermal conductivity of the PCBs. One is normal PCBs and the other is about 18 times larger PCBs than the normal PCBs. The experimental results showed that the thermal conductivity of normal PCBs was not dependent on the wiring patterns. On the other hand, the thermal conductivity of larger PCBs increased with increasing amount of copper wire due to the heat diffusion in in-plane direction by copper wires. We concluded that the effect of the wiring patterns on the in-plane thermal conductivity can be observed with our measurement method. We also performed Computational Fluid Dynamics (CFD) analyses and clarified the correlation between amount of copper wire and in-plane thermal conductivity of the PCBs.Copyright

Experimental study for method to measure terminal part temperature of micro-electronic devices using infrared thermograph and image processing

Concerning surface mount resistors, which are one of micro electronic devices, measurement of the terminal part temperature measurement is important from the standpoint of thermal management. When the electronic device designer uses the infrared thermograph to measure the approximate value of the terminal part temperature of the surface mount resistor, the surface hotspot of the resistor will form a so striking contrast in the thermograph image that the terminal part will be hard to be measured. The possible method for overcoming this problem is to apply a proper cutoff frequency filter to the obtained temperature distribution image, and then smooth the hotspot of the resistor (locally high temperature portion of resister surface) down near the terminal part temperature level, and finally read this hotspot temperature as the terminal part temperature. The possibility mentioned above will be discussed in this study.

Measurement Technique of In-Plane Thermal Resistance of PCB

Nowadays, thermal design of electrical packages is a key issue for the development of electronics. One of the most important thermal design is the estimation of temperature distribution on Printed Circuit Board (PCB). However, estimated temperature distribution by Computational Fluid Dynamics (CFD) analysis has not enough accuracy because of unknown in-plane thermal conductivity of PCB. PCB has complex wiring pattern and the effect of copper wire on heat diffusion has not been investigated. In this study, we developed measurement technique of thermal resistance of PCB without and with in-plane heat diffusion effect. Measured PCB consists of Glass-Epoxy of 1.6 mm thickness and various copper wiring patterns of 35 μm thickness. Measurement results showed that thermal resistance without inplane heat diffusion effect was not dependent on the wiring patterns. On the other hand, thermal resistance including in-plane heat diffusion effect became smaller with increasing the amount of copper. Further, we compared the experimental results with the results of Thermal Network Method (TNM).Copyright

Application of Excel and thermal network method to thermal analysis of electronic equipment

In the field of scientific computing, Basic, C, and Fortran are widely used all over the world. Lots of beginners and students, however, suffer problems in making simulation programs due to inexperienced and complicated computer programming rules. Under such circumstances, some practical examples solved by using the spreadsheet software like Excel have been reported recently [1–3], where just by using equations written in each cell, calculations are performed without making any program. In the Excel analysis, writing and debugging are visually conducted by simply making use of worksheet analysis, and iterative calculations are carried out automatically until the prescribed convergence criterion is satisfied. Concerning calculated results, numerical values shown in the computational domain are immediately visualized using the chart wizard. Therefore, by using the spreadsheet software like Excel, even the students and younger researchers who have just started to study numerical analysis can perform calculations easily and quickly without studying any special knowledge about the computer language, and in addition without writing lengthy computer programs. In this study, for the purpose of introducing the abovementioned useful method to thermal analysis of electrical apparatus, electronic equipment and so on, the calculation process by thermal network method has been shown by using the concrete example of a compact fluorescent lamp.

Fundamental study of surface roughness dependence of thermal and electrical contact resistance

For thermal management of electrical equipment, thermal contact resistance is one of the important parameters. However, thermal contact resistance is dependent on various factors, for example surface roughness, the contact pressure and the hardness of the material. Therefore, quantitative evaluation is difficult. Nowadays, CFD (Computational Fluid Dynamics) analysis is widely used in thermal design of electronics. However, unknown thermal contact resistance is always a problem for accurate temperature estimation. In this study, we examined surface roughness and material hardness dependence of thermal contact resistance and electrical contact resistance for simple estimation of thermal contact resistance. Measurement of thermal contact resistance takes a long time and electrical resistance measurement is much shorter. If thermal contact resistance can be estimated from electrical contact resistance, thermal contact resistance can be known in short time, and this method can support accurate CFD analysis. The materials to be measured are Al1070 and S45C, and three patterns (Ra = 0.2, 3.2, 12.5 μm) of surface roughness are examined. After the measurement of thermal and electrical contact resistance, we examined the ratio between electrical contact resistance and thermal contact resistance for the faster estimation of thermal contact resistance using the concept of Wiedemann-Franz law and Lorentz number like experimental constant.

Approaches for improving peak temperature detection capability of infrared thermograph

Recently, measurement of surface temperature with thermocouples has become difficult because of the advanced downsizing of electronic components. On the other hand, the infrared thermographs became cheaper and easier to handle. As a result, the electronic device designers, who use the infrared thermograph to measure the surface temperature of small electronic components, have been increasing. However, the relation between the photographic subject area corresponding to one pixel of the infrared thermograph detector and the minimum area in which the peak temperature can be detected (here, it will be referred to as a peak detection capability) is not well known, and some improper use of the infrared thermograph are seen occasionally. In our previous study, we proposed a method by which the peak detection capability of the infrared thermograph can be easily clarified, where the differences in emissivity of a flat plate with uniform temperature was applied and the peak detection capability was evaluated by the cut off spatial frequency of the Gaussian filter. The present study is its application. That is, a simple method to enhance the peak detection capability has been proposed. In the method, the inverse filter suitable for the infrared thermograph is applied.

On simple prediction method for thermal contact resistance between wavy surfaces with thermal interface material under low mean nominal contact pressure (fundamental study based on 1-D model)

The thermal contact resistance (TCR) is the crucial issue in the field of heat removal from systems like electronic equipment, satellite thermal control systems, and so on. To cope with the problem, a lot of studies have been done mainly for flat rough surfaces. However, as pointed out so far, there are still wide discrepancies among measured and predicted TCRs, even for similar materials. To investigate the key factors for the abovementioned discrepancies, a fundamental analysis was conducted in our previous study [1] using a simple contact surface model, which was composed of the unit cell model proposed by Tachibana [2] and Sanokawa [3]. Furthermore, by introducing a 2-D microscopic surface model, which consists of random numbers and Abbott’s bearing area curve, the effects of surface waviness and roughness on the temperature fields near the contact interface have been investigated microscopically [4]. In this study, based on a 1-D wavy surface model, a fundamental study has been conducted to predict TCR and the thermal contact conductance (TCC), which is a reciprocal of TCR, between wavy surfaces with the thermal interface material (TIM) under a relatively low mean nominal contact pressure of 0.1–1.0 MPa. From comparison between the calculated and measured results, it has been shown that, in spite of a simple 1-D analysis, the present model predicts the temperature drop at the contact interface, which is obtained as the product of TCR and the heat rate flowing through TIM, within some 10 to 60% error for a TIM with the thermal conductivity of 2.3 W/(m·K) and the initial thickness of 0.5, 1 and 2 mm.Copyright

Fabrication of a Vapor Chamber on a Plastic Board

A vapor chamber is a flat-plate heat pipe, where a cooled (condenser) section is much larger than a heated (evaporator) section, and has been used as a heat spreader to enhance the cooling of electronic devices. An objective of this study is to integrate the vapor chamber into a polycarbonate board. Plastic materials are easy to manufacturing, light weight, low cost, flexible, and then the present study aims at performing a phase-change heat transfer and a heat spreading inside the polycarbonate board. A sintered copper powder and water are used as a wick structure and a working fluid, respectively. In experiments, the heat is applied by a heater while the cooling water is circulated between a thermostatic bath and a cooling jacket. The experiments are conducted changing a liquid volume and a heat input, and the transient temperature distribution of the vapor chamber is measured by thermocouples. For comparison, the experiment is also conducted where the working fluid is not charged into the vapor chamber. Moreover, based on a thermal resistance network, an analytical model of the vapor chamber is made and the analysis is performed on the phase-change heat transfer inside the vapor chamber. From the experimental and analytical results, the heat transfer characteristics of the polymer-based vapor chamber and the effectiveness of the phase-change heat transfer are confirmed.Copyright

A capillary-wick heat pipe fabricated on a plastic board (Fundamental experiments on heat transport characteristics)

A capillary-wick heat pipe having the dimensions of 5.0 mm × 5.0 mm × 100 mm (length) is fabricated on a surface of a plastic board, and the experimental investigations are conducted on the operational characteristics of the heat pipe. Plastics are easy to manufacturing, lightweight, low cost, flexible, and besides, the present study aims at the phase-change heat transfer inside the plastic board. A sintered copper powder and water are used as the wick structure and the working fluid of the heat pipe, respectively. In experiments, an evaporator section of the heat pipe is heated by a heater while a condenser section is water-cooled by a heat sink. A heat input and a liquid volume inside the heat pipe are changed, and the temperature distribution of the heat pipe is measured by thermocouples. Moreover, a one-dimensional thermal circuit model is made to evaluate the effective thermal conductivity of the heat pipe. From the experimental results, the continuous phase-change heat transfer inside the plastic board and its effectiveness are confirmed. It is also revealed that the effective thermal conductivity of the heat pipe is 854 W/(m·K) in maximum under the present experimental conditions.Copyright