REPORT SUBJECT RESEARCH METHOD TOPIC MICRO CHANNEL

ABSTRACT This research is to introduce the electronics design workflow and show the problem of heat transfer for electronic devices.. FEM Solvers Thermal analysis is used to determine t

MINISTRY OF EDUCATION AND TRAINING HCMC UNIVERSITY OF TECHNOLOGY AND EDUCATION

REPORT

SUBJECT: RESEARCH METHOD

Lecturer : PhD Dang Hung Son

PHAN HOANG BUU

LY TU CO

LECTURER REVIEW

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TABLE OF ASSIGNED WORK

Work

Word PowerPoint

1 Đinh Thế Duy Abstract, Introduction Abstract, Introduction

2 Phan Hoàng Bửu Problem statement Problem statement

3

Đào Quốc Duy

20147150

Theoretical approach (page7,8), Conclusions (page 11)

Theoretical approach (slide:8-13), Conclusions

(slide 22)

4 Lý Tự Cơ

20116016 Case studies ( page 6-8 ) Case studies ( slide

15-20 )

HEAT TRANSFER SIMULATION FOR THERMAL

MANAGEMENT OF ELECTRONIC COMPONENTS

Đinh Thế Duy Phan Hoàng Bửu Đào Quốc Duy Lý Tự Cơ

Ho Chi Minh City University of Technology and Education Viet Nam

Faculty of High Quality, No 1 Vo Van Ngan Street, Linh Chieu Ward, Thu Duc District,

Ho Chi Minh City, Vietnam.

ABSTRACT

This research is to introduce the electronics design workflow and show the problem of heat transfer for electronic devices Physical, economic, environmental, ergonomic or performance issue all have a big influence on heat transfer Engineering knowledge achievements and their relationship with a PLM platform are schematically discussed The most common heat transfer solvers and the peculiarities for the electronic are presented The concept of athesehis is presented through two examples First, an example

of natural convection transfer for a heat sink The second study was an experimental forced-convection steady-state cooling setup Experimental results will be presented in the report

Keywords: Electronics, design, cooling, heat transfer, knowledge, FEM, CFD.

1 Introduction

The mechanism of heat transfer in electronics is very complex The multi-physics aspect can be divided into three domains: the electric domain, the thermal domain, and the fluid one Heat is generated in most electronic components devices with moderate cooling capacity are cooled by fans liquid can also be used to cool them It is necessary to take care of the temperature of electronic devices to prolong their life and performance Products are getting smaller and smaller, demand is increasing, manufacturer standards are constantly improving, and manufacturers have to keep up the pace Radiator with heat pipe combination Heat sinks that combine heat pipes, micro-channels with highly thermally conductive materials can replace conventional coolers The economical aspect can turn into concern as the final products become expensive and their manufacturers are

no longer competitive All design decisions for an electronic device are made only after the cooling problem for that device has been resolved Therefore, heat transfer is a very important issue in the design of electronic components

Numerical 3D optimization of a heat sink base, Computational Fluid Dynamic (CFD), combining numerical and analytical temperature approximation All these papers describe specific solutions for particular cases Important product information and the relationship with Product Life Cycle Management (PLM) solutions are also discussed These examples contain theoretical and experimental solutions that prove the accuracy of the presented concepts

2 Problem statement

In the field of electronics, competitiveness is a key factor to assure an optimal product price in respect to actual standards Design engineers face multiple challenges in order to take the right decisions Optimal electronics are designed with the least number of

components, placed on an optimal Printed Circuit Board (PCB) layout and assembled in

an ergonomic casing The engineering knowledge is stored at PLM level and used further

to automate simulation tasks, reuse models and share results The conceptual design is tested using Electronic Design Automation software The exterior and interior 3D

assembly is completed by means of Computer Aided Design (CAD) software Optimal thermal assessments require a combination of analytical solutions, empirical analysis and thermal modeling, using all available tools to support each other A wide range of heat transfer solvers are available Thus, numerical solutions are more common because complex parameters are not required

Two types of commercial numerical heat transfer solvers are available: the Finite

Element Method (FEM) and the CFD solver

2.1 FEM Solvers

Thermal analysis is used to determine the temperature distribution and the other heat transfer computations in a body:

the quantity of heat exchanged, thermal gradient and heat flux As the structural response

is influenced by the thermal field, coupled thermal-structural analysis are performed to describe the stress state due to thermal expansion or contraction

In structural FEM based software, heat is transferred by conduction, convection or

through radiation

The major disadvantage of the FEM thermal analysis is the size of the model and the computation time

2.2 Computational Fluid Dynamics

Simulating real flow by numerical solutions of the governing equations Thermal

management can be simulated using the CFD

Solvers instead of approximating convection film coefficients as FEM software

The major advantage on the FEM solution is the dedicated CFD pre and postprocessors available for electronics

Simulation capabilities are expanded due to extended material libraries, components and Integrated Circuit (IC) package databases, multi-layer PCB configurators, thermal

interface materials and others

2.3 Electronic Design Automation

The circuit diagram of the assembly is created using Electronic Design Automation software for circuit evaluation and simulation purposes in the electrical domain

The behavior of the current is captured and its transient flow can be described allowing all heat data to be estimated

3D CAD files independent or with the use of third-party tools, such as macros and add-ons

2.4 Heat Transfer Calculator

The behavior of the electronic components can be used to compute 2D surface heat flow

on 3D internal heat generation for components and printed circuits Due to the

complexity of the heat flow mechanism in electronics (i.e switching circuits, power losses, thermal characteristics, junction temperature, joule heating) a complete physic description is still not available Therefore, methods of computing thermal

characterization parameters are based on simplified assumptions a

2.5 Computer Aided Design

Not all Electronic Design Automation applications have CAD generation features or extended component libraries (i.e complex heat sinks, heat pipes, blowers, fans, specific connectors).Therefore, in order to complete the product assembly, a CAD system is required

A final layout is proposed, than minor design changes are considered

For example, passive components can be placed in the vicinity of active components to act as heat exchangers Also, small heat sinks can be positioned on the PCB under the

active components, to enhance the thermal behavior around hot spots This is only a preliminary design scenario where the 3D assembly is parameterized

2.6 Heat Transfer Solvers

Heat transfer solvers are used to predict the temperature of the components and parts within an assembly The central role of the heat transfer code capability of such software

to identify any issues Both hot spots within the PCB temperature distributions that exceed operational limits can be visualized The choice of the heat ware ranges from simple analytical code numerical solvers Results from the heat transfer to decide if the design is optimal or an optimization scenario has to be considered The concern of

electronics heat transfer computer is that of the active components and their cooling components (resistors, transistors, integrated circuits transformers)

3 Theoretical approach

The equations for conductive heat transfer are described by:

where represents the specific heat matrix, time derivative of the nodal

temperatures, - thermal conductivity matrix, and - the effective nodal heat flow vector The primary unknown values are the nodal temperatures Other thermal

parameters can be computed based on the nodal temperatures There are two types of FEM thermal analysis: steady-state and transient thermal analysis

3.1 Steady-state thermal analysis

Steady-state thermal analysis is used to determine the temperature distribution in a structure at thermal equilibrium Steady-state solvers assume that the loaded body

instantaneously develops an internal field variable distribution to equilibrate the applied loads The analysis is generally non-linear because the material properties are

temperature dependent The governing equations for a non-linear regime are:

where i represents the iteration step number The first iteration is used to solve the initial temperature conditions and the solution proceeds to the next iteration until the result convergence is achieved The necessary number of iterations for a precise solution

depends on the non-linearity of the problem For solving the non-linear problem Newton-Raphson algorithm is used

3.2 Transient thermal analysis

This type of analysis is used to determine the temperature distribution within a structure

as a function of time, to distribution within a structure as a function of time, to predict the rates of the heat transfer, or the heat stored in the system The transient thermal analysis

assumes the evolution of a new field variable distribution from a set of initial conditions via a set of transition states, evolving through time Because most of the thermal

phenomena have a transient evolution, this is the most common type of thermal

simulation Material properties for a transient thermal analysis are: the density, the thermal conductivity and the specific heat The last characteristic is used to consider the effect of the stored heat:

where: is the specific heat matrix and matrix of the thermal conductivity Loads are functions of time The effects of numerical integration are activated using the Crank-Nicholson, Euler and Galerkin or Backward stiffness methods When the solution

is done, post-processing of the temperature evolution in time can be presented as tables, graphs or contour plots

3.3 CFD thermal analysis

The CFD (Computational Fluid Dynamics) simulation solves the conservation equations for mass and momentum For flows involving heat transfer, an additional equation for energy conservation is required

The equation for mass conservation, or the continuity equation, can be written in a general form as follows :

where is the fluid density, - speed vector, and source mass

The conservation of momentum in an inertial reference frame is described by

where is the static pressure, stress tensor, gravitational body force, and - external body forces Also, contains other model-dependent source terms

The stress tensor is given by

where is the molecular viscosity, I - unit tensor, and the second term on the right hand side represents the effect of volume dilation

For the heat transfer, the energy equation is solved in the following form:

Domain Parameters

Thermal domain Activeflow [W]component heat Non-linear(Fig 3) Fluid

domain

Stagnant air natural convection cases [W/m 2 °C] 7.151

Results Transient nodal

where is the effective conductivity, is the turbulent thermal

conductivity, defined according to the turbulence used model, and - the diffusion flux

of species The first three terms on the right-hand side of Eq represent the energy

transfer due to conduction species diffusion, and viscous dissipation, respectively includes any other volumetric heat sources Additional transport equations are also solved for turbulent flow

4, CASE STUDIES

4.1 Transient Thermal Analysis for Heat Sink performance evaluation

In this first example, the transient temperature behavior of natural convection cooled electronics was simulated using a FEM transient thermal analysis

4.1.1 Simulation model setup: The model comprises a single layer FR-4

Epoxy board that has been attached two IC silicon based chips, cooled by natural convection using a fined aluminum alloy heat sink this simulation

is to study the temperature distribution within the heat sink for a transient heat flow, such performance of the cooling solution can be evaluated The simulation requires the definition of three domains: electrical domain (current flow constraints within the circuit), thermal domain (heat generated due to the cu rent flow) and fluid domain (stagnant air heat transfer between the heat sink and the exterior) The simulation domains and parameters are described in the below

Fig 3 Non-linear IC heat cycles used in the simulation Fig 3 Non-linear IC heat cycles used in the simulation

Fig 4 Heat sink temperature distribution

4.1.2 Results and discussions: After completing the solution, time-temperature distributions graph (Fig 4) and the nodal temperatures for certain analysis time steps were processed to evaluate the performance of the heat sink The time-temperature graph shows three distinct regions: a linear temperature growth region and two parabolic ones, that describe the response of the heat sink after conduction is achieved

The temperature non-uniformity is clearly depicted as the heat remains concentrated at the bottom face of the heat sink, while the fins remain essentially at the reference temperature

4.2 Steady State forced convection cooling analysis

First of all, general remarks have to be done regarding Figure 4 In most cases, the heat flow of the active electronic components is non-linear However, due to the specific heat of each material found in the path of the thermal conduction, a steady-state temperature will be achieved for the non-linear heat flow that has a constant behavior in time Solving the transient CFD heat transfer problem can generate a black-box behavior of the product Further, specific convergence guidelines for transient CFD problems are not available, because the accuracy of the results relays most on the experience of the analyst Moreover, solver output files become large and the time required to achieve a solution increases dramatically

4.2.1 Simulation model setup : In this second study ANSYS ICEPAK pre and post processors, together with ANSYS FLUENT were used to simulate a steady-state forced convection cooling problem The active components were two MOSFETS installed on two aluminum heat sinks with horizontal fins (Fig 6) An exterior circuit comprising four resistors for each MOSFET caused the active components to generate a constant level of heat Cooling is achieved by an axially

Installed fan as the air flows from the case back (called inlet) to the front (outlet) Two precision LM-35 temperature sensors were installed in different positions on the heat sinks and using an external micro-controller, the temperature was measured considering the time increment, until the steady-state temperate is achieved

The experiment took place in three stages:

1. circuit power ‒ ON;

2. transient temperature monitoring;

3. ////Steady state temperature achieved