Heat Transfer Analysis for Vehicle Electronic Parts
by Keiji Mashimo *, Yukihiro Saita *, Ken Koizumi * and Hideki Nakazato *2

This paper focuses on Junction Blocks (JBs) composed of circuit boards. Fuses
and relays are built into a JB with high density. They are connected to electricalcircuits which are also built into the JB. Circuits can be constructed of bus bars, wires, or circuitboards. In some cases using a bus bar is superior, while other cases circuit board is superior.
The choice depends on the specification of each car. Vehicle electronic systems are getting larger in scale every year. This is increasing the importance of thermal management and thermal
countermeasures applying heat transfer. One of the most important requirements for a computer
simulation of a JB is the compatibility of easy modeling and accuracy 1), 2). This paper is intended
to provide that compatibility. A new approach to modeling and transformation software for CAD
files has been developed successfully. The error level of the simulation is approximately ±5 % inthis case.

ABSTRACT1.

INTRODUCTION

Progress in the vehicle electronics field has been remarkable. The number of circuits increased by four times andthe length of harnesses increased three times from 1980
to 1990. As a result, the heat generated from the connected loads, the junction parts and the circuits has increased
dramatically. If the heat generated increases simply fourtimes, four times the surface area is needed to release it.
Actually, the surface heat transfer coefficient depends on
shape and size. However, the former assumption is trueas an approximation. Thus, a four times larger area isrequired, which is equivalent to eight times greater volume
with similar figures. In practical applications, heat generated has been successfully reduced. Therefore, the heat to
be released is not actually four times. The future expansion of vehicle electronics systems means countermeasures for heat transfer issue will remain important. In particular, JBs with relays and fuses which are generators ofheat, cannot release the heat easily into the ambient
atmosphere. The components (relays and fuses) are
packed into a compact box, and design consideration toheat transfer is especially important. The appearance of a
JB with a comparatively large circuit scale is shown in
Figure 1. Historically, since the margin was large and theheat generated was small, the design could initially be on
the safe side. In such a situation, the design does notneed to be based on a precise heat analysis. On the other
hand, an analytical approach has become indispensable
in recent years. Moreover, since it is necessary to respondquickly to a minor design change during the development* R&D Dept., Automotive Technology Center*2 LM Project Team, Automotive Products Div.

period, it is required that satisfactory accuracy should bereconciled with simplicity and practicality of operation.
In this paper we report the results of a heat analysis
technology developed based on these factors. An analytical approach using a computer was adopted for the circuitboard type JB. However, efforts are made to develop ananalytical tool that is as flexible as possible and is not limited to a specific structure. The trade-off to develop a simple tool was understood in the process. Finally, a moderately general-purpose and specialized pre-processor isadopted. Specifically, limitations of form are applied. For
example, if software can respond easily to changes in thearrangement of parts or the number of boards, even if it isapplied to an object composed of two or more circuitboards with several parts, the software is still useful inpractice. If further software development is performed, avery flexible software module group can be built in the
future.

Figure 1

Appearance of a junction block.

1

2.

CIRCUIT BOARD-BASED JB DESIGNWITH HEAT TRANSFER ANALYSISThe thermal design of a typical JB is based on following
conditions.
• Relation between total heat generated and heatreleasing ability of a JB.
• Local temperature rise induced by Joule effect in circuit leads.
• Local temperature rise induced by heat generated
from components.
The following are descriptions of a circuit board-based
JB.
(1) Relation between Total Heat Generation andReleasingAbility.
Let Q [W] be the estimated value of total heat generation,
α [W/m2K] be mean heat conductance, A [m2] be effective
surface area, and ∆T [K] be mean value of temperaturerise. .Then we get
Q=α A∆T
That is .
∆ T = Q /α A
∆T should be smaller than design value of the temperature rise. Otherwise you should modify the original design.
(2) Local Temperature Rise Induced by Joule Effectin the Circuit Lead
Figure 2 shows the permitted current for the cross-sectionof a conductor in circuit leads. The essential design policy
is to know the temperature rise under a balanced state.
The thermal calculation approach is almost the same forcircuit boards and electric wires. However, a board itself
can release heat as a heat sink. A thicker Cu foil is more
effective for releasing heat. The effect also depends onthe gaps between circuit leads. These factors should beconsidered when deciding the optimum thickness of Cu
foil. However, in an actual design, ease of supplying material etc. is required to be considered when determining Cu
foil thickness.
(3) Local Temperature Rise Induced by Heat
Generated from Components
Mounting components usually have a complicated internal
structure. Therefore, too much time and effort is need forexact modeling. Moreover, it is troublesome that not only
heat generated but also heat resistance depends on connection method. Typical connection methods are plugsocket, soldering, and welding. The following section
explains this in a little detail in terms of simplification ofmodeling 3).

3.

MODELING OF COMPONENTS

Relays and fuses are the main components of a JB. In
particular, relays have relatively complicated internal
structures. Thus, inputting geometry data is a time-consuming process, and model simplification is necessary.
Moreover, relays are the main heat sources in some typesof car due to their advanced electronic systems. In suchcases, an accurate simulation is required with a simple
calculation model. The following two concrete subjects
have been taken up for modeling relays. These are"Simple modeling of an internal structure" and "Modelingof a terminal connection".
As a result, we have succeeded in a calculation model,
which is verified for accuracy. Each is described below.
(1) Simplified Modeling of Internal Structure
If the internal structure of a relay is disregarded, calculation accuracy does not reach a practical level. Our investigation into the internal structure of a relay led to the following conclusion. That is, a thermal resistance network
can describe the actual thermal resistance of a relay.
Accuracy is sufficient for practical use. The thermal resistance network is shown in Figure 3. Overall thermal resistance is calculated from each divided ingredient. Let R1
and R2 be heat conduction resistance of current terminals.
R5 is the heat conduction resistance of the shell of a relay.
Let R3 and R6 be radiation heat transfer resistance and R4
and R7 be convection heat transfer resistance. Overall
resistance Roverall can be calculated by the following formula.
1
1
1
=
+
+
R2
Roverall
R1

1
1
1
1
+
R4
R3

+ R5 +

1
1
1
+
R6
R7

Permissible current (A)60

Heat source

∆T=50

50

R3

∆T=40

40

R5

∆T=30
∆ T=20

30

R4

∆T=10

20

R1

10

R2

R6

R7

0
0

50

100

150

200

250

Conductor cross-section (x 10-8m2)

Figure 22

Permissible current vs. cross-section of conductor.

Figure 3

Thermal resistance network of a typical relay.

Furukawa Review, No. 22

2002Relay inside2

57.8[°C]
153.4[°C]

4 75.7[°C]
5 (Rear side)

46.9[°C]370.5[°C]
12[V]Heat
sourceSubstrate & terminal

R3
R5
R4

8 31.9[°C]
7 33.4[°C]
6 37.8[°C]

11 32.5[°C]
10 33.0[°C]
9 35.3[°C]

Thermal resistance estimatedfrom measurement: 23.7[K/W]

Figure 4

R1

R2

R6

R7

Thermal resistance derived
from calculation model: 24.9 [K/W]

Comparison between measured and calculated thermal resistances.

Conduction resistance is strictly defined by sizes andmaterial properties. While natural convection resistancedepends on the temperature difference between an object
and ambient atmosphere, radiation resistance depends on
the temperature itself. Therefore, a device is required forthe calculation. Generally, heat transfer resistance is calculated by assuming the temperature of each part, andthen the temperature of each part is calculated from thevalue of heat transfer resistance. Furthermore, it is necessary to repeat the exercise until the temperature values
converge.
Nevertheless, the accuracy level of the temperature
required for the resistance calculation is not very high.
Actually, a rough temperature distribution value is sufficient for practical use. Because the atmosphere temperature itself does not change significantly, especially in acase of the same kind of JB, heat transfer resistance canbe calculated easily.
Comparison of heat transfer resistance based on themeasured temperature and the heat transfer resistancecalculated from a heat resistance network is shown in
Figure 4. The error is less than 10 %. Therefore, the modeling concept of relays has been justified.
(2) Modeling of the Solder Joints
Solder joint is one of various connecting methods for components and a board, and it is popular in many applications. There are roughly two types of solder joint: leadthrough type and surface mount type. The surface mounttype is better for high density packaging, while the leadthrough type makes it easier to achieve a reliable design.
In particular, some heavy components are not suitable forthe surface mount type. Although the situation is expected
to change in the future, the lead through type joint will still
be required for a while. That means modeling joints istroublesome, because the structure of a lead-through joint
has a complicated three-dimensional structure. The joint iscomposed of current lead, solder material, and pad on a(a) Lead-through model(b) Virtual surface mount model

Figure 5

Model construction of a joint.

circuit board. Thus we propose a virtual surface mountjoint, which is applicable for design modifications. Figure5(a) shows an actual lead-through model and shown in
Figure 5(b) is a virtual surface mount joint. These twomodels have the same heat transfer resistance. By adopting such a model, the construction process of geometricshape and also design modification process can be executed smoothly.
With this method, the microscopic heat flux in a joint is
not very reliable. This is the weak point of the method. On
the other hand, macroscopic temperature distribution presents no problems, even when using this virtual surface
mount joint model. Therefore, it can be used for a thermal
simulation, except if a microscopic view at the joint segment is required. The problem should be improved in thenext stage.

3

CAD data set(file from each layer)

Thermal Sheet

Filtering software

Set of corner point data
Thermal Sheet
Automatic meshgeneration software

Board

Mesh data
Board

Thermal Sheet

Figure 6

4.

Automatic mesh generation flow.

AUTOMATIC MESH GENERATION PROGRAM

CAD drawings are required for manufacturing. The drawings are desired to be materials that can be used directly
for a computer analysis. An essential problem at thisstage is as follows: a fact that is very clear to a human is
not always clear to a computer. For example, automatic
recognition of a structure with a narrow crevice or a laminated structure is very hard to perform. A human canmake a judgment about a crevice, and a decision onwhether or not to fill it, and if it is the premise that a 3dimensional form is given by a point set that can beunderstood by a computer, automatic division can be performed based on developed predecessors 4), 5). If there isneither a narrow crevice nor an overlap structure (whenthe lid is put on a box etc.), commercial software that can
carry out automatic generation of a mesh from a quite
complicated CAD drawing has also appeared on the market recently. However, it is not necessarily completely satisfactory. That is, commercial automatic mesh generation
software fundamentally applies tetrahedral mesh generation. There are some disadvantages with the finite element method using a tetrahedral mesh. At least, a hexahedral mesh enables us to make a model with fewer
nodes. Moreover, in some kinds of analysis, a tetrahedralmesh cannot provide sufficient accuracy. Automatic meshgeneration of a hexahedral mesh is still being developed
by several groups and has not yet been completed. Since
our present subject is mainly constructed of rectangles, ahexahedral mesh is more suitable and a tetrahedral is notintended to to be used.
We developed the procedure shown by the flow in
Figure 6 for mesh generation. The method can be applied
for some objects whose character is limited in a finite
range. The CAD drawings to begin with are somewhat different from conventional CAD drawings. Two-dimensionalline drawings are provided from each layer. This data setof line drawings is defined as a "CAD" here. Filtering software can provide the minimum geometrical informationfrom the CAD file. The minimum geometrical information
can be defined using "corner points". Mesh generation
software can generate a calculation mesh from the corner
points. The filtering and mesh generation software are ouroriginal product. The role of filtering software is to purge

4Heat Pipe

Figure 7

Inner structure of a junction block.

Figure 8

An example of mesh generation.

unused information and to provide useful geometricalinformation from each layer of line drawings. The automatic mesh generation software reconstructs a three-dimensional structure from the vertices and generates a hexahedral mesh using a structured method. The structured
method is suitable for a rectangular based object 6), 7) .
Figure 7 shows the inner structure of a JB.
It is confirmed that this method is effective for circuitboard-based JBs and similar cases. A mesh generation
example is shown in Figure 8. In spite of limited applicableobjects, this method is suitable for many types of JB
because the components used in JBs are mostly rectangular. For other types of JB, other types of software
should be applied.

5.

ANALYSIS RESULT(1) Heat Transfer Analysis
The temperature of the several points located on theboard surface is compared between measured and calculated values. Figure 9 compares calculated and measured
temperature distribution on a board. Figure 10 takes calculated values along the horizontal axis, and takes actual
measured values along the vertical axis, and plots somepoints on a board. Also in the Figure, ∆T expresses a temperature rise value, and the unit is Kelvin. In this example,
the maximum error between measurement and calculation
is approximately ±5 %, which is sufficient for practical pur-

Furukawa Review, No. 22

2002

54

50

46

44.0
42.0

Calculation
54

50

46

44

Figure 11 An example of design optimization.

42.0

with the process. Even in the worst case, it can be shortened to 1.5 days with the developed software from 3 days
with conventional software.

Measurement

Figure 9Comparison between calculated and measured temperature distributions with numerals corresponding
to ∆T.

60

Calculated ∆ T (K)

55
50
45
40

6.

DESIGN OPTIMIZATION

The developed software tool is applied to the design optimization of a circuit board. With a manual calculation, thetemperature distribution cannot be easily obtained.
Therefore, predicting the optimum arrangement of thecomponents is not easy. On the other hand, a threedimensional computer analysis makes this possible.
Figure 11 shows an example of design optimization. In
this case, the projection area of the board is reduced by
20 %.

35

7.

30
30

35

40

45

50

55

CONCLUSION

60

Measured ∆T (K)

Figure 10 Comparison between calculated and measured temperature distributions.

poses. However, in actual application, we may be interested in the temperature of a very fine structure of about 1
mm, and may be unable to use the present model.
Although it is in the main factor for having simplified themodel, making a precise model will produce problems ofdisagreements with a simple tool. This should be solved in
the following stage.
(2) Calculation Time
Analysis time can roughly be divided into model creation
time and calculation time. Among commercial solvers, calculation times are not so long in normal cases. Moreover,as model creation time is usually overwhelmingly taken fora long time rather than calculation time in a heat transfer
analysis, it is meaningful to shorten this. Although calculation time also depends on the quality of a mesh, about 30
minutes is typically considered and it does not interfere

In this paper, we present a new modeling method and atransformation software from CAD for a JB FEM analysis.
This simulation tool is based on the structured mesh generation method and we obtained the following conclusions.
(1) Error level is ±5 % or less in thermal analysis of acircuit board based Junction Block.
(2) Total simulation time is reduced by 50 %.
(3) Design optimization reduces weight by approximately 20 %.
In the future, the software tool should have a simpler
interface that is compatible with sufficient accuracy. The
authors also intend to get higher performance for a wider
range of applications.

ACKNOWLEDGEMENT
The authors would like to thank the staff of Taniguchi
Laboratory of Okayama University, Fujitsu Nagano
Systems Engineering and Furukawa Information
Technology for their cooperation.

5

REFERENCES
1) T. Ikeya et al., Thermal Simulation of the Electrical Distribution
Box, SAE 2001-01-0690, 2001.
2) Y. Oka et al., Investigation and Evaluation of the Wire Harness for
the High Performance Components in the Vehicle, The Instituteof Electronics Information and Communication Engineers,
Technical Report R97-32, pp19-22, 1998.
3) K. Mashimo et al., New Approach of Computer Simulation for
Heat Transfer in Automotive Parts, IPC2001D053, Nov. 2001,
Shanghai.
4) T. Taniguchi, Automatic Mesh Generation Methods of Arbitrary
3D Domain, J. of Simulation Vol. 18, No.2, 1999.
5) T. Taniguchi and E. Fillion, Numerical Experiments for 3-dimensional Flow Analysis in a Fractured Rock with Porous Matrix,
Advances in Water Resources Vol. 19, No. 2, pp 97-107, 1996.
6) M. de Berg et al, Computational Geometry, Springer, 2000.
7) Okabe et al., Spatial Tessellations 2nd ed., John Wiley & Sons,
19988) K. Mashimo et al., Development of the Simulation Tool for
Vehicle Electronic Parts, SAE 2002 World Congress, Mar. 2002,
Detroit6

Furukawa Review, No. 22

2002