Development of a Metal-Core Wiring Board J/B for Vehicles
by Jutaro Mukai *, Mitsuo Tanaka *, Hirokazu Kakuta *, Toshitaka Hara *2,
Shin’ichi Kawasaki *2, Hisao Honma *2, Kazuyuki Moriyama *2,
Hideki Nakazato *2, Hirokazu Takewaki *2 and Keiji Mashimo *3

Toyota’s Land Cruiser is positioned in the firm as one of the Lexus brands, sothat a drastic equipment upgrading was carried out --although it was defined as aminor change-- in the model change the car underwent in August 2002.
With regard to the junction block (hereafter denoted as J/B), the requirement for installation
space inevitably became much more stringent due to the dilemma of increased electrical components and in-vehicle livability. To solve this problem, the authors have developed, through realization of high-density components mounting, an innovative J/B with a metal-core wiring boardthat can be installed in a space equivalent to the conventional. This report describes the development.
It is believed that this development will contribute to solving the problem of installation spacethat is expected to grow in severity in future.

ABSTRACT1.

INTRODUCTION

Recently, functions in automobiles tend to be enhanced to
upgrade user-friendliness, so that the number of wire harnesses and electric components that support this tendency behind the scenes also increases even more.
On the other hand, car users require a large indoor
space and comfortable livability, rendering it even moredifficult to secure ample space for installing electric components. Accordingly, in addition to components downsizing, some of the components conventionally installedindoors are going to be transferred into the engine room,making it necessary that these components should be
upgraded in terms of low heat generation and/or heat
resistance to cope with the harsh environment. Moreover,car manufacturers strongly demand weight reduction
aimed at improving fuel consumption as well as reduction
in developmental investments and shortening the time fordevelopment.
Against this background, we have sought for a new J/B
configuration, unlike the conventional busbar J/B or wiring
J/B, and have arrived at the solution of a metal-core wiringboard J/B. Below will be reported its development.
In the area of J/B for automobiles, there is a technological tendency to replace mechanical relays conventionallyused with semiconductor switches, which is aimed at function upgrading together with downsizing and weight reduc*Electrical Components Engineering Dept., Automotive
Components Div.
*2 LM Development Project Team., Automotive Components
Div.
*3 Automotive Technology Centertion. In view of this tendency, the use of wiring board inthis development is probably advantageous.
Whereas certain precedents for the installation of wiringboards within engine rooms are known so far, they use afactitiously prepared environment resembling that ofindoors. Thus, there are no precedents for the present
case where J/Bs are installed directly in the engine room
to control, in particular, large electric currents.

2.

DEVELOPMENT OF METAL-CORE
WIRING BOARD J/B

2.1 Background
J/Bs are generally used to streamline the electric wiring in
automobiles. Conventionally, J/Bs are renovated at thetime of full model change of a car to accommodate themselves to body form changes and upgrading of equipment
specifications. At the time of minor model change, meanwhile, there would be a small-scale upgrading of equipment in a short period of time without changing the bodyform and J/Bs.
However, it was decided to substantially upgrade theequipment of the Land Cruiser --on which a new J/B was
to be installed, although this was a minor change car, inview of customers’ needs and trends of rival cars, andthus Toyota presented the following goals for the development of the J/B.
1) Reliability of ranking first in the world2) Improvement of functionality and scalability in support of large-scaled car electronics
3) Compatibility between accommodation to largescaled car electronics and ease of production

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Table 1Comparison between existing and required specifications.
Item

Upper caseExisting specification Required specification Difference

Number of circuits

153

206

+ 53

Number of fuses

27

32

+5

Number of relays

14

21

+7

Installation space (mm)

L160 × W250

L160 × W250

±0PowerPlate
TM sheet for
wiring board

A metal-core wiring board J/B was thought to be effective in realizing a J/B with reduced size, weight and development time period, so that in response to car manufacturers’ needs, its development program began earlier todifferentiate ourselves. The development at this time succeeded this program and accelerated its progress in order
to finish ahead of schedule.
2.2 Required Specifications of J/B
Table 1 shows a comparison between the existing andrequired specifications of J/B.
2.3 Development Goals of J/B
The development goals were established as shown below
based on the customer’s requirements such as weight
reduction, development cost reduction and shortening ofdevelopment period, together with those shown in Section
2.2.
1) Size: equivalent to that of existing J/B
2) Mass: minus 30 % over the conventional busbar J/B
3) Productivity upgrading4) Quality upgrading
5) Reduction of mold investment: minus 50 % overbusbar J/B
6) Reduction of development time period: minus threemonths for electrical circuitry over conventional busbar J/B
2.4 Structure of Metal-Core Wiring Board J/B
Figure 1 shows the exploded perspective view of a metalcore wiring board J/B, excluding the upper and lower covers.
The developed J/B consists of the upper and lower covers, case, base block and a PowerPlate ® Assembly --amajor component developed this time. The PowerPlate
Assembly includes a PowerPlate, i.e., a heat-dissipatingplate, on both side of which metal-core wiring boards
mounted with relays and the like are assembled using a
TM sheet, i.e., a heat-conductive sheet as an intervening
layer. The metal-core board efficiently equalizes the heatgenerated by electric components such as relays; the TM
sheet conducts the generated heat to the PowerPlate witha minimal loss and provides electrical insulation as well;and the PowerPlate transports the conducted heat instantaneously to the heat-dissipating side, thereby dissipating
the heat to the car body via TM sheets.
In the following Sections, major constituents of thedevelopment will be described including the metal-core

Metal-core wiring board
Foamed urethane

TM sheet for body
Relay forwiring board

Heat dissipation
onto bodyConnector for
wiring board
Fuse holder
Rivet
Base block

BusbarLower case

Figure 1

Exploded perspective view of developed product.

wiring board, TM sheet, PowerPlate and thermal simulations.

3.

METAL-CORE WIRING BOARD

3.1 What is Metal-Core Wiring Board?
Metal-core wiring board is a wiring board with a metal
sheet for its core layer. Its general features are heatequalization, mechanical strength and shielding performance, and heat equalization is most remarkable among
these.
The schematic structure of a metal-core wiring board isshown in Figure 2.
Ordinary printed circuit boards using glass-epoxy substrates have disadvantages such that, because the circuit
pattern layers are insulated by the insulation layer electrically as well as thermally, the heat generated by themounted components and the circuitry is likely to be concentrated causing temperature rises, and that bulk dissi-

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Development of a Metal-Core Wiring Board J/B for Vehicles

Metal coreThrough hole

Figure 2

350

Copper foil

Solder resist

Schematic cross section of metal-core wiring board.

Temperature rise (°C)

Insulation layer

300

Glass-epoxy

250

Metal-core

200
150
100
50
0
0

3.2 Application to Wiring in Automobiles
Because the installation space for the J/B is limited, thecomponents had to be mounted in high density, which
inevitably resulted in the following requirements:• Making the circuit pattern finer, leading to downsizing• Gathering and dissipation of heat
We directed our attention to the heat equalizing propertyof metal-core board, and adopted the board to satisfy theabove mentioned requirements. With respect to the corematerial, we selected aluminum that weighs about one
third of copper for its lightness.
With conventional technology, circuit configuration using
a lamination of busbar and insulation would be a solution.
What is problematic with this is that the circuit pattern
becomes large in size in order to suppress temperaturerises in individual circuits, and also to make the over-current resistance satisfactory. In particular, in terms of theover-current resistance, the circuit patterns can not be
downsized since the pattern width is subject to the current
capacity of upstream fuses even in case of small-current
circuits. In addition to the use of heavy copper alloys forthe conductor, large circuit patterns result in increased
mass, making it difficult to meet the customer’s require-

10

15

20

25

Electric current (A)Figure 3

Temperature rise vs. electric current load of metalcore board and glass-epoxy board. Conductor widthis 1 mm.

70

Temperature rise (°C)

pation of the heat is difficult to be done because the heatsources are diffusely distributed. On the other hand, themetal-core boards can easily be configured to effect bulk
dissipation of heat, because the heat generated by thecomponents and circuitry concentrates on the metal core.
Figures 3 and 4 compare the thermal performance ofglass-epoxy board and metal-core board. It is seen thatthe metal-core board can suppress temperature rises better than glass-epoxy board under the same electric current, and that it can suppress local temperature rises aswell. Metal-core boards are finding major applications insuppression of temperature rises due to components withlarge heat generation such as CPUs and power transistors.
Metal-base board is another board known for its heatequalizing performance. Whereas the structure of a metalbase board is such that layered circuits are laminated on ametal sheet via an insulation layer, mounting of components is limited to one surface only. In contrast, because a
metal-core board has a structure such that the circuit patterns on both surfaces of the board are connected using
through holes, the board permits components mounting
on the both surfaces. In addition, the board permits solder
mounting of components having lead wires, thus allowing
for a large degree of freedom in circuitry design.

5

60

Glass-epoxy

50

Metal-core

40
30
20
10
0
-60

-40

-20

0

20

40

60

Electric current (A)Figure 4

Temperature distributions of metal-core board andglass-epoxy board. Conductor width is 3 mm, electric
current is 25 A.

ment of weight reduction. Moreover, with respect to dissipation of heat generated by the circuitry and mounted
components, it is very difficult to gather the heat from distributed places to one place, so that these heat sources
have to be dealt with individually.
On the other hand, because its wiring pattern and metal
core are thermally connected while they are electricallyinsulated, the metal-core wiring board enables circuit
wiring with a minimized wiring pattern width, so that theJ/B can be downsized due to the following reasons:• Local temperature rises on the circuit pattern can besuppressed because the heat is equalized.
• The heat shock due to overcurrents is absorbed andalleviated by the heat capacity of the metal-core.
However, irrespective of the reduced temperature differences within a board, the board in totality rises in temperature due to such factors as the interaction of heat generated from each circuit pattern and the heat concentration
caused by high-density wiring, so that it became necessary to provide some means of heat dissipation. Thus,taking advantage of its heat-gathering and -conducting
property, the metal-core board was connected with aPowerPlate, which was subsequently jointed to a heat-dissipating plate on the outside, to dissipate the generated
heat to the exterior thereby lowering the overall temperature.

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Through these means of weight reduction, we were successful in reducing the mass of the new automotive J/Bwith heat-dissipating components by 37 % over the conventional busbar J/B.
(a) Metal-to-metal connection

4.

TM SHEET (HEAT-CONDUCTIVE INSULATION SHEET)

4.1 What is TM Sheet?
The TM sheet is a heat-conductive insulation sheet comprised of acrylic rubber including a highly heat-conductive
compound. This was an essential material as a heat-dissipating component of the developed J/B, but it was anticipated that the harsh environment in the engine room
would degrade the rubber sheet considerably. We weresuccessful, however, in developing the sheet with satisfactory characteristics by optimizing the compound ingredients and their compounding ratio.
Below will be described the main features of a TM
sheet.
1) Superior heat-conductivity
High heat conductivity is achieved due to the use of
a compound with high thermal conductivity. Moreover,whereas metal surfaces generally have microscopic
irregularities resulting in small effective areas for heat
conduction as shown in Figure 5 (a), the TM sheet is
flexible enough to absorb irregularities and fill the gap
thus suppressing heat resistance as shown in Figure
5 (b).
2) Superior electrical characteristics
Electrical insulators are also compounded in the TM
sheet, so that electrical characteristics such as withstand voltage and insulation performance have beenachieved without deteriorating its thermal performance.
3) Excellent workability and adhesivenessSince the TM sheet is adhesive in itself, it can beeasily fixed on the surface simply by removing theprotective film.
4) Siloxane free
Since the TM sheet uses acrylic rubber, it has nofear of generating low molecular-weight siloxane.
Conventional heat-conductive sheets made of silicone
materials often cause problems such that low molecular-weight siloxane contained in the material isvolatilized at high temperatures. This material
adheres to the electrical contacts of relays andswitches to produce silicon dioxide --an electricalinsulator-- and eventually causes defective electricalcontacts.
4.2 TM Sheet on the Wiring Board SideAs illustrated in Figure 1, the heat generated on the wiringboard is conducted to the PowerPlate for outside heat dissipation, so that the heat resistance in-between should be
as small as possible. Generally speaking, small heat
resistance leads to small electrical resistance, making it
difficult to assure sufficient electrical characteristics such

Figure 5(b) Connection using TM sheet

Conceptual model for connection of metal surfaces
with and without using TM sheet.

as withstand voltage and insulation performance between
these two parts. If the wiring on the board is electricallyconnected with the PowerPlate, troubles such as errors in
automobile control may arise. Thus, it is necessary to satisfy a basically contradictory requirement of insulating
electrically the board from the PowerPlate but simultaneously combining thermally them together.
We have been successful, as mentioned in Section 4.1,
in obtaining low thermal resistance while maintaining sufficient electrical characteristics, and applied the technology
to the connection of board with PowerPlate.
4.3 TM Sheet on the Car Body SideThe end of the PowerPlate is connected to the car body,which is utilized as a heat radiator. If these are connected
directly, since the former is made of aluminum and the latter mild steel, there is a possibility of pitting of thePowerPlate leading to malfunctions due to the contact corrosion between different metals. From the standpoint ofthermal performance also, the use of TM sheet is very
effective.
However, unlike the wiring board side, the TM sheet on
this side is exposed to the exterior when the J/B isshipped, raising concerns about sticking of dust. So, it
was decided that the TM sheet to be adhesive on one side
only, and that the adhesive surface to be attached on thePowerPlate. Except for this single-sided adhesiveness,
the TM sheet on this side has the same characteristics asthat for the board side described in the preceding Section.

5.

PowerPlate ®

5.1 What is PowerPlate?
The PowerPlate has been employed as a device for transporting the heat generated by the components mounted
on the board to the outside of the J/B.
The PowerPlate is a heat-dissipating device consistingof a hollow plate of aluminum, in which a small amount ofvolatile liquid called working fluid is sealed after the spaceis evacuated of air at an ultra-high vacuum. Just like normal heat-pipes, the device can efficiently transport heat, inthe lengthwise direction as well, by the latent heat of theworking fluid, which repeatedly vaporizes and condenses
as a result of the temperature difference between theevaporator and condenser sections.

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Upper plate of PowerPlate

Upper plate of aluminum spacerLa

4 φ emboss

Figure 7

Lower plate of aluminum spacer

Strength calculation model.

Lower plate of PowerPlate

Exploded perspective view of PowerPlate.

5.2 Structure
Figure 6 shows an exploded perspective view of thedeveloped PowerPlate.
The PowerPlate comprises four stamped sheets of aluminum, which are laminated and bonded together by brazing.
The heat-absorbing portion of the PowerPlate is provided with a space to avoid interference with the terminals ofboard-mounted components having leads, and except for
that, the dimensions of the PowerPlate are almost equivalent to those of the wiring board so as to efficiently absorb
the heat that is equalized over the board. The heat-emitting portion is designed to closely contact the car body, sothat the heat is directly dissipated into the body thus suppressing temperature rises in the J/B.
5.3 Selection of Working Fluid
Furukawa’s conventional PowerPlates of aluminum used
R123, i.e., a CFC substitute for its working fluid. Since thismaterial belongs to global warming substances, however,
we have newly adopted cyclopentane in consideration ofenvironmental concerns. This material raises no global
warming problems, while offering sufficient performance
as a volatile refrigerant.
5.4 Mechanical Strength
Because the J/B is installed in the engine room of a car,the surface temperature of the metal-core board is expected to reach 140°C maximum in some places under the
worst conditions of the scorching sun in the summer. The
vapor pressure of cyclopentane is 969 kPa at this temperature of 140°C, while its boiling point is 49°C at atmospheric pressure, so that the PowerPlate has to have a sufficient strength against this inner pressure.
In the strength study, the calculation model shown in
Figure 7 was used, i.e., a disk with a fixed perimeter undera load of uniform distribution 1).
When an inner pressure p acts uniformly on this model,
the deflection of disk w and the tensile tension σ both
become a maximum at the center reaching:w (max) =pa4 /64 d
σ (max) = 3a2 p/4h2

1.2

Heat resistance (°C/W)

Figure 61Volume of working
fluid over internal
volume (%)

0.8

9%

0.6

19%

0.4

24%
45%

0.2
0
0

20

40

60

80

100

Heat transfer rate (Input heat) (W)Figure 8

Relationship between heat transfer rate and heat
resistance with working fluid volume as a parameter.

where, a is the disk radius, d the shape factor, and h the
sheet thickness.
Letting the sheet thickness h be 1 mm, w is 0.0096 mm
and less --a negligible value-- in the range of a≤8 mm.
Thus, considering only the tensile stress σ due to theinner pressure is enough to calculate the load on the disk,
and a result of a≤8 mm is obtained based on the yieldstrength of A3003 aluminum at 140°C, which is 34.8
N/mm2.
The actual PowerPlate used has a margin of strength
over the calculated value mentioned above, since it isreinforced with a 3-mm thick spacer integrally soldered inaddition to the peripheral ribs appropriately arranged.
Whereas the interval of ribs was set to 7 mm to minimize
influence on the thermal performance, it has been confirmed that this structure permits the device to be practically used in the environment up to 160°C.
5.5 Thermal Characteristics
Heat resistance and maximum heat transfer rate are two
important factors to represent the thermal characteristicsof the PowerPlate. Figure 8 shows their relationship.
The J/B at this time has an input heat of about 20 W
generated by the board-mounted components. Against
this heat, we have made the channel cross section of thePowerPlate for vapor passage as large as possible andoptimized the volume of working fluid, thus achieving aheat transfer rate of 80 W or higher for horizontal installation. For the actual heat transfer rate of 20 W, the heat
resistance slightly increases to 0.4 W/°C --a sufficiently
low value, thereby proving its ample performance as aheat-dissipating device for a metal-core wiring board J/B.

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5.6 Long-Term Reliability
One of the problems with respect to the long-term reliability of PowerPlate is generation of hydrogen gas andingress of air through microscopic defects. The hydrogengas and air may act as incondensable gases to hinder theheat transportation of the working fluid based on vaporization and condensation, thereby causing degradation of thethermal performance of PowerPlate.
The hydrogen gas within the PowerPlate is generated
mainly by a chemical reaction where the moisture molecules adhered to the inside walls chemically react withaluminum due to thermal activation, as shown below:

54

50

46

44.0
42.0

Calculation

Figure 9

Temperature distributions calculated by simulation.

2 (Al)+ 3 (H2O)= Al2O3 + 3 (H2)60
55

Calculated ∆T (K)To prevent this reaction, the moisture inside thePowerPlate is completely vaporized by high-temperatureprocessing at the time of brazing, and furthermore, theproducts are stored in a vacuum drying furnace until justbefore the sealing of working fluid. Thus, the inner surfaceof PowerPlate is assured to be completely dry.
The ingress of air is often caused by leaks due to thedifference between the internal and external pressures,
when microscopic defects are present at the welds formedby brazing. Because welding of aluminum is likely to beaccompanied by defects, and because it is basically difficult to assure to be free of defects merely through controlling the welding process, leak tests are carried out
throughout all the products. The test instrument measuresleak volume of the working fluid in a vacuum environment,
converts the volume into that of ingress air at normal temperature and pressure, and judgement is done whether or
not the thermal performance of the product can beassured for at least 10 years.

THERMAL SIMULATION

6.1 Outline
Recent years have seen concentration of heat-generating
components on J/Bs, making it essential to incorporate
heat dissipation into the structural design. Thus requirement for thermal simulation is increasing day by day. It is
required that thermal simulation at the time of structuraldesign can respond to design changes quickly and thatthe results be accurate enough for practical design objectives. We have conducted thermal simulations of themetal-core wiring board J/B developed here to confirm its
design 2).
6.2 Simulation Results
Figure 9 shows the surface temperature distributions of
the J/B board obtained by thermal simulation, and Figure
10 shows a comparison between calculated and measured temperatures.
It can be seen that the difference between calculated
and measured temperatures are around 3°C. Given thisaccuracy, it may be concluded that the simulation canaccurately predict the thermal performance of products at
an early stage of design, enabling provision for counter-

45
40
35
30
30

35

40

45

50

55

60

Measured ∆ T (K)

Figure 10 Comparison between calculated and measured temperatures.

measures beforehand, and that this is an effective tool for
J/B designs.

7.
6.

50

ACHIEVEMENTS OF METAL-CORE
WIRING BOARD J/B

7.1 Features of Developed J/B
The overall features of the metal-core wiring board J/Bdeveloped here are summarized as follows:
1) Using aluminum for the core, the weight of themetal-core board has been reduced.
2) With respect to the fuse holder, a slope has been
provided to discharge water entering through the
opening for mini-fuse insertion, thereby improving
leak-current resistance.
3) Taking advantage of ease of wiring on the board,the fuses have been arranged in such a way that theplus and minus potentials do not take adjacent positions at the time of IG-ON and -OFF, thereby improving leak-current resistance against water.
4) Removable rivets have been used to joint thePowerPlate with the wiring board, thereby improving
recyclability of PowerPlates made of aluminum.
5) A sliding mount structure has been employed tojoint the base block with the lower case, thereby
improving ease of components mounting and serviceability.
6) With respect to the relays on the board, only the terminals for plug-in relays have been changed, in order

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Table 2

Achievements of metal-core wiring board J/B developed.

Item

Conventional product *1(L160 × W250)

Developed product
L160 × W250

Achievement- (L500 × W100)

Size+ (L50 × W150)

Mass *2

100%

63%

12

4

70%

90%

+20%

100%

46%

-54%

6 month

2.5 month-3.5 month

Number ofconnector joints
Automated
process ratio
Investment inmolds *2
Development
time period(-19%)-37%-8(-67%)*1: Assuming bus-bar structure for the conventional product.
*2: Represented in relative values.

to reduce the time period of development as well as
the cost.
7) In an effort to reduce developmental costs, thesame size as for the current product has been realized, enabling appropriation of the lower cover of thecurrent product.

8.

CONCLUSION

Whereas the new components used in this development
such as metal-core wiring board, PowerPlate and TM
sheet are commercially available, they proved to be insufficient to withstand the harsh environments of automobiles
and in particular of engine rooms, raising a number ofproblems. In an effort to solve these problems, the authors
put together the wisdom and power of those concerned to
improve the product. Thus, it is the authors’ greatest pleasure that they could manage to launch this metal-corewiring board J/B into the marketplace.
We plan to further study and solve the remaining tasks
of: 1) Compatibility with Pb-free process, 2) Additional
downsizing and weight reduction, 3) Cost reduction and 4)
Upgrading thermal analysis technology, thereby becoming
successful in establishing new technologies that differentiate ourselves from other companies.
Lastly, the authors would like to express their gratitude
to those individuals who were of assistance in this development including those in Toyota Motor Corporation,
ARACO Corporation and Furukawa Electric.

REFERENCES

7.2 Performance of Developed ProductIn terms of leak-current resistance against water, it has
been proved at water spray tests that the leak current has
been suppressed to one hundredth of the criterion value,so that sufficient long-term reliability has been assured
through preventing solder precipitation and fracture of circuit patterns due to leak currents.
Table 2 shows the achievements of the metal-corewiring board J/B developed here with respect to the development goals.

1) Ichiro Nakahara: Strength of Material, Vol II, p.133~141 (inJapanese)
2) Keiji Mashimo, Yukihiro Saita, Ken Koizumi, Hideki Nakazato:
Heat Transfer Analysis for Vehicle Electronic Parts, Furukawa
Review, No.22, 2002.

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