Application of PowerBoard® , Large Current
Composite Printed Circuit Boards in Automobiles
by Takao Kobayashi *, Naoyuki Toyoda *Hiroyuki Shibata *2 and Kunio Negishi *Many types of large-current composite printed circuit boards have been developed

ABSTRACT to meet various needs. The use of electronic circuitry in the automobile industry isgrowing too, and equipment that was mechanically driven and controlled is now operated more
precisely by electronic circuitry. Furthermore, to cope with fuel shortages due to fossil fuel depletionand environmental problems such as acid rain, the development and marketing of electric andhybrid electric vehicles is proceeding apace, raising a need not only for conventional control circuitry but for power circuits as well. The structure and features of products that have been introduced for automotive applications are described.

1.

INTRODUCTION

A variety of composite printed circuit boards for large current use, the PowerBoard®, have been developed to satisfy the requirements brought about by the introduction of
inverter circuitry into such industrial fields as robots, working machinery, power supplies, and air-conditioners.
PowerBoard features, while ordinary printed circuit boards(hereafter called PCB) use general purpose copper foils
having a nominal thickness of 70µm or less, its circuitry
structure consisted of copper foils having a thickness thatis several times as much. This enables PowerBoard to integrate power circuitry with signal circuitry on a single PCB,
permitting conduction of large currents, which was impossible for conventional PCBs.
Automobile industry, which is believed to be one of themost promising market areas for PowerBoard, has seen
many cases where mechanical driving or controlling isreplaced by electronic circuitry in search of more precisecontrol. For instance, hydraulically powered steering including auxiliary equipment is actually shifting to electrically powered steering aiming mainly at fuel saving.
What is more, as represented by the PRIUS of Toyota, ahybrid-electric car mass-produced and commercialized forthe first time in the world at the end of 1997, development
and marketing of hybrid-electric or electric car is advancing rapidly to cope with fuel shortages due to fossil fueldepletion and environmental problems such as acid rain.
This report describes PowerBoard that has been com* Electronic Components Engineering Sec., Electronics
Products Div., Information Systems Gr.
* 2 Electronic Components Production Management Sec.,Electronics Products Div., Information Systems Gr.

mercialized so far with its structures and characteristics,
focusing on its application to automobiles.

2.

DEVELOPMENT TARGETS

PowerBoard has been developed to complete the following targets.
. To integrate the power circuitry, which users have hitherto been assembling using wire harnesses and busbars, with control circuitry thereby mitigating assembly
work as well as inspection work. This will also lead users to easy process management by reducing the quantity of components to be taken care of.
. To be effective in making equipment compact and lightweight. Compared with wire harnesses, PowerBoard
dissipates heat more efficiently, thereby reducing thecross section of conductors.
. To realize stabilization in electrical characteristics ofequipment through the elimination of wrong wiring andfluctuations of power circuitry pattern.
In order to achieve the above mentioned targets, studies were conducted in terms of structure of components
to be mounted as well as mounting method and current
Controlling circuitry(general purpose copper foil)Power circuitry(thick copper foil)Basematerial

Through-holeFigure 1

Cross section of PB-L

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Table 1

Current capacities of selected PowerBoards (board temperature rise = 10°C, conductor pattern width = 10 mm)

Type of circuit board

PCB for general purpose

PowerBoard

Type of conductor

General purpose copper foil

PB-M

Thick copper foil (PB-F, PB-L)

Short bar (PB-S), Bus bar (PB-B)

Thickness of conductor(power circuitry)

35 m

70 m

245 m

210 m

500 m

1 mm

1.5 mm2 mm

3 mm

Continuous current rating

10 A

14 A

25 A

25 A

38 A

54 A

66 A

76 A

93 Acapacity, and PowerBoard of five types have been developed as described below.

3.

STRUCTURES AND FEATURES OF VARIOUS POWERBOARD

While each type has its own features, PowerBoard can beselected, as a rule, according to the current capacity andcomponents to be mounted.
In determining the current capacity, two parameters, i.e.,the steady conduction current (DC value or rms value)
and the allowable temperature rise of PowerBoard at current conduction should be known.
Table 1 gives an example of the current capacity ofPowerBoard, which assumes an allowable temperaturerise at conductor of 10°C and a conductor width of 10 mm.
100

Temperature rise ∆T (˚C)

Conductor width =5 mm 10 mm

30 mm

806050 mm
Conductor
width =70 mm

4090 mm
110 mm
130 mm
150 mm2000100

200Conduction current (A)Figure 2

Measured temperature rise of inner conductor(thickness = 210µm, horizontal disposition)

100

Temperature rise ∆T (˚C)

Conductor width =5 mm 10 mm

30 mm

50 mm

80Conductorwidth =70 mm

604090 mm
110 mm
130 mm
150 mm2000100200300

400

Conduction current (A)Figure 328

Measured temperature rise of inner conductor(thickness = 500µm, horizontal disposition)

Note that the table refers to a case of test pattern and thatthe current capacity changes depending on peripheral
conductor patterns (for example, with or without a widepattern).
3.1 PB-L (thick copper foil multi-layered type)
PB-L (thick copper foil multi-layered type)1) consists of, as
shown in Figure 1, inner layers of thin PCBs using 6 ounceelectroplated copper foil (210µm nominal thickness) on
which power circuit patterns are formed, and outer layers(four layers or more) of general purpose copper foil layered by using prepreg (semi-hardened composite sheet
made of glass cloth substrate which is soaked with epoxy
resin and dried), which subsequently undergoes such processes as through-hole forming, circuit patterning, solderresist coating, and characters printing using silk screen.
For large current applications, another type is available
that includes inner layers implemented by 500µm thickrolled copper foil, which is embedded by prepreg into layers of general purpose copper foil to make a multi-layeredproduct.
One of the greatest merits of PB-L is that it enables not
only conduction of large currents (typically 30~70 A) thanconventional PCBs of general purpose copper foil due toits employment of thick copper foil for inner layers, but also
mounting of surface mounting components due to its employment of general purpose copper foil for outer layers,thus ensuring readiness for use just like ordinary PCBs.
Moreover, since insulation between power circuits (largecurrent conduction circuitry) of different voltages can beeffected by using resin, the insulation distance can bedecreased reducing the size of the substrate compared tothe case where power circuitry is placed in the outermost
layer. Similarly, circuits of different voltages can be broughtnear reducing inductive reactance that causes spike voltages (surge voltages).
PB-L eliminates soldered connection within the power
circuitry, is manufactured in the same way as ordinary
PCBs resulting in high reliability, enables mounting of surface mounting components, and thus is capable of downsizing of substrates. Because of this, for automobile applications, PB-L is best suited for various controlling andpower circuitry such as electric powered steering and electric powered seats.
Figure 2 and Figure 3 show temperature rises of two
types of PB-L respectively, namely with copper foil 210µm
and 500µm thick, with conductor width taken as a parameter. At the time of pattern design, the conductor width
given is normally multiplied by a factor of 1.5 to mitigate
the current load.

Furukawa Review, No. 18. 1999

Power circuitry(thick copper foil)

Controlling circuitry(general purpose copper foil)

Controlling circuitry(general purpose copper foil)Power circuitry(short-bar)

BasematerialBasematerial

Through-hole

Through-holeFigure 4

Figure 6

Cross section of PB-S

Controlling circuitry(general purpose copper foil)

Insulating sheet
Power circuitry(bus bar)

Figure 5

Burring

Cross section of PB-F

Power circuitry(thick copper foil)

Through-holefor screw

Cross section of PB-B

Base
material

Through-hole

3.2 PB-F (thick copper foil double sided type)
Having a cross section as shown in Figure 4, PB-F (thickcopper foil double sided type) is a double sided substrate
based on copper clad laminate (CCL) which uses, like
PB-L, 6 ounce electroplated copper foil. Since PB-F utilizes only the CCL of thick copper foil for circuit patternformation, it has limitations in the attainable density of circuit patterns. Thus PB-F makes it suitable for power circuitry applications (application examples having continuous ratings of 15~30 A) which occasionally incorporates
some signal circuitry.
One of the merits of PB-F is that it is economical because it saves material costs and layering costs. Moreover, its thick copper foil provides PB-F with a large heat
capacity permitting it to be used as heat radiator (heatleveling), as well as a large conductor cross section compared to general purpose copper foil enabling the reduction of voltage drops within the circuit pattern.
In automobile applications, PB-F is suitable for the following purposes: to connect e.g. a plurality of relays using
the power circuitry only; to equalize e.g. the line impedance of a bridge circuit that employs a plurality of FETs(field effect transistor).
3.3 PB-B (laminar bus type)
PB-B (laminar bus type), having a cross section as shownin Figure 5, consists of copper sheet conductors (toughpitch copper or oxygen-free copper) and such insulating
sheets as mica, aramid paper, and PET, which are multilayered through adhesive layers. The insulator material isdetermined considering the insulation requirements of theproduct in addition to environmental temperatures in use.
One of the advantages of PB-B is that, similarly for PBL, insulation distances between different voltages are shortand uniform thereby reducing spike voltages. This makes
PB-B suitable for the high-speed switching and high frequency uses, enabling simplification of snubber circuit andheat sinks than conventional technologies.
What is more, PB-B is structurally suited for assembling
such screw-fastened components as IGBT (insulated gate

Figure 7

Cross section of PB-M

bipolar transistor), IPM (intelligent power module), diodemodule, and magnet switches as well as for connecting
the circuitry between power supply terminals and the like.
In the area of automobile applications, PB-B has been
employed in the inverter circuitry of the PRIUS of Toyota,
a hybrid-electric car mass-produced and commercialized
for the first time in the world at the end of 1997.
3.4 PB-S (short-bar type)
Having a cross section as shown in Figure 6, PB-S (shortbar type)2) consists of a double-sided or multi-layered PCB
using general purpose copper foil and power circuitry. A
copper sheet, stamped out to a required shape using press
dies and bent if necessary, subsequently solder plated tomake a short-bar, is fastened onto the pattern on PCB bysoldering or riveting to constitute the power circuitry (application examples having continuous ratings of 30~70 A).
The fastening method by riveting has been developed
to improve its vibration resistance considering installation
on vehicles. Short-bars are fastened to the substrate byriveting, whereby conduction to substrate patterns is effected by inserting the short-bar ends into through-holes
after bending and subsequently soldering it at the time
when other components having leads are flow-soldered.
3.5

PB-M (components side/solder side composite
thick copper foil substrate type)
While other PowerBoards pursue their own merits respectively, PB-M (components side/solder side composite thickcopper foil substrate type)3) has been developed with amajor target of reducing costs under an assumption ofmass production. Its cross section is shown in Figure 7.
PB-M features its automated manufacturing process, inwhich, thanks to the successful development of automatic
machinery, copper foil is stamped out using dies, automatically mounted on general purpose substrates throughthe use of adhesive, thereby enabling continuous forma-29

tion of partial power circuitry.
For automobile applications, PB-M is suitable for building up power circuitry (application examples having continuous ratings of 15~30 A) of small circuit quantities onto
the signal circuitry made of general purpose substrates.

4.

ENVIRONMENTAL TESTSPowerBoards undergo various environmental tests sinceeach type provides its own service, and major test items
include thermal shock test, humidity test, hot-humid cycle
test, and high temperature test.
Tests were carried out based on the methods specified
in JIS, MIL, and the equivalent, in which appearance, electrical conductivity, insulation characteristics, and peelstrength were evaluated in addition to other items specificto each type.
Test conditions and results are omitted from this report,
but PowerBoard characteristics proved to have no abnormalities, like general purpose substrates.

5.

CONCLUSIONS

We have described the outline of advantages ofPowerBoard, composite printed circuit board for large current use, together with its applications in automobile industry. Realizing keen competition for lower costs in thisbusiness area, we plan not only to reduce costs of developed PowerBoard products, but also to develop new products suited for this industry, while maintaining product reliability comparable to that of conventional general purpose PCBs.

REFERENCES

1) Composite printed circuit board for large current use (II),Furukawa Electric Review, No.87, 1990. (in Japanese)2) Composite printed circuit board for large current use (I),
Furukawa Electric Review, No.87, 1990. (in Japanese)3) Composite printed circuit board for large current use, Furukawa
Electric Review, No.101, 1998. (in Japanese)
Manuscript received on August 4, 1998.

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Furukawa Review, No. 18. 1999