Development of Flexible Flat Cable Harnesses
for Automobile Roof Modules
by Atsushi Fujisaki * and Norikazu Samitsu *There has in recent years been a marked trend in the automobile industry towardmodular construction, and each manufacturer has applied a variety of approaches in the effort to reach a solution. As a parts manufacturer, Furukawa Electric has come to grips
with this trend and is proceeding with the development of more effective modular elemental parts.
Specifically we can anticipate that wire harnesses, which have traditionally taken the form of bundles of conductors, will take on a flat profile, and we have for some time been proceeding withstudy and development work. In the work discussed here we have developed flat harnesses foruse with flexible flat cables (FFCs), and succeeded in using them in vehicles. The adoption of theflat harness profile greatly reduces the use of protective parts and gives greater routing freedom,but the cost of parts goes up and connecting problems arise. The point is how to reach solutions,and by solving these problems and launching mass production we were able to gain the technicalexpertise to derive specifications responsive to vehicle requirements and develop manufacturing
equipment.

ABSTRACT1.

INTRODUCTION

As automobiles have become more sophisticated, harnesses have, through multiplexing, junction boxes andelectronic control units in the body, seen a reduction in
wiring, and advances have been made in the downsizingof wires using CAVUS ultra-thin low-voltage cables forautomobiles. Harnesses are, however one of the productsthat has undergone no major change in the last 50 years
or more.
There is, meanwhile, a trend favoring rapid introductionof structural modules and integration, so that from thestandpoint of the structure of the vehicle as a whole it isessential that changes be made in the configuration of theharness.

even if they interfere with other parts is to cover them with
tube-like protectors, but this just makes the harness evenmore bulky. What is needed is to form them with a dimensional accuracy that is as good as that of other parts.
A means used in the past to solve the problem of forming with increased dimensional accuracy was the use of a"forming protector". However such protectors and forming
components are a waste of both money and weight fromthe standpoint of the harness's function of providing electrical conductivity.
One concept that is fundamental to solving these problems may be stated, "Forming without waste a harness
that is without form = make it parasitic on objects having
form".

3.
2.

CONFIGURATION OF HARNESSREQUIREDThe higher sophistication of the motor vehicle in recent
years has meant an explosive increase in the number ofcircuits it contains. It is not at all unusual today for a vehicle to have upwards of 2,000 circuits.
Since harnesses take the form of bundles of conductors
secured by tape their dimensions vary greatly and it
becomes inevitable that dimensional tolerances for thembe an order of magnitude greater than for other structuralcomponents. One way to assure that there is no problem*

Development Promotion Dept., Automotive Components Div.

THE NEED FOR A FLAT HARNESS

Configurations that can made parasitic may be divided
broadly into: first, those means by which the wires aremounted directly to a structural member having grooves
that accommodate bundles of conductors; and second,
those means by which the conductor is made as thin as
possible and then either attached to or integrated with thestructural member.
The first of these is possible with the conventional configuration, and is applicable even with large numbers ofcircuits. The second requires a flat-type conductor configuration and the number of circuits is limited, but it has theadvantage that there is basically no need to secure aroute for the harness.

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In the work reported here, a roof module harness for
FFCs using flattened conductors has been accepted for
the Toyota Alltezza.

4.

HOW THE FLAT CONFIGURATION WASACHIEVEDOne can imagine flat-type conductors having a variety ofconfigurations. These may be divided broadly into two
types: first, the "circuit board type" in the form of a surface; and second, is the continuous "cable type".
Typical of the first is the flexible printed circuit (FPC), forwhich fabrication techniques that may be mentioned as
most typical are a) etching and b) printing, although theauthors have also considered c) shearing, d) plating, e)laser cutting, f) ultra-high speed routering, g) simplified
press forming, and so on.
Since all of these are circuit board types, however, theirmajor disadvantage is that when a large area is taken up
by a harness, etc. yields are extremely poor and costs are
high.
With respect to the second type, we think not only of
FFCs, having flattened conductors, but of conventional flat
cables, in which single cores or twisted conductors arelaid out flat and extruded or bonded.
These characteristics allow manufacture in long, continuous lengths, and therefore make for higher productivity
and improved yields, and provide a solution to the problems of the first type. On the other hand, in actual use it isnecessary to subject them to further processing, by bending, etc.
In this work we selected an FFC with flattened conductors both in an effort to reduce thickness to the maximum
extent possible, and in recognition of the future integrationof auxiliary equipment functions.

5.

DEVELOPING THE FFC HARNESS FORROOF MODULES

5.1 Objectives
The FFC harness the development of which was
described in the preceding section was conceptually congruent with the roof module being planned by Toyota
Motor Corporation, and it was decided to proceed with a
Photo 1

Complete roof module.

Table 1

Specifications for roof harnesses.
Auxiliaries

Specification
MAP LP88

S/R SW

VTYLP

DOME

Japanese normal roof

Small

Japanese sliding roof

Small

Japanese normal roof ab

Small

Japanese sliding roof ab

Small

Left-hand drive normal roof

Small

Left-hand drive sliding roof

Small

Left-hand drive normal roof c

Small

Left-hand drive sliding roof c

Small

Left-hand drive normal roof dLarge

Left-hand drive sliding roof dLarge

Left-hand drive normal roof cd

Large

Left-hand drive sliding roof cd

Large

European/Australian normal roof eLargeEuropean right-hand drive sliding roof eLargeAustralian sliding roof eLargeEuropean/Australian normal roof ce

Large

European right-hand drive sliding roof ce

Large

Australian sliding roof ce

Large

EC-MIR

Furukawa Review, No. 21

2002Photo 2

Photo 3090 female terminal. Photo 4

Photo 5

Cross-joint terminal.

040 female terminal.

FFCs.

Photo 6

Cross-joint.

0.33
1.2

1.5

1.0
6.4

0.33
1.2

2.5

1.5
8.9

Figure 1

FFC dimensions.

joint development. At that time Toyota Motor was carrying
forward a study on the use of a roof harness module using
an ordinary wire harness for a model other than the
Alltezza, for which a model change resulted in a change inthe body configuration, so that harness routing could besecured. The Alltezza was given model year improvements so major changes in the body were not approved,
so that with an ordinary wire harness, there would beparts that seriously interfered with the body. The fact thata slim profile was the prime advantage of the flat harness
spurred them to further consideration of Furukawa
Electric's FFC harness.
5.2 Overall Specifications
Photo 1 shows the roof module in its entirety, and Table 1
shows the product line-up and the specifications.
At the first stage of development it was proposed to use
FFCs throughout, with the exception of circuits in the sliding roof. However with the introduction of curtain shield
airbags the minimum space for the accommodation of theharness in the front pillar was only 12 × 12 mm. Thus theFFC, because of its width, could not be used and ordinary
wires were used in the front pillar, resulting in the co-presence of both ordinary wires and FFCs.
5.3 Component Parts
With the assumption that the FFC would be generally
applicable, allowing it to serve as a replacement for conventional wires, we adopted specifications with a strong
bias on the side of safety (see Photo 2 and Figure 1).

Photo 7

Cross-joint after lamination.

With the terminal connectors there was the matter of thetiming of their introduction, so it was decided to adopt general-purpose connector interchangeability, to assure interchangeability with conventional auxiliary equipment. The
housings were also such that conventional connectors
could be used without modification.
Terminals were newly developed for FFCs. For connection with the FFCs, a new concept known as piercing wasintroduced, assuring the electrical and mechanical stabilityof the connection. Two sizes were used: 2.3 (090, see
Photo 3) and 1.0 (040, see Photo 4).
For splices, a cross-joint terminal was developed to connect two FFCs overlaid in a cruciform configuration.
Piercing technology was also used here as in terminal
connection (see Photos 5 and 6).
So as not to sacrifice the advantage of a slim profile, thecross-joints were not protected by a case, etc. but were
insulated by aligning and laminating the cross-joint withfilms from above and below. In addition it was possible bythis method to obtain a simplified water-proofing effect(see Photo 7). Because of the need to connect wires to
FFCs, a wire-to-FFC joint terminal (WF), with piercing andcrimping on both ends, was also developed, as will be
described in the next section (see Photos 8 and 9).

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Photo 8

Wire-to-FFC joint
terminal (WF).

Photo 9Appearance afterconnection.

The basic idea behind connections for the FFC was toallow terminals to be fitted to the FFC without removing its
insulation.
5.4 FFC Harness Configuration
For normal roofs, a mixed configuration was used, with
FFCs attached to the roof and ordinary wires accommodated in the front pillar. The WF terminal was used to connect the FFC to the wires, and was accommodated in a
WF holder protecting short-circuiting between terminals,
and a saving in space compared to the conventional wire
harness-to-wire harness connector was also a consideration.
The points of differences in sliding roof specifications
from those for the normal roof are that the sliding roof
switch and sliding roof motor circuits use an ordinary wire
harness, and that FFCs and wires are present in the roof
portion.
Problems of Use in Vehicles, and Measures
Taken
Preconditions were the fact that, as a minor change, itwould obviously not be possible to make major changes inthe body, etc., and, with respect to electronic components,
mass-produced parts were to be used without change, sothat no change in circuit position for electronic components was possible.
A requirement was also made that mass-produced connectors be used so that they could be used in common
with wires and FFCs. This in itself was a major theme inthe development process, but conversely it was also possible to establish the elemental technologies that were an
indispensable step in FFC development.
In terms of routing, wires are attached to the roof, sothat the route can be shorter that with the conventional
configuration in which they are attached to the body. On
the other hand for the circuit leading to the vanity mirror
lamp, it has conventionally been possible to have a connector emerge from an aperture on the body but since theharness was attached to the roof head lining, the circuit isfastened in together with the lining, sandwiched in thespace between the body and the sun visor stay. With conventional wires such a configuration cannot be achieved,
making it necessary apply some process such as to form
an indentation in the body.
In the case of the FFC, even if sandwiched in it has noeffect, and no change in the body is required. Specificallythe brace of the header panel has a major effect onstrength and unnecessary irregularities have a major
effect on its performance, so that a configuration like the
FFC offers major advantages.
The means by which the FFC is fixed to the roof is common with other models that are running with the process
for attaching the roof head lining of the Alltezza, so it washandled by applying two-sided tape over the whole surface. The FFC has good adhesion properties, and, unlike
conventional wires, is flat, making the adhesive area
greater and resulting in an excellent bond. The FFC has
no left-right bend and snaking does not occur so that if thereference position shifts, the shift persists to the end. The
process of attaching was therefore studied by all those
involved based on a full understanding of these properties.

6.

MANUFACTURING THE FFC HARNESSThe manufacture of harnesses using FFCs was also
caused to be extensively changed from that of conventional wire harnesses.
The basic objective was to change from the traditional
manual processes to processes using automatedmachines.
Also the concept of completely single-unit processingthat has been conventionally practiced was continued,
with computers controlling of product numbers batch bybatch, with production being order-driven, and carried out
on a just-in-time basis.

5.5

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6.1 Cutting and Bending of FFCs
Since with FFCs excess length cannot be absorbed, theaccurate cutting and bending are of great importance.
Accordingly FFCs were bent automatically at the sametime as they were cut.
6.2 Trimming
Once the FFC has been cut and bent, it is then trimmed--aprocess that involves making slits between conductors at
the ends. This means insertion into the conventional housing, and is practiced to avoid partitions between cavities.
6.3 Piercing
Since piercing must be carried out precisely in the centerof the conductor, its position is detected. The process
known as piercing is virtually the same as conventional
crimping or insulation-displacement connection. The FFC
that has been pierced is then terminal-inserted in the
housing.
6.4 Splices
The cross-joints are points at which FFCs intersect, andrequire that terminals be driven in at the intersection. To
do this each FFC is fixed to a single flat jig, restricting theposition and forming a splice. After terminals have been
driven in at all splices they are sandwiched between upperand lower films and heat sealed.

Furukawa Review, No. 21

20026.5 Connections to Wires
As was described above, the harnesses for pillars use
conventional wires, and for this reason connections
between FFCs and wires are made. Sub wires are manufactured by automated machines, and the point of connection to an FFC is also automatically set on a jig once thewire-side is crimped.
The jig used for the cross-joints is then shifted as is to
the wire connecting machine, and piercing is carried out.

On the other hand there is an urgent need for slim-profile solutions like the FFC, and advanced development
must be carried out so that it can replace conventionalwires with significant advantages in all areas, including
cost, design, and quality.
Finally the authors would like to take this opportunity to
express their deep appreciation for the great assistanceoffered in this work by the personnel concerned at Toyota
Motor Corporation and Kanto Auto Works Ltd.

6.6 Finishing, Testing and Shipment
Finally the work is finished, focusing on the wires, a continuity test and visual inspection are carried out, and the
product is shipped.
Since here FFC length is already assured by the equipment, dimensions are not checked on the assembly stand.
After passing through these process steps, the FFC harnesses are complete.

Manuscript received on December 25, 2001.

7.

ANTICIPATING FURTHER DEVELOPMENTS

These roof modules using FFCs will not be confined to theAlltezza, but should be extended to other models in the
future.
A cost comparison between wire only and FFC only,
however, shows that it is not yet competitive, and as
things stand now savings in cost can are found only in the
reduction in protection due to the stability of configurationand the reduction in line length due to use of the shortest
routes. In future, there will obviously be further reductionsin the cost of the FFC itself, production processes will berevised and connectors reappraised. There are many
avenues that must be pursued, such as design revisions
suited to the FFC.
Modification for larger current and finer pitch are also
under consideration not only for the roof modules, andways of use that take advantage of the features of the
FFC will be extended to all components.
Further, harnesses can also in the future be provided,for example, with "integration of termination parts" such as
unitized switching functions, and with the inclusion ofsome joint-box and electronic control unit functions by"surface mounting technology", and it is thought that this,in itself, will have a variety of functions. With this in view
we intend to proceed by utilizing the advantages of theflattened conductor.

8.

CONCLUSION

This technology has been tied in with mass production,
but this is not the end.
This achievement is only the first step to methods ofchanging harness configuration; further developments
must be made. And not all will take the configuration of
the FFC; each must have its own configuration using
appropriate materials in the appropriate place.

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