Using Flex Cable in Cars

 Auto makers want low "applied cost"

Flex cable has been used in consumer products for a long time, but despite
its major benefits of lower weight and reduced size compared to conventional
wire, it has not yet been adopted for volume car manufacture. The key reason
is that the automotive market demands high reliability in a harsh environment,
but at low "applied cost".  Rather than look at component cost in isolation,
"applied cost" examines the total cost of providing a given function on the
finished vehicle. There is no point reducing cost on parts, and then increasing
total cost by requiring a more expensive assembly process, or more difficult
quality checks.  The need to consider the whole manufacturing process has
limited the adoption of flex cables.

The problem requires more than just a new connector.  Specialist harness
manufacturers require a complete support system to enable them to
assimilate any new interconnect technology into their production lines,
and deliver harnesses to the required level of quality, day after day, and in
large volume.

FCI adopted an approach to overcoming the barriers preventing widespread
use of flex circuits in automotive applications that addressed the whole
manufacturing process. In addition to a new flex termination technology,
the company developed technologies for automating the feed of
connectors, and for terminating multi-way connectors in a single
automated pass. By simultaneously developing the connector and
application tooling, the company made it possible for harness makers
to migrate to the new technology all at once. 

FCI's Director of Application Tooling,
Flavio Fantini

First, the termination problem

Flex cable consists of copper tracks, held between two layers of
flexible insulation, typically only 0.25mm thick.  The assembly can
be installed under carpets, or behind roof & door panels, without
requiring special space allowances within the car body.  This freedom
is particularly attractive since a modern car can contain up to 5km of
cable, weighing as much as 50kg.  These traditional harnesses have
limited flexibility to fit tight corners, and are a potential source of
noise as they vibrate and rub within the structure of the car. 

Historically the challenge with flex cable has been the termination. 
Whether it is a splice which joins two wires together, a shunt bridging
wires within a cable, or termination to a connector, the method of
terminating the flex conductor to the outside world determines the
overall system reliability.  It also affects the assembly methods and
costs significantly.  Unfortunately, most existing termination systems
for flex cable require the preparation of the cable and individual
conductor termination.  This process is difficult to automate, and
results in relatively high "applied cost".

"StarCrimp" requires no flex cable preparation, since it pierces the
insulation as the connection is made. The four point star pattern
also provides mechanical strength and integrity, which is critical
for all stages: from the assembly of the connector; through its installation
in the vehicle; and on to its in-service life. Punch and die tooling
ensures that termination is consistent and automated, and can be
integrated to existing press tools. 

Traditional harness assembly terminates each conductor to a terminal,
which is then individually inserted into a connector housing.  This is
very flexible for manual assembly, but leaves room for errors, and
requires significant amounts of labour for large volumes.  FCI's
decision to aim for controlled quality and minimum applied cost
by automating the assembly process therefore required several
additional developments.

Each connector module is supplied pre-loaded with customer
specified terminals, removing one step from the traditional assembly
process, with the StarCrimp tooling enabling every terminal of a
multi-way connector to be made at the same time, greatly speeding
up production rates.  Remembering that no preparation is needed to
the flex cable, a way of speeding up the presentation of each housing
for termination was needed.  In another first for automotive connection
systems, FCI has developed reel-based packaging, which enables the
automated delivery of connector modules to ensure high throughput. 
Once again, this capability has been developed as a simple upgrade
to existing press tooling.

FCI's ModuPack™ system enables designers to do more with flex
technology than simply replace traditional wires.

Next, Strain Relief

The harsh environment for an automotive connector starts as soon
as it is assembled on the harness. Stresses are placed on the
termination at all stages of handling and not just during installation
on the vehicle production line.  It is not unusual for operators to
pull a connector as a quick check that assembly is correct, so the
"ModuPack" system incorporates an integral strain relief mechanism
that clamps the flex cable at the same time the terminations are made.
This protects the connector from the moment the flex cable is terminated,
and ensures that the consistent quality of the automated termination is
not compromised by downstream processing.

Automotive connectors are not only high density, but often are
required to combine multiple individual sub-harnesses. This system
features a range of single row and multi-row housings to accept these
automatically terminated connector modules, providing flexibility in
configuring for model variants and revisions.

Flex cable gives designers the option of  scaling each conductor to
match the specific electrical requirements of each route, which can
further improve the weight benefit of flex cable over the finite size
steps available with conventional wires. To provide the flexibility to
combine signal and power routes in one connector, this system permits
both 0.64 and 2.8mm terminals to be mixed in single row and multi-row modules.

New system design opportunities

This systems' fundamental benefits of reduced weight and space
utilization are expected to have a significant impact on the cost
and performance of existing designs. However, the real opportunity
may be for future generation designs. The car is becoming an
increasingly 'electric' environment. Whether it is a powered seat
remembering the preferred driving position of several different users,
or an integrated GPS navigation system showing you how to get home,
more and more interaction is expected with the vehicle.  Each function
means at least one extra wire, and probably more. The "ModuPack"
system  enables designers to do more with flex technology than simply
replace traditional wires.  For example, consider the LED as an indicator:
it features low power, high reliability, and low cost.  The technology is
nearly ideal, except that it does not like 12V.  Adding a resistor becomes
a problem for a conventional wiring harness, but the electrical characteristics
of flex technology provides a proven route to introduce not just resistors,
but a whole range of intelligent electronics. Further, flex interconnects
offers the ability to mix and match between flex cable, flex circuits and wires. 
Designers can consider a versatile design that offers an option for a heavy
wire harness to be replaced by a simple flex cable, which in turn can be
replaced by an added value flex interconnect sub-system.  The same
flex interconnects are used, and there is no extra 'box' to mount, but
significant extra functionality can be added. This technology can even
support sections of the harness moving from wire to flex cable to flex
circuitry at different times, since the system permits all formats to be
mixed and interconnected.

Harness manufacturers have become expert at designing modular harnesses,
which optimize the splitting and joining of groups of wires. This provides
flexibility within a model range, and simplifies subsequent design changes.
This system of flex interconnects has been designed to offer the same ability
to splice conductors onto flex cable, and link together conductors within a
cable using shunts. In addition, for high volume applications, manufacturers
can take advantage of the company's precision overmolding capabilities to
incorporate significant mechanical and electrical complexity into a single,
precision component.

Complementary technologies

As more systems are incorporated into each vehicle, the automotive
industry's demands for high reliability, ease of installation, lower cost and
design flexibility mean that continual development is necessary.  For example,
safety-critical applications such as seat-belt pre-tensioners, and the direct
connection of airbag pyrotechnics, include innovations such as the
secondary lock to increase the retention force of mated initiators, and
mechanical codings for multi-airbag systems. Designers may consider
the use of precision insert-molding, enabling connectors to be produced
in high volume, to tight tolerances but at very competitive cost.  This
ability to combine the processing advantages of plastics with the
mechanical and conductive properties of metal is also available to
volume customers as a completely customised service.  Using this
technology, complex components can be integrated into an encapsulated
finished part, delivering consistently high quality and significant
miniaturization benefits.  Since the process can incorporate metal,
specialist plastic and even silicon sealing devices, it is possible to
optimize the mechanical and electrical performance to match a wide
range of shock, vibration, moisture, dust, heat, cold or corrosion
environments, while reducing both space and cost. 

In addition to connectors and internal wiring, the technique can also
incorporate switches, resistors, capacitors, coils and heat dissipation
devices, enabling complex sub-systems to be produced to intricate
geometries, and in confined spaces.

Flex cable has been used successfully in several different industries
and has proven itself a reliable and flexible interconnect. Now the
auto industry is poised to take advantage of the technology.  

For further information visit Internet: http://www.fciconnect.com