Section: Technology Focus

The technology of flexible printed circuits has developed over many years from its roots in the Aerospace and Defence industries to become a major growth sector of the printed circuit industry. Applications now extend across almost all sectors of industry including "motor-tronics". In any modern car you will find EMUs (Engine Management Units), ABS systems, air bag electronics and much more waiting to be connected. The choice open to the designer is to stay with wiring looms that have been used since the car was invented or use flexible circuits which provide the most cost-effective option.

Virtually every electrical and electronic circuit in "motor-tronics" is suitable for employing flexible circuit technology, including headliner and door panel interconnections, fuse boxes, sensors and audio circuits.

The naturally planar construction of a flexible circuit enables it to be simply fixed to a variety of surfaces simplifying modular assembly, interconnection and eliminating bulky cable ties. This also has the major added benefit of eliminating annoying rattles and opens up the prospect of much smaller connectors being employed in the vehicle than has traditionally been the case. Again, this affords considerable benefits in terms of space utilisation and, among others, weight. An average cable harness weighs around 10 times that of an equivalent flexible system with connectors and the benefits are obvious.

Historically one of the major factors limiting the application of flexibles in the motor car was that of low cost manufacture and capacity. With the much higher volume utilisation of flexible circuits, the ongoing development of materials and the introduction of high volume reel to reel manufacture, these limiting factors have now been removed.

The modern approach being adopted by automotive manufacturers means that flexible circuits are the interconnection of the choice as the annual growth in usage until the year 2002 is projected to be some 55%.

There are, of course, key factors to be considered in designing flexible circuits if the lowest total cost of ownership is to be achieved, toward that end it is essential that the design engineer seeks advice from the circuit manufacturer before building in what might turn out to be costly constraints. Material utilisation, material selection, cover-coat selection and proposed assembly methods, among others, are all factors and innovative techniques which enable circuits to be designed in a small envelope and folded to produce the ultimate shape and which contribute to the overall installed cost of the circuit.

The material selection will depend upon the application, with the higher specification options being selected for the most demanding circumstances such as use in the hostile environment of the engine compartment. In these instances it is not unusual to find Polyimide (Kapton) selected for reasons associated with its thermoset characteristics and tolerance of higher temperatures. In less arduous applications, such as instrument clusters, headliner circuits and door panel circuits, Polyester is the normal choice as it is the lower cost option and, while being thermoplastic in nature, it is rarely subjected to high temperatures. A relatively new material, PEN (Polyethelene Napthalate), is gaining popularity because it fits between Polyester and Polyimide from a performance point of view.

In some instances it may be necessary to provide EMI/RFI screening and this can be accomplished a number of ways. The most cost effective approach, and coincidentally the one which provides the most flexible construction, is to use silver Polymer screens encapsulated with a screen-printed or photo-imageable covercoat. In some applications, however, it may be necessary, because of the frequencies involved, to resort to cross hatched screens. This obviously has higher cost implications (typical dimensions are shown in table 1). The assembly techniques, when using flexible circuits, vary according to the material and the type of assembly.

Simple crimp-on or ZIF (zero insertion force) mechanical connectors for interfacing with the outgoing world are more than adequate and the connector industry is responding to the need for lower profile, lower mass, reliable connector systems more befitting this technology than is the case with traditional connectors.

The weight savings when comparing flexible circuits with a conventional wiring harness are very considerable with flexible circuits on its own often weighing less than 20% of their more traditional counterparts. This, coupled with the prospect of lower mass connectors, means a massive gain for the automotive industry.

The current carrying capability for any allowable temperature rise depends on both resistance and the ability to dissipate heat by conduction, convection and radiation. The thin, flat form of flexible circuit conductors makes them better heat dissipators than round wires of equivalent cross section, see figure 1. Typically, an electric sunroof or electric window motor requires around 8A and the flexible circuit will easily handle these relatively heavy currents, or indeed signal currents in alarm detector circuits, see table 2.

Automotive application examples

Whilst flexible circuits have been in existence for over 30 years, application has been limited in most cases to general electronics and not until recently has the car become another highly complex electronics package. For example, when one of the world's leading automotive electronics manufacturers wanted to produce a new electronic lock for a luxury car maker the problems and costs involved in joining four small printed circuit boards with connectors or a flex rigid multilayer almost resulted in the project being abandoned.

The electronic lock, see figure 2, comprises four small printed circuit boards, manufactured as a "biscuit" of two sets of four per board, that are linked together and can be folded into a three dimensional form for compactness.

The difficulties arose in trying to link these boards together with connectors or a flex rigid multilayer.

Standard products available at the time required plated through holes on each of the printed circuit boards and hand soldering of each connector or jumper while a flex rigid multilayer meant exceeding the budget. This combination meant that the project was in danger of becoming prohibitively expensive and being scrapped completely.

Following extensive research the SMI (Surface Mount Interconnect) flexible "sculptured" surface mount jumper was developed.

The SMI jumper is designed for pick-and-place machine assembly and reflow soldering, addressing many of the assembly cost problems encountered. Its design provides good mechanical support while its co-planar profile enables flawless electrical connection with the board surfaces.

After passing through the reflow soldering process, the printed circuit board "biscuit" can be cut as required and reformed to the desired compact shape, without the need for a hand soldering process.

The SMI jumper features flat copper conductors that are protected by a Polyimide dielectric film to withstand reflow solder and board washing conditions. The jumper is available in up to 33-ways and is designed to ensure that each exposed finger is positioned exactly for soldering to the printed circuit board. The SMI jumpers are supplied on tape and reel to enable complete automatic placement in the correct position.

1: Typical dimensions

                                    Dimensions in microns

Base laminate thickness                   25-125
Copper thickness                           18-70
Coverlay (film)                            25-50
Covercoat (liquid)                         12-20

2: Conductor material weight/thickness

Copper weight                               Approximate
thickness
oz/ft[sup 2]       gms/m[sup 2]             inches       mum

1/4                     76                   0.0003        9
1/2                    152                   0.0007       18
I                      305                   0.0014       35
2                      510                   0.0027       70
3                      915                   0.0040      105
7 1/4                2,211                   0.0100      250

GRAPH: Figure 1: Conductor resistance

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Adapted by MD

Notebook by Brian Shorrock, MD, Flextronic Ltd