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Flexible Printed Circuits: a multitude of innovative applications
November 21, 2003 |Estimated reading time: 6 minutes
Flexible Printed Circuits open up a multitude of innovative applications
In industry, the Flexible Printed Circuit (FPC) may be seen as an expensive and possibly exotic forms of interconnect, used only in specialised and individual cases. This impression may be partly attributable to their extraordinary characteristics and specialist manufacturing requirements. In the following article, these areas of consideration will be explained and some examples of their application will show how the unique characteristics of an FPC can be used to provide beneficial and innovative solutions.
What is different?
In principle, the number and sequence of the processing steps in the manufacture of a Flexible or Printed Circuit do not significantly differ from that used to fabricate a rigid PCB. The real differences for the manufacturer of the FPC are the considerations of handling and transporting very thin and flexible material through conveyerised process equipment, and other production plant.
These thin materials typically necessitate specialised transport systems for horizontal (conveyer) machines to avoid damage. Manufacturers of rigid PCBs appreciate the special considerations in the processing of very thin laminates for rigid multilayer PCB production. However, flexible circuit base material can be even more flexible than these thin laminates Ð their greater flexibility can further complicate automatic handling processes. To overcome these problems single-sided flex material may be handled by a roll-to-roll process, with specialised roll-handling equipment.
A particular material characteristic of a flexible circuit that requires special consideration during the manufacture process is its dimensional stability, particularly after the etching process. To achieve acceptable tolerances, material movement can be characterised and the size of a process panel may have to be limited. Although smaller panel sizes often lead to higher costs, Freudenberg overcome this through the production efficiencies offered by reel-to-reel automatic processing, coupled with low material costs, as their parent company Nippon Mektron manufacture their own flexible base material.
Despite FPCs being perceived as possibly being more expensive per unit area than rigid PCBs, they have gained a market share of 20 percent in the Far East, growing substantially each year through finding many new applications that cannot be realised with rigid PCBs. For example, optical cameras and computer hard drives (picture 1) are typical of the many products that rely on FPCs to achieve their current form and function.
In addition to the electronic interconnect function of the FPC, it usually performs an essential mechanical function. Assembled parts of a flexible circuit provide interconnection in three-dimensional arrangements, for example in a camera, or they have to survive bending or dynamic stress such as in a computer hard-drive.
In most applications the FPC does not stay in any one plane, but makes accessible a third dimension of interconnect. The FPC uniquely combines the electronic interconnect and mechanical function and as result may be perceived as a classic example of a mechatronic component. The combination of high functionality and the possibility to solve complicated wiring tasks add special appeal to FPCs.
Picture1. optical camera and hard disc drive are typical applications for FPC
Examples of applications for flexible printed circuits
As previously mentioned, FPCs have made a unique contribution to the realisation of achieving the miniaturisation of electronic interconnection.
Numerous new applications for FPCs are opening up through new developments in electronic interconnection and packaging, providing solutions to systems integration and complete wiring assemblies. Moreover, increasing functionality leads to the FPC becoming an active 3-dimensional component carrier combined with a complete means of connectivity.
All assembly and connection technologies are possible with FPCs. The range of applications ranges from chip carrier for chip-scale packaging (CSP) through LCD-display driver chips utilising chip-on-flex, to the flexible micro-substrate for multi-chip modules (MCMs).
The following application examples have been realised at Freudenberg Mektec Europa GmbH, the respected Germany-based manufacturer of flexible printed circuits. Freudenberg Mektec is a joint venture of the worldwide operating FPC manufacturer Nippon Mektron (Japan) and Freudenberg. Through this business relationship, Mektec has access to worldwide production plants, which can offer its customers greater flexibility of technology, applications and local support and logistics for delivery requirements.
1. Flexible Printed Circuits with stiffeners to substitute Rigid Printed Circuits
Many rigid PCB applications can be substituted productively and less expensively by flexible circuit solutions. For example, for a LCD-display driver chip application (picture 2), a rigid PCB and flex circuit had been used originally. The circuit was double-sided with assembly of SMD and chip on board components. Through some minor changes, this application was successfully converted to one double-sided flexible circuit. In the assembly area, the flexible part was supported by a stiffener. In addition to the far better product characteristics, for example temperature considerations, there was huge potential cost saving.
Picture 2 LCD application
2. Flexible Circuit carriers as construction elements
In a particular automatic gearbox, an aluminium plate divided different sectors (picture 3). Through flexible circuit technology, this plate was used to carry the electronic circuitry - which previously had been outside of the gearbox. The FPC, laminated onto the aluminium support plate, provides support for and wiring to this function unit of the gearbox. A thick-film ceramic hybrid is attached to the FPC, with the FPC being completely surrounded by gear oil, which resists a temperature range up to 150¡C.
Picture 3 Gearbox control unit
3. Combination of FPCs with sealing components
For an application in the automotive industry sector, the FPC serves as a component carrier for a rotation angle sensor and as wiring connection to the control electronics (picture 4). By injection moulding the FPC within an elastomer seal the wiring becomes an integrated component.
One FPC assembly fulfils the functions of signal transfer and sealing. The FPC is exposed to extreme conditions such as:
- Acceleration up to 1000g in the sensor area
- Immersion in diesel fuel with 30 bar pressure
- Environmental temperatures from Ð40¡C till 125¡C
Picture 4 Diesel injection pump
4. FPCs in Data Technology
In data technology FPCs are well established in many applications (DVD, CD-Rom drives etc.) One example shows how the FPC serves as a carrier for a finger print sensor (picture 5) and at the same time as wiring to the analysis electronics Ð this is designed to replace a conventional automotive ignition key. In a window of the FPC a naked chip is installed, supported by a stiffener and encapsulated. Wire bonding and SMD-assemblies are used for interconnection.
Picture 5 Fingerprint sensor
5. FPCs in Telecommunication
In an application in the telecommunication field an FPC is used as a keyboard and interconnect wiring to the display via a flexible hinge area (picture 6). Further interfaces to additional components are included on this single FPC design. The speciality of this application is a silver paste print on one side, used as a very low cost second wiring level and the combination of several stiffener materials like steel, polyimide and polyester. This application results in significant space saving and is very cost competitive, through achieving a three-dimensional connecting technology.
Picture 6 Cellular phone
Summary
All technologies used to produce printed circuit boards (SBU, HDI etc.) can be used for the manufacture of flexible printed circuits. It is obvious that a FPC can be much more than a connecting element between two circuit carriers. The range of applications for FPCs is increasing dramatically Ð there seems no limit to how effective FPCs can be in providing the ultimate solution to interconnection and packaging for current and future products. Extensive three-dimensional wiring can be handled in one plane with conventional assembly technologies and then complexly fitted into products. Very significant cost efficiencies and higher functionalities are increasingly being realised by the use of flexible printed circuits.
Article source: Freudenberg Mektec Europa GmbH