OKI Electric Cable’s "Long FPC" is a flexible printed circuit achieving lengths of up to 100 meters. Featuring an ultra-thin profile of approximately 0.1 mm and a weight only 1/100th of conventional round cables, it optimizes wiring efficiency in space systems, large FA equipment, and large-scale lighting. This technical column objectively details its structure, comparison with standard cabling, moisture absorption precautions, and a unique production system that supports custom orders from a single piece.
In the design of electronic devices and industrial machinery, reducing wiring size and weight, saving space, and enhancing reliability are critical challenges. For large-scale Factory Automation (FA) equipment, large lighting installations, and space development hardware, conventional round cables often create bottlenecks due to their weight, bulkiness, and the risk of miswiring during assembly.
This column provides a technical overview of "Long FPC" (Flexible Printed Circuits) from OKI Electric Cable (Oki Electric Cable Co., Ltd.) as an innovative solution to these cabling bottlenecks. We explore the technology behind manufacturing FPCs up to 100 meters long, compare them with traditional cabling methods, highlight the critical design consideration of moisture absorption, and present real-world applications including JAXA's space sail demonstrator. This article is compiled to assist design, development, purchasing, and production engineers with objective, practical data.
Basic Mechanism and Structure of FPCs (Flexible Printed Circuits)
A Flexible Printed Circuit (FPC) is a circuit board made by forming conductive metal electrical circuit patterns on a thin, insulating base film. Unlike standard rigid boards, FPCs possess excellent flexibility, allowing them to be folded, bent, and arranged in complex three-dimensional spaces. Electronic components can also be directly mounted on them, just like traditional rigid printed circuit boards.
An FPC is constructed from the following primary layers:
- Base Film (Insulating Material) A thin plastic film that provides electrical insulation and mechanical strength. OKI Electric Cable's FPCs primarily utilize polyimide, which offers outstanding heat resistance (withstanding temperatures of 120°C and above), radiation resistance, and electrical insulation. This ensures stable performance under extreme thermal cycles and harsh environments like outer space.
- Conductive Metal (Circuit Layer) Generally made of copper foil, high-precision circuit patterns are chemically etched onto the base film.
- Coverlay and Stiffeners An insulating coverlay film is laminated over the etched copper circuit to protect it from environmental exposure. Additionally, in areas requiring local mechanical strength—such as connector insertion points or component-mounting areas—stiffeners (reinforcing plates) are bonded to the backside of the FPC.
Technical Features and Specifications of OKI’s "Long FPC"
Conventional FPCs have typically been limited to a maximum length of approximately 0.5 meters (50 cm) due to the size constraints of standard manufacturing equipment. To span longer distances, engineers had to connect multiple FPCs using inter-board connectors or transition to conventional round wire cables.
OKI Electric Cable has overcome this limit by developing proprietary production technologies and process controls at its Gunma Plant, enabling "Long FPCs" to reach continuous lengths of up to 100 meters. This allows for seamless, single-piece cabling over long distances.
Key technical specifications and features include:
- Structure (Layer Count): Available in single-sided (1-layer) and double-sided (2-layer) configurations.
- Maximum Wiring Length: Supports continuous lengths of up to 100 meters for single-sided FPCs and up to 10 meters for double-sided FPCs.
- Ultra-Thin Profile: The total thickness is exceptionally thin, measuring approximately 0.1 mm (or 0.1 mm to 0.2 mm depending on the layer stackup).
- Significant Weight Reduction: When comparing cables of equivalent current/signal carrying capacity, the Long FPC reduces wiring weight to approximately 1/100th of conventional round cables.
- High Durability and Motion Support: Shaping the FPC into a bellows-like accordion geometry allows it to handle sliding, twisting (torsional), and expansion/contraction movements. In standard linear sliding applications, high-flexibility configurations can achieve a durability of over 100 million cycles (depending on the bending radius and specific design parameters).
Technical Comparison: Long FPC vs. Round Cables vs. Standard FPCs
The table below highlights the technical differences and trade-offs of implementing OKI's Long FPC compared to standard wiring methods:
| Evaluation Metric | Conventional Round Cables | Standard Short FPCs | OKI Electric Cable's "Long FPC" |
|---|---|---|---|
| Max Wiring Length | Hundreds of meters (Virtually unlimited) | Approx. 0.5 meters max | Single-sided: 100 m / Double-sided: 10 m |
| Thickness / Envelope | Bulky (Requires large installation space) | Ultra-thin (0.1 mm to 0.2 mm) | Ultra-thin (Approx. 0.1 mm, high space efficiency) |
| Weight | Heavy (100% baseline) | Light (But restricted to short lengths) | Extremely light (Approx. 1/100th of round cables) |
| Inter-board Connectors | None required for single runs | Multiple connectors required for long runs | None required (Continuous, seamless wiring) |
| Miswiring Risk | Yes (High manual connection errors) | None (Circuits are printed and fixed) | None (Printed circuit prevents wiring errors) |
| Flex / Dynamic Life | Moderate (Risk of fatigue and wire break) | High (But limited by short length) | Extremely high (Up to 100 million sliding cycles) |
Key Benefits of Adoption
- Unmatched Weight and Space Savings The reduction in weight to 1/100th of round cabling and the ultra-thin 0.1 mm profile directly contribute to lighter systems. This is a game-changer for aerospace applications, such as rockets and small satellites, where weight dictates launch costs. It is also beneficial for high-speed moving parts in manufacturing machinery, reducing inertial momentum.
- Elimination of Assembly Miswiring Manually routing and terminating multiple round wires is labor-intensive and prone to human error. Because the Long FPC integrates all conductors into a pre-defined printed layout, assembly is simplified to plugging in connectors. This completely eliminates miswiring risks and speeds up installation times on the production line.
- Higher System Reliability via Fewer Junctions Chaining multiple standard FPCs to span a long distance introduces points of failure at every connector or sub-board junction. These joints are vulnerable to vibration, shock, and contact wear. By utilizing a single continuous Long FPC, all intermediate connectors are eliminated, maximizing physical and electrical system reliability.
Critical Design Consideration: "Moisture Absorption"
While FPCs offer excellent electrical and mechanical properties, engineers and production managers must account for a key material characteristic during assembly: Moisture Absorption.
Polyimide—the core material used in the base film—and the adhesives used in laminate layers naturally absorb moisture from the surrounding atmosphere. If an FPC undergoes sudden, intense heating (such as during reflow soldering, hand soldering, or thermal pressing) while holding absorbed moisture, the trapped water rapidly vaporizes and expands. This can cause severe physical defects, including:
- Measling: Localized delamination at the resin-circuit interfaces, appearing as white spots.
- Delamination: Separation of the layers (base film, copper foil, or coverlay).
- Blistering (Voiding): Visible bubbling or puffing on the FPC surface.
Engineering Countermeasures
To prevent these defects, FPCs must undergo a Baking (Dehumidification Drying) process immediately prior to any high-temperature operations like reflow soldering. A typical baking procedure involves heating the FPCs in a dry oven at 100°C to 120°C for several hours. Because dried polyimide reabsorbs ambient moisture quickly, assembly must be completed within a specified time frame post-baking. Unused components should be sealed in moisture-barrier bags with desiccant (silica gel) and stored in a temperature- and humidity-controlled environment.
Target Industries and Real-World Applications
① Space Systems and Satellites (Contributing to "New Space")
A prominent historical milestone for OKI Electric Cable’s Long FPC was its deployment on JAXA’s (Japan Aerospace Exploration Agency) Small Solar Power Sail Demonstrator, "IKAROS" (launched in 2010). IKAROS utilized a massive, ultra-thin sail measuring approximately 14 meters square to propel itself through space using solar radiation pressure. OKI Electric Cable supplied a 14-meter-long continuous FPC that was mounted directly on the sail surface. It served as the crucial power and signal transmission lines connecting the thin-film solar cells and various scientific sensors. The FPC successfully withstood the extreme vacuum, harsh thermal cycles, and intense radiation of deep space, enabling the world's first successful solar sail flight.
In the rapidly growing "New Space" commercial satellite market, Long FPCs are increasingly sought after by private startups and venture firms. They provide a vital solution for reducing satellite mass, cutting component counts, streamlining manufacturing, and ultimately lowering rocket launch costs.
② Large-Scale Factory Automation (FA) and Industrial Machinery
In massive factory automation equipment, such as flat-panel display (FPD) manufacturing lines, semiconductor handling systems, and large-scale inspection stages, moving parts travel over several meters. Long FPCs provide highly durable signal and power transmission within cable carriers. Their low profile minimizes the required space inside moving tracks, and their proven durability of over 100 million sliding cycles dramatically reduces machine downtime.
③ Large-Scale Lighting and Architectural Installations
For linear LED lighting, large advertising displays, and architectural accent lighting that span long distances, the Long FPC serves as both the mounting substrate and the long-distance wiring board. Eliminating discrete connectors along the run allows for slimmer profiles and significantly higher long-term reliability.
OKI's Production Strength: Hybrid Manufacturing at the Gunma Plant
For purchasing departments and development engineers, initial tooling costs (NRE) and Minimum Order Quantities (MOQ) are major barriers to adopting custom FPCs, especially for prototype development or academic research.
At its Gunma Plant (located in Isesaki City, Gunma Prefecture, Japan), OKI Electric Cable operates two distinct, optimized production lines to support customers from initial prototyping through full-scale mass production:
- Sheet-fed (Piece-by-Piece / "Maiyo") Line for Prototypes and Small Lots This line processes FPC materials in individual sheets. It features low initial tooling costs and high operational flexibility, making it ideal for highly customized, low-volume designs. By leveraging the Sheet-fed line, OKI Electric Cable can manufacture custom Long FPCs of up to 100 meters with a Minimum Order Quantity of just a single piece (1 unit), accommodating spot prototype requests and early-stage R&D.
- Roll-to-Roll ("Roll-to-Roll") Line for High-Volume Mass Production This line continuously feeds raw materials from large rolls through etching, lamination, and processing steps. It provides excellent throughput and material efficiency, substantially reducing unit costs for high-volume production runs once designs are locked.
This hybrid capability—prototyping a single custom long board on the sheet-fed line and seamlessly transitioning to the roll-to-roll line for high-volume commercial production—makes OKI Electric Cable a versatile partner for both New Space startups and established industrial OEMs.
Conclusion
OKI Electric Cable’s "Long FPC" represents a major leap in wiring technology, shattering the traditional 0.5-meter limit to deliver continuous flexible printed circuits up to 100 meters long.
By shrinking cabling weight to 1/100th of conventional round wires and maintaining an ultra-thin thickness of 0.1 mm, it offers a dramatic solution for system weight reduction and spatial optimization. The ability to span long distances without intermediate connector joints eliminates primary electrical failure points while streamlining assembly and preventing manual miswiring.
Backed by JAXA's rigorous IKAROS mission and supported by proper moisture baking protocols, the FPC delivers elite reliability under severe operating conditions. Through the Gunma Plant’s dual-line setup, OKI Electric Cable accommodates everything from a single custom prototype to high-volume manufacturing, offering an invaluable tool for modern design engineers in aerospace, factory automation, and beyond.
◇Manufacturer's Website
https://www.okidensen.co.jp/en/prod/fpc/flexible/tyouzyaku_fpc.html


























































