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IEC TR 62658: Roadmap of Optical Circuit Boards and Their Related Packaging Technologies
✅ Standard at a Glance
IEC/TR 62658:2013 is a Technical Report that provides a comprehensive technology roadmap for optical circuit boards (OCBs) and their related packaging technologies, including optical backplanes, board-level optical connectors, and optoelectronic modules. Published by IEC TC86/JWG9 (Joint Working Group with TC91), this document maps the current state of the art, identifies technology gaps, and outlines the standardization path needed to enable widespread adoption of embedded optical interconnects in high-performance computing, data centre, and telecommunications systems.
IEC/TR 62658:2013 is a Technical Report that provides a comprehensive technology roadmap for optical circuit boards (OCBs) and their related packaging technologies, including optical backplanes, board-level optical connectors, and optoelectronic modules. Published by IEC TC86/JWG9 (Joint Working Group with TC91), this document maps the current state of the art, identifies technology gaps, and outlines the standardization path needed to enable widespread adoption of embedded optical interconnects in high-performance computing, data centre, and telecommunications systems.
🔌 1. The Case for Optical Circuit Boards
1.1 The Bandwidth Bottleneck
The volume of network traffic is dramatically increasing due to the amount of data being captured, processed, conveyed, and stored as digital information. Personalized content drives network traffic growth of 20% per month, resulting in a doubling of network traffic every 1.5 years. However, this growth is out of step with the input/output (I/O) performance of servers, which doubles only every 2 years. This creates an increasing gap between the performance evolution of network equipment and the growth in network traffic — a gap that traditional copper-based electrical interconnects are increasingly unable to bridge.
The fundamental problem lies in the physical limitations of copper transmission lines at high frequencies. As signal data rates increase beyond 10 Gbps, electronic channels suffer from crosstalk, dielectric loss, skin effect, and electromagnetic interference (EMI). The maximum permissible density of electronic transmission lines is determined by crosstalk: at 10 Gbps, the required line pitch between adjacent channels is three times larger than at 3 Gbps. This means that as signal speeds increase, PCB trace density must decrease — exactly the opposite of what the bandwidth demand requires.
💡 Engineering Insight
The power consumption implications of this bandwidth bottleneck are equally severe. By 2020, power consumption in network routers in Japan alone was projected to reach the gross power generation capacity of Japan in 2005. An energy saving of 3 to 4 orders of magnitude in network router technology is required to meet Kyoto Protocol targets. Optical interconnects offer a path to achieve this: a comparison of 10 Tbps electrical and optical routers shows that the optical router consumes 20% less power while providing superior bandwidth density. For data-intensive applications, optical interconnects can reduce power consumption per transmitted bit by 10-100x compared to copper at equivalent data rates.
The power consumption implications of this bandwidth bottleneck are equally severe. By 2020, power consumption in network routers in Japan alone was projected to reach the gross power generation capacity of Japan in 2005. An energy saving of 3 to 4 orders of magnitude in network router technology is required to meet Kyoto Protocol targets. Optical interconnects offer a path to achieve this: a comparison of 10 Tbps electrical and optical routers shows that the optical router consumes 20% less power while providing superior bandwidth density. For data-intensive applications, optical interconnects can reduce power consumption per transmitted bit by 10-100x compared to copper at equivalent data rates.
1.2 Advantages of Optical Interconnects
Optical interconnects address the limitations of copper through several fundamental advantages. Optical waveguides neither produce nor are affected by electromagnetic interference, eliminating the EMI constraints that impose severe cost burdens on high-speed copper PCB design. The layout advantages of optical waveguides reduce the functional area and layer count of PCBs, with the most I/O-intensive applications seeing the greatest reduction in PCB volume. Optical channels also eliminate the need for adaptive equalization at board-level distances, reducing both power consumption and design complexity.
🔬 2. Optical Circuit Board Technologies and Classifications
2.1 Types of Optical Circuit Boards
IEC/TR 62658 classifies optical circuit boards according to their construction and application. An optical circuit board (OCB) has arbitrary optical transmit patterns with straight, crossing, bent, or tapered optical channels with input/output optical ports. The optical channels may comprise optical fibres (glass or polymer) or planar waveguides fabricated directly on or within the board substrate. OCBs can be categorized as follows:
| OCB Type | Substrate | Optical Channel | Key Characteristics | Typical Application |
|---|---|---|---|---|
| Rigid OCB | Rigid PCB (FR-4, polyimide) | Embedded polymer or glass waveguides | Standard PCB processing compatibility | Server motherboards, router line cards |
| Flexible OCB | Flexible substrate (polyimide film) | Polymer waveguides on flex film | Bendable, suitable for dynamic applications | Wearable electronics, robotics |
| Flexi-Rigid OCB | Hybrid rigid + flexible | Waveguides on flexible section | Compliant section for board-to-board connections | Direct passive board-to-board optical connects |
| Electro-Optic PCB (EOCB) | Combined electrical + optical layers | Integrated optical + copper layers | Replaces separate electrical and optical boards | High-performance computing, data centres |
2.2 Optical Backplanes
An optical backplane is a circuit board that supports optical connectors to which two or more daughter cards can be connected, forming optical pathways between them. Daughter cards are usually connected orthogonally to the backplane in a bookshelf-type arrangement. Optical backplanes typically include electrical layers for power distribution, control signals, and low-speed bus signals, along with optical layers for high-speed data transmission. IEC/TR 62658 identifies four categories of optical backplanes based on optical interconnect type, connector topology, and daughter card arrangement.
2.3 Board-Level Optical Connectors
Board-level optical connectors differ fundamentally from fibre-optic connectors used in telecommunications. They must accommodate the tolerances and alignment challenges of PCB manufacturing, including board warpage, thermal expansion, and vibration. IEC/TR 62658 covers both in-plane connectors (where the optical path remains in the plane of the board) and out-of-plane connectors (where light is redirected perpendicular to the board surface using micro-mirrors or prisms). The standardization of these connectors is essential for enabling multi-vendor interoperability in optical backplane systems.
⚠️ Technology Note
Board-level optical connectors face unique alignment challenges compared to telecom fibre connectors. While a telecom fibre connector typically requires sub-micron alignment precision, board-level connectors must accommodate board-level tolerances of 50-100 microns. This is achieved through expanded-beam designs, self-aligning features, or active alignment mechanisms. The choice of connector technology significantly impacts the overall system cost and reliability, and should be evaluated against the specific application requirements.
Board-level optical connectors face unique alignment challenges compared to telecom fibre connectors. While a telecom fibre connector typically requires sub-micron alignment precision, board-level connectors must accommodate board-level tolerances of 50-100 microns. This is achieved through expanded-beam designs, self-aligning features, or active alignment mechanisms. The choice of connector technology significantly impacts the overall system cost and reliability, and should be evaluated against the specific application requirements.
💡 3. Engineering Design Insights and Standardization Roadmap
3.1 The Role of IEC TC86/JWG9
The standardization of optical circuit boards falls under the purview of IEC TC86 (Fibre optics) in collaboration with TC91 (Electronics assembly technology). The Joint Working Group JWG9 was established in 2009 specifically to address the standardization of in-system optical circuit board packaging, including performance and reliability requirements, optical interconnect interfaces, and test methods. The scope of JWG9 covers three levels of system-embedded optical interconnect: system level (daughter cards communicating across an optical backplane), board level (chip sets communicating across an optical PCB), and chip level (components within a multichip module optically connected).
💡 Engineering Insight
The standardization approach taken by JWG9 is deliberately technology-neutral, covering both glass fibre and polymer waveguide technologies, as well as rigid, flexible, and flexi-rigid board constructions. This ensures that the standards remain relevant as the technology evolves. However, the current state of the art favours polymer waveguides for board-level integration due to their compatibility with standard PCB manufacturing processes, lower cost, and the ability to fabricate complex routing patterns using photolithographic techniques. Key academic contributors to polymer waveguide research include the University of Cambridge, University College London, and Fraunhofer IZM, while key industrial contributors include IBM Research Zurich, Xyratex, and TE Connectivity.
The standardization approach taken by JWG9 is deliberately technology-neutral, covering both glass fibre and polymer waveguide technologies, as well as rigid, flexible, and flexi-rigid board constructions. This ensures that the standards remain relevant as the technology evolves. However, the current state of the art favours polymer waveguides for board-level integration due to their compatibility with standard PCB manufacturing processes, lower cost, and the ability to fabricate complex routing patterns using photolithographic techniques. Key academic contributors to polymer waveguide research include the University of Cambridge, University College London, and Fraunhofer IZM, while key industrial contributors include IBM Research Zurich, Xyratex, and TE Connectivity.
3.2 Performance Trends and Technology Gaps
IEC/TR 62658 maps the performance trends for optical circuit boards and identifies the technology gaps that must be addressed through standardization. The key performance parameters include optical channel density (waveguides per millimetre), insertion loss (dB per channel), crosstalk between adjacent channels (dB), bandwidth-distance product (Gbps times metres), and operating temperature range. The Technical Report identifies that while individual technology demonstrations have achieved impressive performance metrics, the lack of standardized test methods, reliability qualification procedures, and interface specifications is the primary barrier to commercial adoption.
| Parameter | Current State of Art (2013) | Target Performance | Technology Gap |
|---|---|---|---|
| Channel Density | 10-20 waveguides/mm | >50 waveguides/mm | Fabrication precision, coupling efficiency |
| Insertion Loss | 0.1-0.5 dB/cm (polymer) | <0.05 dB/cm | Material absorption, sidewall roughness |
| Bandwidth | 10 Gbps per channel demonstrated | 25-40 Gbps per channel | Modulator/detector speed, dispersion |
| Operating Temperature | 0 to 70 degrees C | -40 to +85 degrees C | Polymer thermal stability |
| Reliability | Limited qualification data | 15+ year field life | Standardized test methods needed |
✅ Green ICT Impact
The adoption of optical interconnects in network server and storage systems is an effective path to realizing green information and communication technology. Among prevailing network technologies, servers account for 50% of all power consumption, followed by data storage systems at 35%. Optical interconnects can reduce the power consumption of signal drivers (which must ramp up power to overcome dielectric loss and skin effect in copper traces), eliminate the need for power-hungry adaptive equalization at board-level distances, and reduce the number of PCB layers required for high-speed routing. Collectively, these improvements can reduce system-level power consumption by 15-30% for bandwidth-intensive applications.
The adoption of optical interconnects in network server and storage systems is an effective path to realizing green information and communication technology. Among prevailing network technologies, servers account for 50% of all power consumption, followed by data storage systems at 35%. Optical interconnects can reduce the power consumption of signal drivers (which must ramp up power to overcome dielectric loss and skin effect in copper traces), eliminate the need for power-hungry adaptive equalization at board-level distances, and reduce the number of PCB layers required for high-speed routing. Collectively, these improvements can reduce system-level power consumption by 15-30% for bandwidth-intensive applications.
3.3 Planar Embedded Optical Waveguides
The primary technology for integrating optical channels into PCBs is the planar embedded optical waveguide. Research and development activities span a wide range of fabrication techniques including photolithographic patterning of polymer waveguides, laser direct writing, inkjet printing, and embedded glass fibre ribbons. The key advantages of planar waveguides over discrete fibre integration include compatibility with existing PCB manufacturing processes, the ability to create complex routing patterns (crossings, bends, tapers), and significantly lower assembly cost. European research institutions have been particularly active in this field, with contributions from universities in the UK, Belgium, and Germany, supported by industrial partners including IBM, TE Connectivity, and Dow Corning.
🚨 Critical Challenge: Coupling Efficiency
The most significant technical challenge for optical circuit boards is coupling light efficiently between the optical waveguide and the optoelectronic transceiver chip. Two coupling approaches exist: in-plane (butt) coupling, where the waveguide end-face aligns directly with the photodetector or laser facet; and out-of-plane (grating) coupling, where a diffractive grating element redirects light perpendicular to the board surface. In-plane coupling offers lower loss (typically <1 dB) but requires precise alignment. Out-of-plane coupling provides more relaxed alignment tolerances but introduces 2-4 dB additional loss from the grating element. The choice of coupling method significantly impacts system cost, yield, and long-term reliability.
The most significant technical challenge for optical circuit boards is coupling light efficiently between the optical waveguide and the optoelectronic transceiver chip. Two coupling approaches exist: in-plane (butt) coupling, where the waveguide end-face aligns directly with the photodetector or laser facet; and out-of-plane (grating) coupling, where a diffractive grating element redirects light perpendicular to the board surface. In-plane coupling offers lower loss (typically <1 dB) but requires precise alignment. Out-of-plane coupling provides more relaxed alignment tolerances but introduces 2-4 dB additional loss from the grating element. The choice of coupling method significantly impacts system cost, yield, and long-term reliability.
❓ Frequently Asked Questions
Q1: When will optical circuit boards become commercially available?
A: Optical backplanes and board-level optical interconnects have been commercially deployed in niche applications (military/aerospace, high-performance computing) since the early 2010s. Mainstream commercial adoption in data centre equipment began accelerating in the 2020s, driven by the insatiable demand for bandwidth. However, the technology is still evolving: the standardization work initiated by IEC TR 62658 and continued by subsequent editions and related standards (IEC 61300 series for fibre optic connectors, IEC 62074 series for fibre optic WDM devices) is essential for enabling the multi-vendor ecosystem needed for mass-market adoption. The technology is best described as “early mainstream” with rapid growth expected through 2030.
Q2: Are polymer waveguides reliable enough for telecom/datacom applications?
A: Polymer waveguide reliability has improved significantly over the past decade. Key concerns include thermal stability (polymers can degrade at elevated temperatures), moisture absorption (which increases optical loss), and photostability (UV exposure can cause yellowing). Modern fluorinated polyimide and acrylate-based polymers achieve operating temperature ranges of -40 to +85 degrees C with projected lifetimes exceeding 15 years under typical data centre conditions. IEC/TR 62658 identifies the need for standardized reliability qualification test methods, which are being developed by IEC TC86/JWG9 based on the established IEC 61300 series of fibre optic test procedures.
Q3: How do optical circuit boards compare to silicon photonics?
A: Optical circuit boards and silicon photonics address different levels of the optical interconnect hierarchy. Silicon photonics integrates optical transceivers (lasers, modulators, detectors) onto silicon chips, providing high-speed electro-optic conversion at the chip level. Optical circuit boards provide the optical waveguide layer that connects these chip-level transceivers across a board or backplane. The two technologies are complementary, not competing: silicon photonics provides the active devices, while OCBs provide the passive optical “wiring.” A complete system requires both technologies working together. IEC/TR 62658 focuses on the board and backplane level, while silicon photonics standards fall under the purview of IEC TC47 (Semiconductor devices).
Q4: What are the main barriers to adoption of optical circuit boards?
A: The main barriers are: (1) Cost — optical PCB fabrication currently adds 30-100% to the board cost compared to all-electrical PCBs, though this premium is decreasing as manufacturing processes mature; (2) Standardization — the lack of standardized interfaces, test methods, and reliability qualification procedures creates risk for adopters; (3) Supply chain maturity — the ecosystem of materials suppliers, PCB fabricators, and connector manufacturers is still developing; (4) Design tools — commercial PCB design tools do not yet natively support optical waveguide routing, requiring specialized software or manual layout. IEC/TR 62658 addresses barrier (2) by providing the standardization roadmap, and the subsequent work by JWG9 is progressively addressing the remaining gaps.
