Optical PCB Manufacturing

Moving Data With Light Instead of Copper

For most of the history of electronics, data has moved as current along copper conductors. The approach works, but copper has limits: the losses rise with frequency, the signal degrades over distance, and the energy required to drive a long high speed link becomes a significant part of a system’s power budget. An optical board takes a different route, embedding a waveguide inside the board so that data can travel as light between components.

The result is a hybrid platform that combines conventional electrical routing for power and control with optical channels for the high bandwidth links. It is the kind of technology that appears first in data centres and telecommunications equipment and then moves outward as the cost falls.

What the Board Is Made Of

The optical channels are waveguides formed within the board structure, acting as paths that carry the light between the points that need to be connected. The board itself uses a substrate that supports the waveguide, which means high performance polymers, glass or composite materials rather than the ordinary laminate of an electrical board.

The active elements sit at the ends of the waveguides: a laser or modulator converts an electrical signal into light, and a photodetector converts it back on the receiving side. Couplers and connectors move the light between the board and any external fibre, and they are often the most delicate part of the assembly because the alignment tolerance at a fibre interface is measured in microns.

How It Works in Practice

The principle is photonic integration. Data travels as light pulses rather than as voltage transitions, which means the signal does not suffer the same attenuation with distance and is not affected by the electromagnetic interference that troubles a copper link. Data rates above a terabit per second are achievable on a single link, which is why the technology is aimed at the interconnect between processors, switches and storage in large systems.

An optical path also resists crosstalk, because the light is confined to the waveguide and does not couple into neighbouring conductors. That is a significant advantage in a dense system where the electrical channels interfere with each other.

Why It Is Used

The first driver is bandwidth. The second is power. Driving a high speed electrical link over distance costs energy, and as data rates rise the energy per bit becomes the dominant constraint in a large data centre, so an optical interconnect that reduces that cost is valuable even before its bandwidth advantage is counted.

The third is size and weight, which makes the technology attractive in aerospace and wearable applications where every gram matters. The fourth is immunity to electromagnetic interference, which matters in satellite and defence electronics and in any dense system with many high speed channels. Our notes on PCB manufacturing describe the precision processes involved.

Where It Is Used

Data centres and cloud infrastructure use optical interconnect between equipment and increasingly within equipment to keep up with the traffic. Telecommunications systems use it in 5G and future generations, where the data flow from the radio access network to the core has to be handled with low latency. Aerospace and defence applications use it for radar, avionics and secure communications, where reliability and interference immunity are critical.

Medical imaging equipment uses it where the accuracy of the data path matters, and consumer products such as high end augmented and virtual reality headsets are beginning to use it. The common thread is an application where the bandwidth or the interference requirement has pushed electrical interconnect to its limit.

Manufacturing

Manufacturing an optical board is more demanding than manufacturing an electrical one. The waveguide structures are formed with lithographic or laser processes, or in some cases with additive printing, and the thickness and the geometry of the waveguide have to be controlled precisely because the optical loss depends on them. The photonic components are then integrated with the electrical components on the same substrate, which requires assembly processes that handle both the electrical joints and the optical alignment.

Testing is the other difference. An optical board has to be tested for signal loss and bandwidth as well as for electrical function, and the environmental testing has to show that the optical path survives thermal cycling and vibration without a loss of alignment. Our notes on PCBA testing cover the test planning.

Design Considerations

Designing an optical board means planning two routing systems at once. The electrical layers carry power, control and the lower speed signals, while the optical layer carries the high bandwidth links, and the two have to be arranged so that the waveguides do not pass under the switching supplies and the optical components can be aligned without being disturbed by the thermal expansion of the board. The thermal design is therefore part of the optical design, because a board that moves as it warms will lose the alignment that the coupling depends on.

The mechanical envelope matters as well, because the bend radius of a waveguide is far larger than that of a copper trace, which constrains how the optical path can be routed and how compact the board can be. Our PCB design and layout group works with customers on this early planning, and PCB capabilities describes the advanced processes involved.

Cost

Optical boards are expensive because the materials are specialised, the processing is unfamiliar, the yields are lower and the test content is heavier. A prototype board costs several times what a high end electrical board of the same size would cost, and the price falls with volume as the process matures, but not to the levels of conventional manufacture.

The cost calculation for a programme should therefore compare the board against the system savings, not against a copper board on its own. If the optical interconnect removes a large amount of power consumption and allows a simpler system architecture, it can be justified even at a higher board price.

Trends

The technology is moving towards tighter integration of the photonic devices with the electrical circuit, so that the optical path becomes a normal part of the board rather than a separately assembled element. Co-packaged optics, where the optical interface is placed alongside the switch silicon, is one expression of this. Lower cost materials and processes, and the growth of the telecommunications and data centre markets, are pulling the technology towards volume production. Our notes on quality management describe how these processes are controlled.

FAQ

What is an optical PCB? A board that carries data as light through embedded waveguides in addition to conventional electrical routing.

What bandwidth is achievable? Optical links can carry more than a terabit per second, far beyond what a copper interconnect can support over the same distance.

Why not use copper everywhere? Because the loss and the power consumption of very high speed electrical links become unmanageable as the data rate and the distance increase.

Is it immune to interference? The optical channel is not affected by electromagnetic interference, which is one of its advantages in dense or electrically noisy systems.

Why is it expensive? Specialised materials, unfamiliar processes, lower yields and heavier testing all contribute to the cost.

Conclusion

An optical board combines the electrical routing that power and control require with optical channels for the data links that copper can no longer carry efficiently. Waveguide accuracy, photonic alignment, hybrid assembly and optical as well as electrical testing are what make it work, and the justification comes from the system level benefit rather than from the board price alone.