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How electro-optical circuit boards are made

Posted: 05 Jan 2015 ?? ?Print Version ?Bookmark and Share

Keywords:electro-optical? PCB? crosstalk? Xyatrex? film processing?

We keep hearing that copper has reached its speed limit, and that optics will take the place of copper for high-speed signals.

We've also heard that active optical cables can replace copper in links between switches, routers, and servers. Furthermore, silicon chips can now have some optical components. But that still leaves circuit boardsor does it?

Enter electro-optical printed circuits, which combine copper and optical paths on the same board. The IEEE Components, Packaging, and Materials Technology (CPMT) Society sponsored a webinar on the subject titled "Electro-optical Printed Circuit Board and Interconnect Technologies and their Application to Data Centre and HPC Systems." Richard Pitwon of Xyatrex explained the need for the technology and its concepts. Marika Immonen of TTM Technologies explained the manufacture of these boards.

Electro-optical PCBs use copper for distributing power and low-speed data, and optical paths for high-speed signals. Pitwon noted that crosstalk, skin effect, and skew degrade signal integrity in copper systems at high frequencies. Optical signals, he explained, don't have those issues, and they have greater channel density than copper. He also claimed that optical signals could consume lower power than electrical signals, because they don't need equalisation and other signal conditioning. Optical signals could also reduce the number of layers on a board by 50% and surface area by 20%.

Optical technology is migrating to the backplane and to connectors. Optical technology, in the form of SFP and QSFP interfaces, has been around for some time, but, according to Pilwon, "We've developed optical backplanes and optical backplane connectors that include the optical transceiver at the connecting edge" (figure 1). Furthermore, optics can appear within a board, not just at the edge. That means the optics can move closer to the electrical signal sourcethe processor. Waveguides and fibre optic patch cords can be manufactured on a board.

 Electro-optical connections

Figure 1: With electro-optical connections, high-speed data signals can run closer to their electrical signal sources than ever before. (Image: IEEE)

Pilwon said that optical backplane connectors have been developed, as well as a technique to align the small waveguides to transceivers on the board. The next challenge is to develop waveguides on to boards where the tight bends don't degrade performance to unacceptable levels.

A Xyatrex demonstration system connected control cards through a backplane in a storage system using 12 Gbit/s links. But the links don't go all the way to a disc drive. In the future, Pilwon sees moving optics all the way to the disc drive.

Marika Immonen then explained the infrastructure of an electo-optical board. Figure 2 shows a cross-section of a board with both copper traces and optical waveguides. This diagram shows the optical paths embedded in the board, but they can also reside on the top layers.

 Optical waveguides

Figure 2: Optical waveguides can be placed on circuit-board surfaces or in inner layers. (Image: IEEE)

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