Optoelectronics Packaging: A New Manufacturing Era -- The growth of the optical communications market provides challenges and opportunities for EMS suppliers.

While much of the electronics industry has been in a slump, one segment has emerged as a bright new spot-optoelectronics. Optoelectronics are the elements that interconnect photonic components and electric circuitry. Photonics are active and passive components and the interconnects that create the optical path. Optoelectronic packaging is the system integration of photonic, optoelectronic and electronic components comprising many physical constraints such as insertion loss (dB), thermal, material, reliability, form factor, manufacturability and cost. Packaging and assembly includes the device, module, board and subsystem levels.

Why Fiber Optics?

Optical fiber is the alternative to copper wire for carrying signals for voice and data. The use of photons, instead of electrons to transmit communication signals, offers significantly greater bandwidth and speed. The basic concept is simple-to take an optical signal and transmit that signal for a long distance and then receive the signal in a much faster and more economical method.

Growth in Optical Communications

Major manufacturers of optical fiber are reporting to be sold out for the rest of the year. Companies such as Lucent and Corning have increased production and plan future expansions.1 The 10-year roadmap, published by the National Electronics Manufacturing Initiative, Inc., (NEMI, Herndon, VA), shows annual growth rates of 17 percent for optical fibers, 15 percent for lasers and detectors, 67 percent for optical switching components and 34 percent for wavelength division multiplexing (WDM) components. Future growth for optical communications is anticipated due to its adoption in new markets, such as fiber communication, within cities and offices (metro) and fiber to the home.

Assembly Challenges: Smoke and Mirrors?

Optoelectronics products require the assembly of active and passive components. Active photonic devices include items such as edge emitting source lasers (EESLs), vertical cavity surface emitting lasers (VCSELs) and photodiode receivers (PDs). Active device interconnection includes optical alignment (active and passive), mode field matching for lens bonding and tapered waveguides, optical path conversion and multilayer wiring. Complicating matters are optical path conversions, such as curved waveguides and mirrors. Passive devices include filters and fibers.

Among optoelectronic products are high-speed receivers that convert incoming optical signals to electrical outputs. Today, 2.5 Gb/s signals are common, with 10 Gb/s signals in production and 40 Gb/s signals just around the corner. To make these receivers, photodectors must be mounted in a hermetically sealed package with tight tolerances, long-term stability and good electrical and thermal performance-all at low cost. In many cases, the photodiode must be perpendicular to both the optical input and output. Because optical signals can be affected by material outgassing, changes in position (due to alignment issues) or other factors, the use of many materials is prohibited. Most materials must have low moisture absorption characteristics, and tight tolerances make the alignment operations difficult for assembly.

Today's diode assembly methods include wire bonding-with flip chip planned for the future. Ceramic substrates are common, but flex circuit, fused silica, Cordierite, low-temperature cofired ceramic (LTCC), silicon, Teflon and liquid crystal polymer (LCP) materials are under investigation. With the requirement for hermeticity, the typical housing is a precision machined Kovar packaged with a brazed lid. New housing materials and methods are under consideration-including housing made of aluminum silicon carbide, metal injection molding, stamping methods and various ceramic materials. Attachment methods for optical components are typically proprietary, but precision alignment solutions include the use of solder, epoxies, low-temperature glass, aluminum oxide (AlO) bonding to silicon and laser welding.

The assembly of optical fiber, made of glass, presents even more challenges. Single mode glass fibers, which transmit light through a core just six microns in diameter, must be handled, precisely aligned, maneuvered and attached. Each of these steps must be accomplished without insertion loss that would result in degradation of the optical signal.

Printed circuit boards (PCBs) must also keep up with the optoelectronics developments in signal speed and bandwidth. Ultimately, signal bandwidths are expected to exceed the capability of copper and on-board optical transmission will become a necessity. Possible options include: 1) optical waveguides created by lithographic processing or 2) thin polymer films patterned on the surface of the PCB or buried within the substrate by subsequent lamination.

Emerging Trends

Currently, optical printed boards feature the use of optical fibers, and an increased number of layers and smaller feature sizes are anticipated. Many companies are discussing the use of polymer optical waveguides to minimize transmission losses.

Planar lightwave circuit (PLC) platforms will integrate optical waveguides and electrical and optical devices with high-frequency wiring required. Silicon platforms, with glass optical waveguides and arrayed waveguide gratings, divide light into multiple paths of signals.

Optoelectrical printed boards are targeted for high-speed multichip integrated circuit (IC) packaging for router/switch functions. Demand for this option is driven by the need for low transmission loss (< 1dB/m) and high transmission speeds (> 200 Mbps). Substrate options include ceramic or copper polyimide thin films and feature the incorporation of polymer waveguides.

Future Needs

Automated manufacturing is virtually nonexistent, and package and assembly standards are yet to emerge. Yields are low, due to the number of components, and, while potential volumes are large, today's market is small and economies of scale are lacking. For the market to reach full potential, greater communication is required between the designers of optoelectronic products and the companies that will manufacture these components, modules and systems.

Reference

1. TELEWatch, February 21, 2001, www.rhk.com/telewatch.

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E. Jan Vardaman is president of TechSearch International, Austin, TX; e-mail: jan@TechSearchInc.

com.

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Copyright [copyright] 2001 Miller Freeman LLC