Automated Photonic Assembly -- A fertile field full of holes and rocks.

When Circuits Assembly asked Bruce Hueners, director of marketing at Palomar Technologies (Vista, CA), about the often-heard statement that manual assembly is a major deterrent to the growth of the fiber optics industry, he said, "It's currently true that 80 to 90 percent of photonic components are still being manually assembled, but robotic machines are putting together complex fiber optic assemblies every day." When Circuits Assembly asked why every aspiring fiber optic contract manufacturer (CM) is not automated, he said, "Because it's an evolving industry-highly segmented-and because it currently has so many odd-form components with diverse applications and assembly needs. Virtually every fiber optic component manufacturer has a proprietary assembly process that gives their unique product a competitive edge. Component assembly today is largely the province of captive manufacturers, such as Nortel, Alcatel and Lucent, who make components and systems. They may contract out some portion of their component assembly to specialized companies like JDS Uniphase Corp. [San Jose, CA], but optoelectronic component manufacturing is not at the outsourcing level common in the electronics industry. The CMs and electronics manufacturing services (EMS) providers, who reach that level in the fiber optics industry, will be instrumental in driving down its costs."

Hueners' career with systems and technology development firms make him a uniquely qualified authority on photonic devices and processes. Hughes Aircraft (El Segundo, CA), whose scientists invented the laser in the 1960s, chose Hueners in 1983 to commercialize laser interferometers for calibrating wire bonders. A decade later, he was part of the spin-off when Hughes formed Palomar Technologies for commercial developments. Ultrasonic wire bonding and precision component assemblies were quickly brought to a higher level. In 1995, Hueners became a guiding technical and marketing force when he and his fellow employees incorporated as Palomar Technologies. At the company's new facility in Vista, CA, new generations of advanced packaging and assembly equipment are developed for use in aerospace, medical, automotive, high frequency and multichip module (MCM) application-all of which now enable automation in the fiber optic telecom industry.

A Short Course in Photonic Automation

The fiber optic industry's source lasers, Hueners explained, are engines that convert a binary electrical signal into modulated optical signals. The conversion is completed by switching the source lasers on and off-either internally or externally. Functioning as key components in wavelength division multiplexing, both vertical cavity surface-emitting lasers (VCSELs) and edge-emitting indium phosphide (InP) source lasers transmit enormous amounts of optical data on multiple channels. Source lasers usually operate in the 1,550-nm frequency range, where attenuation losses in the silica fiber are lowest. VCSELs are predominantly used for short haul (metro and last mile) transmissions, and InP lasers are predominantly used for long haul transmissions. Often, frequencies of multiple signals are only fractions of a nanometer apart in a dense wave division multiplexer (DWDM), and each laser must be precisely set by manufacturers to exact specifications before delivery. The manufacturer must also ensure that the laser is capable of operating at a single frequency over the working life of the device. Tunable lasers can generate a constant output from within a range of frequencies-a capability that provides some degree of freedom in manufacturing and stocking of spares in the field.

Optical fiber signals need to be reamplified at approximately every 80 kilometers. This process can be completed by employing erbium-doped fiber amplifiers (EDFAs) and raman amplifiers. In an EDFA, a short length of fiber is impregnated with small amounts of the rare earth element erbium, which is excited (or pumped) by a 980-nm laser. Under this excitation, the doped fiber amplifies the source signal at the input and sends the strengthened signal back onto the optical network. Energy transfers to maintain source signal strength in raman amplifiers occurs along the entire optical link, in the direction opposite the traveling optical signal. As the optical signal is attenuated while traveling from the source laser, the signal is also being amplified by the raman gain from the opposite direction.

"A solid-state laser is an extremely efficient device, converting up to 50 percent of its input power to light," said Hueners. "When a very small device is driven with a relatively high current, a large amount of heat dissipation is needed. Diode lasers are capable of extremely high power densities up to 10 MW/cm2. We get the heat out by providing a very good thermal contact to the heat sink inside the package, but the contact has to be completely free of solder voids for the laser to reliably function. Laser packages have to be more reliable than medical devices, partly because lasers cost so much, but mainly because some have to function in submarine systems on the bottom of oceans-making them a good deal harder to replace than a heart monitor."

Hueners continued by saying, "Gold/tin is the most common eutectic alloy for optoelectronics because it provides a clean, high-thermal chip connection. In conventional gold/tin reflow, they spike the temperature up, overshoot the reflow point and reduce [the temperature] at an uncontrolled rate. The result is inconsistent melting, variable heat transfer and voids under the die. This may be okay for many conventional electronic devices, but not for photonics. To avoid damaging gallium arsenide (GaAs) or InP chips, we use an in-situ programmable, conductive reflow process with micron accuracy for preform and pre-tinned devices. We ramp up the temperature at a carefully controlled rate, critically damp at the eutectic point to avoid overshoot, dwell for a programmed interval at the reflow temperature and ramp temperature down at the optimized rate for device performance. Doing it right sounds like it's just a matter of properly using the tools at hand, [but] believe me, it's not that simple. When it comes to photonics, we've learned that nothing is as simple as it first sounds. You really have to understand photonics, material properties and the reasons for the meticulous steps in the process to bring effective automation to photonics assembly."

Rocks and Holes

Hueners explained that, "Automated photonics assembly is a fertile field with many challenges, and CMs are seeking more process capability and production scalability as they're getting into [production]. They begin with manual and semi-automatic assembly, then, as was done when surface-mount technology was new, they concentrate on critical processes and proceed to islands of automation. The next step links the islands into in-line automation."

Many rocks, in the shape of odd-form components, exist in Hueners' fertile field of optoelectronics and photonic automation. Significant holes exist in the areas of design for manufacturing (DFM), design for test (DFT) and experienced process engineering. A virtually virgin automation supplier base, for a highly segmented industry with numerous demanding processes, could be considered a massive sinkhole. Standards are needed for clearing the assembly field of such obstacles to allow the growth of the anxious outsourcing industry.

"Without standards," says Hueners, "it's hard to design devices suitable for the cost-effective automation an industry needs before it can grow. Dimensional standards, for packages and substrates, and handling compatibility standards, like those in the surface-mount industry, are needed. From the semiconductor area, standards concerning metrics for reliability, maintainability and availability of equipment must be presented. Basic standards such as these are largely absent from the optoelectronics industry. Initiatives on these issues are underway, but the development of real working standards can take several years. Standards not only pave the way for commonality in manufacturing techniques, but are also essential in forming partnerships between OEMs and outsourcers-driving costs down.

"Intellectual property issues are also delaying standardization. Companies manufacturing components agree with the industry's need for standards, but are reluctant to work on [formation] committees because they don't want to disclose any of their firm's proprietary developments. [These actions put] equipment manufacturers like us in the position of developing new products to intercept new technologies as they are developed. The only way to stay ahead of the game is to do continuing, in-depth research in optoelectronic and photonic developments and trends, which happens to be a fascinating area."

Conclusion

At one point during his interview with Circuits Assembly, Hueners said, "Getting into automated photonics makes me think of the early pioneers starting on the Oregon Trail. Someone once commented that, 'The timid ones never started, and the weak ones died along the way.'" No matter where companies are going, guidance helps the timid as well as the brave. To guide you on your way to automated photonic assembly, technical journals, global trade shows, symposiums and online courses can offer workshops and lectures on every aspect of fiber optics science and technology-from stilbazolium derivatives to Mach-Zender modulators. I'm starting with the basics by ordering a copy of a book recommended by Hueners, Understanding Fiberoptics, by Jeff Hecht, 3rd Edition, published by Prentice Hall.

Jerry Murray is West Coast Editor; e-mail: [e-mail].

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