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Pathways to Low-cost Optoelectronics
December 31, 1969 |Estimated reading time: 3 minutes
By Alan Rae
In 2002, NEMI and IPC will revise the optoelectronics sections of their roadmaps to include technology options that may lead to low-cost optoelectronic assemblies.
There is a strong interest in low-cost optoelectronic assembly today for several reasons. First, in North America only part of the necessary infrastructure has been established, with just 5 percent of the fiber lit and only 5 percent of the population having access to a broadband connection much lower than Europe and Asia. The density of investment needed at the local level interferes equipment costs must be reduced and investors must see a clear way to gain ROI.
Additionally, increasing computer bus speeds will become a critical factor in the near future. Desktop computers now have clock speeds up to 2 GHz, but some roadmaps to faster processors show 5 GHz bus speeds within the foreseeable future. Data volume and speed will be difficult to obtain with conventional circuitry, so optoelectronics will extend into data processing.
Another issue lies in the complexity of integrated circuits (IC) with pinouts around 2,000, up to 40-layer boards will have to be used. Calculations suggest that 80-layer boards may be necessary to handle the 4,000 I/O devices on the ITRS roadmap.
Finally, current optoelectronic datacom assemblies are built to unnecessarily high reliability telecom standards. Many switches and routers reside in air conditioning rather than on telephone poles. Packages generally are hermetic and high cost, hand-assembled onto circuit boards with manual management of soldering and the fiber "pigtail," which cannot be exposed to reflow temperatures because of an 80°C temperature limit.
The current optoelectronic standards addressing these issues are neither comprehensive nor compatible. Therefore, IPC is putting together STD-040, a comprehensive standard that will be proposed as the international optoelectronics joint standard.
NEMI teams also are doing the groundwork to "make optoelectronics mainstream."
About 90 percent of the board cost currently comes from the optoelectronic components, which were designed for performance rather than manufacturability. This is understandable as data rates rise from 2.5 to 10.0 to 40.0 Gbps and manufacturers are pushed to commercialize the latest technology as soon as it is available. Loss management between transmission laser and receiver also is costly and critical. Active alignment positions and fixes the outgoing optical fiber in six dimensions (X, Y, Z, pitch, roll, yaw) in the tens of nanometers. Commercial package yields can be as low as 10 percent.
In assembly, hand soldering nonstandard hermetic packages to the board is typical, although the newer small form factor transceivers can be assembled using a reflowable plug-in mount with BGA-type pad format with the penalty of 0.4 dB as two extra connectors are needed to avoid attached pigtails. Hand soldering the typical hermetic "butterfly" package is time- and yield-consuming to double the output, manufacturers need twice the operators. Mass-produced, low-cost non-hermetic or semi-hermetic packages are being developed and are starting to come to market.
NEMI has six projects underway or forming in the optoelectronics arena to support the progress needed in these areas: three in fiber management (signal performance, fiber handling systems and splicing); one in materials (optical adhesives); one in assembly (selective soldering); and one evaluating optical interconnects.
NEMI also has identified several "levels" of interconnection technologies to cope with the rapid transfer of data:
Level 0 is the current high-frequency technology. Boards are now produced to handle 10 Gbps using clever high-frequency design. Coaxial structures, broad traces and other techniques have pushed the performance of FR4 materials much further than previously thought possible.
Level 1 represents single fiber component-to-component connection flexible and relatively inexpensive in terms of materials but inefficient and expensive to assemble.
Level 2 describes multiple fibers embedded in flex. Fiber routing is simplified, but where the fibers leave the harness they are vulnerable to damage. One fiber damaged too close to the flex to be spliced means the whole assembly must be discarded.
Level 3, board-level interconnect, shows both the most promise and challenge. The end game in optoelectronics connection is to use SMT, with packaged vertical cavity surface-emitting lasers (VCSEL) firing directly into optical vias with structures that permit interconnection within and between boards.
With the help of comprehensive standards and improved systems, the industry may be less than five years away from the widespread availability of lower-cost, higher-reliability interconnect systems to handle high data rates.
Alan Rae, vice president of technology, may be contacted at Cookson Electronics Inc., 25 Forbes Blvd., Suite 3, Foxborough, MA 02035; (508) 698-7238; Fax: (508) 698-7201; E-mail: arae@cooksonelectronics.com; Web site: www.cooksonelectronics.com.