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Assembling Optoelectronic PWBs with a Standard SMT Assembly Line
December 31, 1969 |Estimated reading time: 3 minutes
By Ronald C. Lasky
In spite of recent setbacks in the optical communications industry, there remains considerable interest in this long-distance, high data rate technology, allowing optoelectronic PWB assembly with current SMT lines.
Optoelectronic assembly causes confusion because of the different levels of assembly that exist. This article clarifies these levels and discusses the skills and equipment necessary to participate in the different assembly levels. It should be understood that optoelectronics is a rapidly evolving field; hence, the discussion that follows is a generalization of technologies that continuously merge and evolve.
Optoelectronic Assembly LevelsThere are at least three levels of optoelectronic assembly: optical subassembly, transceiver module and printed wiring board (PWB).
The optoelectronic optical subassembly (OSA) typically consists of a light source (often a semiconductor laser) or detector, feedback control monitoring electronics, and a lens. The critical task in assembling an OSA is to assure that the laser or detector is aligned with the lens in the optical path so that it can transmit or receive light into or from the optical fiber. This alignment requires expensive alignment and bonding equipment as well as technical skills foreign to the typical PWB and component assembler.
The module is assembled from transmitting and receiving OSAs (TOSAs and ROSAs) and support electronics. There are varying sophistication levels in module designs, but typically they are designed electrically and optically to receive as input electrical and optical signals. The incoming electrical signals are parallel streams, which are transformed into an optical serial data stream to be transmitted. The incoming optical signals are in a serial stream that is converted into parallel electrical data streams.
Module assembly is similar to integrated circuit (IC) component assembly, with the exception that OSAs are delicate because of their precise optical alignments. Therefore, care must be taken not to thermally shock the OSAs in module assembly. Optoelectronic module assembly can be performed by organizations that currently assemble components such as ball grid arrays or quad flat packs, with a moderate amount of additional skill development and capital investment.
The majority (95 percent or so) of optoelectronic PWB assembly consists of assembling electrical components to the PWB with standard SMT processes. There are only two major concerns when performing this assembly: 1) OSA thermal shock and 2) handling of the optical fiber if attached to the module (referred to in the industry as "pigtailed"). Typically, the thermal issue is mitigated by hand soldering the module at the end of the process. Optical fiber is relatively robust as long as it is not bent at a radius of less than approximately 5 cm; however, the end of the fiber can be scratched. Fiber handling is an art that has not been automated, but a reasonably skilled team of manufacturing engineers can develop fiber-handling techniques that will work for a given product. Therefore, a typical SMT assembly facility can assemble optoelectronic PWBs if the organization has an able team of manufacturing engineers to develop techniques to minimize thermal shock and develop fiber-handling techniques.
There is one issue in PWB assembly that can be a showstopper: test. Optoelectronic test arguably is an order of magnitude more difficult than standard product test. If test is a requirement (i.e., must be performed by the assembler vs. being vended out) and the assembling organization has never performed optoelectronic test, the organization will have to hire engineers with this skill. The suite of equipment required can cost more than $1 million. The bottom line is that test requires a major commitment of engineering skill and capital equipment.
ConclusionIt is possible to perform optoelectronic PWB assembly with a standard SMT line; however, the team faced with this challenge will have to learn some new skills. Various resources such as industry organizations and publications are good sources of information to supplement this need.
Ronald C. Lasky, Ph.D., PE, senior consultant to the Indium Corp., may be contacted at 26 Howe St., Medway, MA 02053; (508) 533-5672 or (315) 381-7566; Fax: (508) 533-5678; E-mail: rlasky@aol.com; rlasky@indium.com.