SMT Optoelectronics or Optronics — Promises and Problems
The optoelectronics field is so new that no one can even agree on what to call it. Most people refer to this field as optoelectronics (OE) but there are some who are shortening the term and calling it optronics. Whatever the name, this technology currently is at the same stage as the field of SMT was in 1981. “SMT” had to be spelled out to make sure that people understood it stood for surface mount technology.
SMT may have matured but it is an evolving technology and renews itself with updated packaging technologies to house OE devices as it did to house electronics devices. As we prepare to deal with OE’s problems and promises, we can benefit greatly from the early experience of SMT and its subsequent evolution. The package will remain the same (or slightly different) but its contents will differ. Some OE devices will be mounted on the substrate in bare die form and, hence, will present the same challenges as current electronics devices.
OE deals with two technologies — optics and electronics. Optics or light is photons. The photons take the place of electrons in the circuit to make the connection between devices. Where electrons travel through copper wire or copper line on the board, photons use the optical fibers as the travel medium for interconnection. Because photons travel at the speed of light, they are much faster than electrons.
The cost of optical systems is so high that key customers of these systems barely can afford them. This partly explains the precipitous decline in the stock prices of major suppliers of optical components and systems such as Cisco, Lucent, Nortel Networks, JDS Uniphase, Corning and a host of other companies. The demand for OE systems almost disappeared even though there is great demand for broadband. You don’t have to look further than the type of Internet connection you have in your house. How many houses even in the United States are connected with optical fibers? Not many. Even businesses have to wait for weeks to put in a relatively slow DSL line. We are in a “catch-22” situation. We need this technology but it is very expensive. So we plod along with copper connections and learn to live with 56K modems to connect with worldwide wait. But there is no question that this technology, even though the demand for it appears flat, will explode when the critical mass is achieved. So the cost has to come down significantly for the proliferation of the broadband technology to our homes and businesses. The packaging engineers play an important role in reducing the cost.
Telecom lasers represent 65 percent of the diode laser market with $5.1B market share. There are three types of lasers: transmitter, pump and vertical cavity surface emitting lasers (VCSEL). These lasers are used in different applications depending on the distance of applications. In each application, however, they serve the same purpose: source of light. On each end of the optical connection, we still have to deal with electrons or electronics; at this time, we don’t have a totally optical system. So in OE we have to deal with both optics and electronics. This means that we must learn to deal with the packaging issues as the same signal travels both as photons and electronics or vice-versa to make end-to-end connections between different optical and electronic devices.
Explained differently, in OE we are dealing with both photons and electrons. Transporting data, as photons, through optical fiber is (O), which provides higher speed and bigger bandwidth or pipe than the copper wires used for transporting electrons. We are converting signals from photons (O) to electrons (E) and then back from electrons to photons (O). So these O-E-O conversions are at the heart of optical networks starting from board-level interconnections to system-level interconnections via backplanes to an alphabet soup of LAN, MAN and WAN where L, M and W stand for local, metro and wide area networks. Transmission, amplification and data reception are accomplished through photons (O). Add/drop multiplexing/de-multiplexing (ADM) is done through electronics (E). Photonic devices such as lasers, lenses and attenuators handle photons. They are mounted on optical subassemblies on printed circuit boards (PCB), along with other SMT and through-hole components to perform O-E-O conversions and amplifications, as applicable.
The laser is the key device in producing the light (O) when triggered by electronics. The light is multiplexed in different colors or wavelengths and fed into fiber optic cable. As the light travels through the fiber, it needs to be regenerated and amplified. Amplifiers typically are made from Gallium Arsenide (GaAs), and amplify the signal as it attenuates when traveling along the optical fiber.
Good examples of OE products are optical transmitters and receivers, displays and sensors. In an optical transmitter, electrons are converted in the circuit to photons and transmit the signal in the form of photons through optical fibers. The transmitter generates photons with lasers when driven by electronic drivers and sends the optical signal over the optical fiber. To convert, the transmitter has an optical driver chip and a VCSEL.
The optical receiver on the other end of the circuit receives the photon signals through the optical fiber and photo diode and converts it back to electrons to interact with other electronics devices such as SMT parts and chips on the PCB.
OE Problems and Promises
There are many issues in connecting fibers to each other either as splice or through connectors, including misalignment and separation. Fiber misalignment is a significant source of loss in an optical signal and hence in performance degradation. Any air gap between ferrules will create reflections. Unwanted reflections are a problem in high-frequency transmission line systems. In aligning and splicing fibers, air gaps, misalignment and changing materials cause reflections. Additionally, any water or contamination of the fiber end to be spliced or connected can degrade the signal’s quality.
In any new technology there are issues that must be resolved. Industry organizations must accept the challenge of developing standards. But before standards are developed, issues must be raised and solutions proposed and agreed upon by the industry. When a technology is new, everyone wants to keep it under wraps as intellectual property. However, there are many issues that are not unique to any particular product but rather apply to all products in that industry. One of the best ways to highlight this is through conferences.
I am glad to see this happening. For example, in early May, IPC — The Association Connecting Electronics Industries organized its first OE conference in Toronto. Considering that it was organized hastily under the auspices of the Surface Mount Council during its January meeting in San Diego, the conference was attended heavily. If interested in a copy of the proceeding, contact Jack Crawford at the IPC. Do not be surprised to see a focus on this topic at APEX 2002 by the IPC. IPC is not the only organization to hold conferences and seminars on OE packaging issues. For example the Surface Mount Technology Association (SMTA) is organizing an OE conference in November. For details, contact these organizations at www.ipc.org and www.smta.org.
As an industry, we need to explore the accelerating advances in OE packaging and assembly issues. This is essentially a virgin territory and fortunes will be made in understanding the technology and its needs, and developing products to meet those needs. Following are some examples in which the SMT industry (material and equipment suppliers) can help implement this technology.
One critical issue in OE is automation. OE device assembly processes, especially optical fiber alignment and joining, are slow and labor intensive. Accuracy and speed are two considerations for automation. For example, multiplexing devices and amplifies require 12 mm accuracy. Speeds of 200 to 300 components per hour in current machines can be obtained to place such components. Lasers and photodiodes require 5 mm accuracy. To automatically place such devices, speeds of 50 to 100 components per hour are achievable. And there are some cases where lenses, lasers and amplifiers need an accuracy of less than 1 mm. Such components can only be placed at only 30 to 40 per day. There is a great market for equipment suppliers who can place components precisely and quickly. In some cases, accurate placement alone is not enough to meet the needs of this technology. Components need to be attached right after placement by the same machine. This means that we need integrated assembly systems and not just placement machines.
Newer attachment processes also are needed. Some OE devices may have trouble withstanding reflow temperature cycles. This provides an opportunity for soldering equipment suppliers who do not heat these devices.
There are many material issues for OE components. For example, clear underfill materials with great optical characteristics (not causing signal loss because of refraction and reflection), thermal adhesives for cooling and alternative materials such as conductive adhesive to replace solder are needed. Some OE devices will degrade due to heat in reflow ovens. Dispensing or printing conductive materials on substrates with fine features, developing substrates with fine features, and lines with very smooth edges to improve performance are needed. This is not a comprehensive list of technical issues in OE. You can go through all SMT assembly processes and think of changing materials and processes to address the unique needs of OE devices.
Let us also not forget the need for industry standards to address the unique needs of OE packages. The IPC is ahead of the curve in this area and has started some initial effort in putting together groups to address this issue. If you have some ideas and would like to contribute and participate in this effort, please give me a call.
RAY P. PRASAD is an SMT Editorial Advisory Board member and author of the textbook Surface Mount Technology: Principles and Practice. Additionally, he is president of BeamWorks Inc. (www.beamworks.com) and founder of the Ray Prasad Consultancy Group. Contact him at his Web site: www.rayprasad.com.
The optoelectronics field is so new that no one can even agree on what to call it.