Automating the Optoelectronic Assembly Process

Estimated reading time: 6 minutes

Automating optoelectronics component manufacturing can lead to enormous cost savings due to increased yield and decreases in labor, materials and operation complexity.

By Bruce Hueners and Bradley K. Benton

Optoelectronic assembly is being given considerable attention as momentum is gaining for build-up of the optical network. However, the rules and conditions that electronics assembly manufacturers are accustomed to are altered when dealing with optoelectronic package manufacturing. Materials are different, thermal considerations play a major role and placement accuracy is critical for electronic information transmittal into an optical form.

The package accounts for an estimated 60 to 80 percent of current manufacturing expenses in optoelectronic assembly. Until recently, optoelectronic package assembly has been almost exclusively a manual process because of package complexity and relatively low production volumes. As the demand for increasingly cost-effective photonic components grows, increasing emphasis is being placed on assembly automation processes to drive down component costs and increase yield.

In manual assembly processes the headcount requirement is high, yield and repeatability are generally low, total component cost is high, and time-to-volume production is slow. In automated production, headcount requirement is low, yield and repeatability are high, total component cost is low, and time-to-volume production is short.

Still, industrial engineering approaches common in other technology industries largely are absent from today's photonics manufacturing environment. These practices include process engineering, design for manufacturing, design for test, standardization, outsourcing and automation. These are interrelated and any discussion of automation must include these practices together with the specific types of materials used in optoelectronic components.

The first true automation in the optoelectronics industry has come from adapting existing automated die attach and wire bonding equipment to meet the needs of optoelectronic component manufacturers. The industry has found that the wire bonding and die bonding equipment that was designed over the past decade for the high-performance hybrid, multichip module and microwave industries can be used as off-the-shelf solutions for automating certain processes in the optical industry.

Special Materials, Processes and FunctionsBecause the optoelectronic package is a hybrid processor of both electronic and photonic signals, there are a plethora of unique materials used in device fabrication. This variety of materials is one factor that distinguishes optoelectronic device assembly from conventional microelectronic assembly. Instead of silicon serving as the substrate, compound semiconductors such as indium phosphide (InP) and gallium arsenide (GaAs) are used. These materials are fragile and require special handling and thermal management.

Processes for optoelectronic assembly today include many traditional microelectronic assembly processes and a few special processes with the added complication of fiber alignment and active component alignment. Implementing a high-yield, fully automatic assembly process for optoelectronic packaging requires specialized machine functions. These include precision motion, machine vision and recipe-driven process control capabilities.

Automating the Optoelectronic Assembly ProcessHow does automated optoelectronic assembly work? Optoelectronic automation commonly is used in the manufacture of pump and source lasers. These devices convert electrical signals into light, thus creating the optical stream that is needed to transmit data along the "superhighway" (Figure 1).

Figure 1. A typical source laser butterfly package.

In a typical optoelectronic package, a compound semiconductor typically is attached to a substrate, then mounted inside an optoelectronic package. An automated assembly cell positions the substrate, carrier or submount, accurately places a eutectic preform (optionally a heat spreader/spacer and a second perform), places the laser diode, and pulse heats the entire stack. This submount then is placed on a thermal electric cooler in the "butterfly" optoelectronic package used in transmitters, receivers, transceivers and amplifiers. Once components are attached, the parts are wire bonded, establishing the first-level package interconnect.

Automated optoelectronic component assembly cells are built on a large open architecture optical breadboard, allowing the flexibility to perform many optoelectronic assembly operations. These assembly cells accommodate a variety of presentation tooling, including expanded wafer, 2 and 4" waffle, or gel paks in trays or stacked, allowing the manufacturer to adapt the process for specific requirements. Although these systems can operate in automatic, semiautomatic and manual modes, the entire process can run unattended, picking components from the input tooling, processing them, and placing them in output trays or material handling conveyors. Because the major cause of clean room contamination is human handling, eliminating operator handling contributes to higher quality, reliability and yield of the finished packages.

Thermal ManagementCritical to the optoelectronic assembly process is a void-free eutectic solder interface that manages thermal and electrical connections needed to generate a stable transmission of laser light. Eutectic die attach transfers the heat generated by high-frequency circuits and diode lasers, and draws it away from the package's active areas. If not void-free, small bubbles of nitrogen, or reducing gas, can get trapped in the eutectic media. These pockets do not conduct heat away from the laser diode and "hot-spots" form at the interface, affecting the laser diode's performance and stability.

Figure 2. Programmable pulse heating vs. dumped energy heating methods.

To accomplish the best thermally conducting solder interface, the temperature profile of the attachment process must be very repeatable and have the capability for a high-temperature ramp rate. Figure 2 shows controlled and uncontrolled temperature profiles. Once the interface is brought to the proper eutectic temperature, the heating mechanism must maintain that programmed temperature with minimal overshoot. After maintaining this temperature for the required amount of reflow time, the heating mechanism must cool the carrier down slowly to minimize damage to the laser's cavity and to allow the eutectic material to produce an equilibrium composition through a diffusion process. This is accomplished through simultaneous application of active thermoelectric pulse heating and cooling gases.

This controlled process is key for high-yield and reliability. Until recently, eutectic die attach was one of the most difficult processes to control. The stage was heated quickly to a temperature that was higher than the one required, the heat was shut off and the stage allowed to cool. If these systems were tuned too low, the resultant eutectic interface was either a cold or improperly phased solder interface. If too high, there were adverse affects on the laser diode.

Ramp up the heat too slowly, and you do not end up with the best thermal conductance of the solder. Cool down too quickly and you run the risk of micro fracturing the laser cavity in areas stressed during cleaving, leading to poor laser power output, frequency instability and significant reduction in the life of the laser diode.

Placement AccuracyFinal placement accuracy, i.e., where the laser diode is placed relative to the substrate edge, is vital for the proper transmission of the laser light. For p-side up edge emitting laser diodes, final placement accuracy of ±5 μm, 3 sigma, is required. Proper placement maximizes thermal conduction. Improper placement occludes the laser's emission. Today's automated systems can control placement quality, accuracy and repeatability.

Die placement in relation to other die also can be controlled tightly through automation. "Relative to die" placement algorithms place the die so that the second, third, and each die or substrate thereafter is placed relative to the final location of the last placed component, rather than the package. This provides greater component-to-component accuracy. It also controls the wire length and position, ensuring the wires are parallel and the lengths are the same factors important for high-frequency electrical conductivity.

ConclusionThe precision, accuracy, reliability, repeatability and higher throughput that are benefits of automation hold true for the optoelectronic manufacturing industry as well. Yet, the tight tolerances with little room for error, the operation's complexity, and material and labor costs make automating the optoelectronic manufacturing process even more desirable. Cost savings are tremendous. A component manufacturer demonstrated that one automated eutectic die attach machine could produce the same total output as four manual stations run by 20 operators. When using the automated process, yielded output went up by more than 50 percent, and there was a 67 percent reduction in occupied floor space in the clean room. As a result, return on the equipment investment was realized in less than three months.

The economic downturn has resulted in a reduced labor force and downsizing. Companies can ill afford the expense of costly repair and rework, or the results of low yield. As the demand for optoelectronic components and devices increases, the decision to automate the optoelectronic assembly process becomes a very appealing, and practical, option.

Bruce Hueners and Bradley K. Benton may be contacted at Palomar Technologies, 2230 Oak Ridge Way, Vista, CA 92083; (760) 931-3600; Fax: (760) 931-3444; E-mail: bhueners@bonders.com and bbenton@bonders.com