Designing an Optoelectronic Package
Robert Irvin, VP of Technology, Coviant, 3701 Market Street, Third Floor, Philadelphia, PA 19104, 215-966-6020, Fax: 215-966-6001, Email:
rwi@coviant.com.
Abstract
In the absence of standardization, designers of optoelectronic packages need efficient, reliable, repeatable packaging technologies to make the integrated cost-effective devices that the industry demands. This article steps through the parts of the design process, from understanding the customer’s needs to performing mechanical, thermal, electrical, and optical designs, and understanding how the different parts of the design interact. Most optoelectronic devices will face Telcordia testing, but the manufacturer can ease the process by preceding Telcordia testing with design verification testing. The choice of materials is critical during thermal and mechanical designs, as is understanding the temperature at which the die will operate. A requirement for hermeticity affects the device’s cost and package complexity. The designs of the optical train and the die both require an eye towards tolerances. Finally, design for automation allows a rapid increase in production scaleup if needed.
The major market trends for optical systems are pushing for low-cost packages that integrate two or more discrete device functions in an increasingly small footprint. Current markets also demand efficient and reliable devices. Especially in the optoelectronics (OE) industry, where standardization is yet to come, these market forces challenge designers to use efficient, reliable, repeatable packaging technologies (see Fig. 1). In addition to addressing the mechanical, thermal, electrical, and optical designs, the designers must also consider the device’s manufacturability and reliability.
Because the OE industry is highly fragmented, knowledge about packaging and packaging processes is also highly fragmented. This is where contract manufacturers, such as Coviant, provide a valuable service. Contract manufacturers should be familiar with a number of packaging technologies and can provide economies of scale and scope. This enables them to address a variety of issues during the design process. This article provides a landscape view of designing an OE device, rather than a detailed portrait of part of the process.
Know the requirements
First and foremost: the designer must understand the application and customer’s requirements. Instead of diving right into the design, take some time to understand the customer’s performance and reliability specifications.
Find out how the customer wants to balance performance and manufacturability. Everyone wants better, faster, and cheaper products, but if designers are expected to take all three issues seriously, then upper management must instill a philosophy that improving device performance, decreasing manufacturing time, and reducing costs are all important. Balancing the better/faster/cheaper issues is fundamentally a business decision that must be addressed throughout the design process.
Talk to the customer about the form and fit of the device, which goes beyond just the outside dimensions. It is important to understand the requirements of the customer’s board level designers. They may have specific requirements for the orientation of the electrical and optical leads. This carries through to the style of electrical and optical connectors used.
Understanding the application is especially critical when developing a new product, or when your customer is developing a new product. In the latter case, the customer’s specification document is often a work-in-progress and becomes a moving target. Once the designer clearly understands the acceptable windows of performance and reliability it is time to begin.
Testing and reliability
The final device performance is up to 80% dependent on package design and assembly processes. Most companies focus on wafer fabrication processes and neglect packaging. The packaging, however, will largely determine whether the device can survive testing with sufficient reliability.
What environmental specifications will the device need to meet? Most devices are tested to Telcordia specifications, which involve environmental and mechanical tests. Environmental testing includes damp heat (85°C/85%RH), temperature cycling (-40°C to +70°C), low temperature storage (-40°C) and accelerated aging (70°C). Mechanical tests include vibration, thermal shock and twist, flex and pull testing of the fiber leads.
Some of the tests in full Telcordia testing require up to 2500 hours to complete and some customers may require 5000 hour tests. As a preamble to Telcordia testing, a smart manufacturer sets up design verification testing (or DVT), which is a subset of Telcordia specifications and provides accelerated aging. A good DVT accelerates potential failure modes and enables the designer to pinpoint and fix problems before entering lengthy Telcordia testing.
Find out, as early as possible, if the package must be hermetically sealed. Hermetic sealing is expensive and difficult to make especially if the device must meet Telcordia specifications. If you can design a device that passes Telcordia testing without hermeticity, then you will gain a competitive cost advantage.
Mechanical design
The mechanical design is mostly about managing mechanical stress -- you will design parts that must be welded, soldered, or epoxied together with sufficient reliability to survive environmental and mechanical tests. A main concern during mechanical design is to choose compatible materials, with matching coefficients of thermal expansion. The designer’s choice of materials for housings, substrates, epoxies, and solders, is critical and permeates the design process.
Modeling that includes 3D CAD capabilities is enormously useful for design and evaluation. CAD programs also offer electrical, thermal, and mechanical stress performance simulations and analysis through finite element analysis (FEA) and thermal modeling. Reviewers are far more likely to spot assembly and tolerance problems in a 3-D computer model that can rotate to show all sides than in 2-D drawings. In addition to spotting problems, the review may show that you can loosen some tolerances and reduce the cost of the part.
Thermal design
The first question to answer is: At what temperature must the die operate? This depends on the physics of the device. Devices in general either run at ambient or at precisely controlled elevated temperatures. There are three possibilities:
· For room temperature devices to survive the thermal cycling that occurs during reliability testing, the design shouldn’t allow thermal stresses to build up. Therefore, the main focus of the design is ensuring good thermal conductivity between the die and the package wall.
· Laser diodes, on the other hand, operate at slightly elevated temperatures, around 25°C. These typically have a thermoelectric cooler (TEC) in combination with a heat sink to maintain the die at a fixed temperature. In this case, the design should ensure good thermal conductivity between the die, the TEC, and the package.
· Devices that operate at elevated temperatures of 75°C or higher typically incorporate thin-film heaters. In this case, one wants high thermal impedance between the die and package.
In each scenario, the materials properties, particularly the thermal conductivity and expansion, are important. Thermal properties play a significant part in selecting substrates, bonding materials, and bonding methods. Finite element analysis (FEA) software allows the designer to model thermal gradients across the die and through the package. A design should be verified both theoretically through FEA and experimentally.
Also make sure that the optical design takes thermal effects into account. Thermal gradients can cause uneven expansion of parts in the optical train. Unforeseen movements change the optical coupling efficiencies and degrade device performance.
Electrical design
How will the electrical leads get in and out of the package, and what technology will you use inside the package? Wirebonding is a mature technology for connections within the device, but it won’t always work for high-speed devices that operate at 10 Gb/s or better. These high-speed applications tend to use ribbonbonds because it allows the designer to better match the impedances.
If the package will be hermetically sealed, how will you pass electrical leads through the package walls? Most designers use the industry standard glass-to-metal seals. The seal is a glass bead with a metal wire through the middle. The glass bead is soldered into place in the metal package wall, while the metal wire is connected to the electrical leads on either side of the package wall (see Fig. 2). This provides electrical isolation and hermeticity.
Optical design
A number of issues revolve around getting light into, through, and out of the package. Most optoelectronic devices use fibers to couple light in and out of the package (see Fig. 3). Inside the device you have a choice of two coupling strategies. Freespace coupling uses lensed optics to couple light from one component to another through space. Butt-coupling attaches the fiber directly to the other components in a process called pigtailing or fiberattach.
Pay attention to the design and tolerances of the optical train. Some packages might have isolators, GRIN lenses, ball lenses, or coatings that must be taken into account. As the components are assembled, notice how the tolerances add up. Will meeting tolerances become a problem during manufacture?
How efficient must the coupling be -- in other words, how much of the light do you need to couple in and out? Higher coupling efficiency costs more. Will single-mode fibers be coupled to waveguides? These have the tightest coupling requirements and call for alignment to better than 1/10th of a micron -- the alignment equipment required for this tight a specification is quite expensive. Whenever possible, adopt passive alignment strategies because they reduce cost and improve manufacturability.
As with electrical leads, there is a standard method for passing optical leads through hermetic package walls. Metallized fiber can be inserted into a metal ferrule and soldered in place to create a hermetic subassembly. This subassembly is then passed through the optical feedthrough in the package wall and soldered in place to achieve hermeticity (see Fig. 4).
Die Design
Coupling efficiency can be improved in the die design. One major source of coupling loss is optical mode mismatches between components. The shape of the light emitted from components varies -- a diode laser emits an elliptical mode while fibers emit circular modes, and waveguides emit roughly elliptical modes. The die designer can increase the coupling efficiency by carefully designing the waveguide so that the waveguide mode shape matches the mode of the next component in the optical train. The die designer may use both optical design software to model the tolerances of the components and beam propagation software to analyze the light modes.
Thoughtful die design can also make aligning waveguides easier for pigtailing. In devices that include many waveguides (arrayed waveguide gratings, for example, may have up to 48 waveguides), the designer can leave the first and last waveguides on the die inactive. These waveguides are used specifically to make the alignment easier.
The pitch of waveguides (the distance from center-to-center) is also important to consider while designing the device. Arrays of fibers are typically coupled using V-groove technology, which allows for passive alignment of the fibers on a glass or silicon substrate. The pitch between the V-groove chip and the waveguides must match. A typical pitch for a fiber array is 250 microns, and some are as small as 127 microns. If you design a pitch any tighter than 127 microns, then the outer diameters of the fibers must be decreased, typically by chemical etching, which drives up the cost.
Design for automation
When the market for fiberoptic devices was small, assembly was largely done by hand. When the market boomed, there was no time to redesign devices to lower the cost of packaging and improve manufacturability — manufacturers just added people to the assembly process to build more devices.
Consider what happens if demand for your product skyrockets. If you have designed for automation, you will save time and increase profits because you don’t have to redesign the product for high-volume production.
Conclusion
Good packaging design for optoelectronic devices can minimize costs while providing a product that meets customer specifications — and expectations. The process includes thoroughly understanding the application and the acceptable windows of performance and reliability, and performing mechanical, thermal, electrical, and optical designs while understanding how parts of the overall design interact.
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