Addressing the Technical Challenges of Non-hermetic Laser Packages
June 13, 2019
In today’s competitive atmosphere, optical component manufacturers are facing growing pressure to find cost-effective laser package solutions to help reduce their final product cost. An attractive lower cost option is a non-hermetic laser package – however, non-hermetics introduce their own set of design challenges, particularly when dealing with cooled lasers.
Uncooled non-hermetics are fairly straightforward, because the laser generates heat during operation, so its temperature will be higher than ambient, eliminating the risk of the device dropping below the ambient dew point. However, in cooled non-hermetic packages, the laser CoC (chip-on-carrier) assembly may need to operate at temperatures below ambient, which results in condensation on the cooled side of the TEC and on the laser diode assembly.
The Biggest Challenge: Condensation
Condensation within the laser package is the most critical challenge facing non-hermetic TEC design. It leads to electrical and thermal shorting, as well as corrosion on electrical contacts. It can interrupt the laser’s light emission and coupling into the fiber. All of these outcomes will degrade the overall TOSA (transmitter optical sub-assembly) performance, and impact laser slope efficiency, increase coupling loss (which in turn lowers data transmission rates) and can even shorten the usable lifespan of the laser package.
This is especially important to note because some manufacturers are rushing to the market with a version of a non-hermetic TEC, but these solutions often use “quick fixes” to address failure modes, such as edge sealants for insulation to block condensation. This doesn’t truly solve the failure mode itself, leaving the TEC hard to QA and open to extreme performance degradation in real-world usage, especially in use cases where condensation may occur. It’s crucial to look for manufacturers who have tested their TECs in real-world condensing environments.
Testing: Test in highly-accelerated reliability test conditions, but the failure modes tested must reflect the operating conditions that are present out in the field. In standard THB (temperature, humidity, bias) reliability testing at 85°C/85% RH, the dew point is approx. 81°C. This means that, if the laser requires cooling to 40-60°C, condensation will occur, as the dew point is only 3°C below ambient temperatures. This condensation will happen regardless of TEC design or TEC protection approach. Therefore, if powered 85/85 testing is needed (and operating temperature is between 40 and 60°C), then moisture-proof sealing considerations such as gasketing, ultrasonic welding, or sealing with epoxy or acrylic must be factored in the design process to prevent condensation from entering the laser package.
Application ambient conditions: Relative humidity also depends on ambient temperature, as it decreases as temperature increases. Some applications, such as indoor data centers, pose low risk for condensation. However, other applications such as outdoor wireless access infrastructure offer a potentially high risk of condensation due to the low degree of control over the environment. Designs will need to control for the absolute humidity inside the laser package and for relative humidity, as dew point is a function of ambient temperature.
TEC orientation: If the TEC is oriented vertically, then gravity is on your side, and can help to prevent moisture from accumulating. However, if the orientation is horizontal, that can encourage the collection of condensation.
Assembly processes: Properly-designed non-hermetic TECs should be compatible with most of today’s popular laser package assembly processes, including wire bonding, solder reflow, epoxy cure and Argon plasma cleaning.
Choosing the Right TEC Manufacturer
How can you avoid these design pitfalls? Choose your non-hermetic TEC manufacturer carefully, as the right one will be a partner who can guide you through these decisions. Look for a company with solutions that are already in the field, passing reliability targets and working in real-world cooled applications where they are being exposed to condensation every day. You’ll also want to make sure that the manufacturer you’ve chosen has taken the time to work on solving the root cause of failure in condensing environments — that way their solutions won’t degrade your performance or increase your power consumption over time.
As a good rule of thumb, you’ll want to find a partner who brings extensive thermal design expertise to the table, and who can offer non-hermetic TECs that have no impact on performance. Try to find a true drop-in replacement for hermetic TECs, as it will remove any possible limitations on design or integration approaches. To support the high demand of data centers and the impending deployment of 5G applications, you’ll want to make sure that your selected manufacturer can also support high-volume production of their non-hermetic TECs in an automated manufacturing facility.
Questions to Ask Before You Start
Before you get started on your non-hermetic TEC design process, start by asking yourself these questions:
What is your driving requirement for non-hermetics? Is it purely cost savings or are there other factors to consider?
What are your reliability testing requirements?
What is your operating relative humidity at the maximum ambient temperature? If you’re unsure, do you know your absolute humidity?
What components of your package are at risk if water condenses on them?
What steps will you take to prevent moisture from reaching the transceiver (sealants, gasketing, other approaches)?
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