The robustness of optical transceivers is a key consideration for new designs and implementations. Since the majority of transceivers are designed for data center type applications, Telcordia GR-4681 has been the reference standard that dictates the testing demonstrates both design and process robustness. However, when one considers the industrial, military and aeronautical markets, this qualification is no longer enough to ensure adequate performance.
Instead, the test definitions are covered by pass-down requirements of the original project that are conveyed up the supply chain meaning that the transceiver vendor has little opportunity to propose alternatives. Different applications are covered by different standards with MIL-STD 8102 being most widely cited for military/aeronautical (mil/aero) applications and AEC-Q1023 for automotive. Some of the more challenging tests that these standards cover include salt fog, fungal growth, temperature/humidity and shock/vibration.
These tests required additional processing, compared to the commercial world, in terms of protective, conformal coatings that ensure protection against corrosion, fungus and humidity. Such coatings are generally polymer-based compounds that are applied as a liquid and then cured to provide the protection. IPC-CC-830C4 defines the type and qualification requirements for different materials. Other protective coatings are available that can offer additional properties such as being hydrophobic or thixotropic.
Although protective coatings are commonly used in PCB processing and systems used for mil/aero applications, they are still considered a niche market for optical transceivers. This is because the optical die and lens surfaces need to be sealed against the coating as these lead to problems such as optical loss or reliability issues. Traditional methods for sealing, including TO-cans or hermetic packages, generally lead to larger or more expensive solutions which limit adoption. Therefore, being able to adapt an existing commercial, high volume part, that allows easy sealing and coating has many advantages.