A Navy ManTech funded development program entitled Fiber Optic Interconnect Technology (FOIT) was developed to advance manufacturing technology associated with high reliability/ performance fiber optic assemblies.
This technology consists of the development of an automated work cell made up of 5 discrete stations which perform the major processes with fiber cable manufacture:
As part of this development process, the Epoxy Cure Station was developed to precisely control the thermal curing process of epoxy used in the termination of fiber optics.
The most widespread technique for curing epoxy in the manufacture of fiber optic cables was the use of a thermal block oven.
The block is typically manufactured from aluminum and has embedded heating elements.
Different control schemes are used, some with a simple on-off type controller and some microprocessor controlled to accommodate elaborate multi-step thermal profiles. The use of these devices entails manually placing an assembled, but uncured, termination into the hole and activating the controller.
There are many types of epoxies used in the termination process. In curing the epoxy, the objective is to achieve a glass transition temperature, Tg, that is higher than the most extreme operating temperature that the termination will experience to ensure the utmost mechanical integrity.
The importance of achieving the proper Tg further grows with fiber assemblies used in high reliability aerospace applications with demanding operating temperature extremes.
In the curing process, there are other factors that drive the actual cure schedule as well. It is not as simple as heating the termination to a fixed temperature for a predetermined time. In many cases, controlled ramp rates and “stepping” of the temperature to different plateaus is required in order to minimize stress build-up in the epoxy interface between the fiber and the terminus ferrule.
If the epoxy is cured very fast, the residual stress can fracture the fiber rendering the termination useless. In other circumstances the fiber may not necessarily fracture, but instead the stress is relieved as the fiber cable is used in its applications where it undergoes temperature cycling.
The stress relieving anomaly can cause the fiber to “piston” within the ferrule which results in high loss. This failure mode is not as severe in applications where the fiber assembly is operating in a constant environment, such as an office environment. It is more likely to occur is situations where the fiber assembly is thermally stressed, such as in an aircraft or exposed to harsh environments where cyclic temperatures are experienced.
The high performance curing system method of thermal curing specific for mil/aero applications places the utmost importance on high reliability and quality.
What are the benefits of this high performing curing system?
- This system utilizes a forced convection means of heating the terminus in process and employs a sophisticated computer control system that calibrates offsets for different cure profiles and termination types.
- This method ensures that the epoxy within the termination is experiencing the desired temperature for curing within 2° C.
- The system utilizes precision fixturing of the terminus with respect to a forced-convection heating nozzle, so that consistency from run to run and nozzle to nozzle is maintained.
- The improvements alleviate the variability associated with the use of block ovens and provide a highly consistent cable assembly product.