Last Updated: August 30, 2019
In terms of long-term reliability, the “weakest link” of a connectorized cable assembly is the area within the ferrule which contains the strip “transition point” (where the buffered fiber is stripped down to bare fiber) and the majority of the injected epoxy. In this discussion I will focus on the strip “transition point”—-the epoxy discussion deserves its own separate, dedicated discussion.
The “transition point” is where the buffered fiber is stripped away to reveal the bare fiber. This is often done mechanically, presenting opportunity for damage to the bare fiber from the stripping tool. Very minor damage to the glass renders is very weak, greatly increasing the risk of fiber breakage under normal stresses. This is observed in the process of scribing, where the fiber is “nicked” with a sharp scribing tool, which allows the fiber to break very easily under very minor side or tensile loads.
A fiber that has been damaged during stripping may not break under normal handing during production assembly, but the defect remains in the finished product—–a potential “ticking time bomb”, ready to break at any time in the future, after the customer has already installed the product.
To ensure maximum long-term reliability in the finished product, it is important that the manufacturing site considers this potential by taking preventive measures to help reduce the chance of fiber damage during stripping, and measures to identify such damage, should it exist, before connectorizing. Such measures may include:
- Performing a “bending test” on each stripped fiber, by securing the buffer just below the strip point and bending the fiber 45-degrees in each of 4 directions. If the fiber has been scored during the stripping, it is likely to break during this test. Better to break now and have to re-strip the product than to have it break later in the process or, in the worst case, have it break after receipt by the customer.
- When cleaning the fiber with alcohol before connectorizing (which should always be done), make sure the operator cleans the fibers “squeaky clean”—–the vibrations of the fiber which generate the “squeak” may be sufficient stress to break a fiber which has been damaged during stripping.
- Having all strip tools on a strict Preventive Maintenance program, examining the stripping blades for wear, and evaluating the stripping effectiveness. The Maintenance program can use the results of Bending Test failures to trigger immediate blade replacement, and also be used to determine a suitable time-based or strip-count-based blade replacement schedule.
- Always ensure stripper blades are cleaned and free of debris before stripping each fiber.
- Prior to inserting into the ferrule, ensure that stripped fibers do not contact anything other than the lab-wipe used for cleaning. Even resting stripped fibers on a production table surface can result in damage to the bare fiber. If the operator needs to put down the stripped cable before inserting into a ferrule, the stripped fiber end should be suspended in air and not allowed to contact and surface.
Even if the stripped fiber is kept free of damage during stripping and handling prior to inserting into the ferrule, it is still very susceptible to breaking if it is not completely encapsulated in epoxy within the ferrule. All termination operators are familiar with connector “strip charts” provided by connector manufacturers. Such strip charts should always show the target strip length, as well as tolerances. The buffer strip length is critical, particularly in jacketed (crimped) assemblies, to ensure that the “transition point” is deep enough within the connector to ensure full encapsulation by the epoxy after insertion. The cured epoxy helps protect the “transition area” from any bending or tensile forces that may be introduced during the products working life (i.e. stresses during installation, stresses due to environmental variations, prolonged spring-force pressure between two mated connectors).
Ideally, the “transition point” will ultimately be located either just touching, or just behind (less than 1mm), the rear of the ceramic ferrule. It is an excellent practice to periodically take samples from the production line and cross-section them to look inside a finished connector to validate the stripping template lengths and the connectorization process——to see where the “transition point” is located, and if it is sufficiently encapsulated within the cured epoxy.
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