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Last Updated: August 5, 2022

Fiber Optic Center NewsThe processes involved in Termination play a CRITICAL role in determining the long-term reliability of the product, and more importantly, errors made in the Termination processes often cannot be detected by subsequent production-line tests / measurements!   Even though effective termination procedures have been developed, an operator error on the production line may very well produce a product that will pass all normal product tests and inspections, but will have dramatically increased chance of future field failure.   Thus, robust process controls, though important in any manufacturing stage, are essential in all Termination processes, to ensure maximum product reliability and performance.

A typical fiber is “terminated” through the following steps:

  • Fiber is stripped and cleaned.
  • Epoxy is prepared and injected into a ferrule assembly (ferrule pressed into a ferrule holder)
  • Ferrule (loaded with epoxy and fiber) is loaded into a Curing Oven for a period of time to cure the epoxy.

Fiber Stripping:

Fibers must be stripped of all protective coatings prior to inserting into a connector.  Mechanical stripping, by far the most common method, consists of using tools with sharp, precisely machined metal cutting blades to cut into the protective layer and strip it away from the fiber.   It is important for the blades to cut a clean, neat edge on the protective coating, and that the blades do not contact or scratch the bare fiber during stripping.

If the blades are dull, the coating edge at the stripped boundary may be rough and jagged.  Such a rough coating edge can lead to lateral forces on the fiber when it is inserted into the ferrule holder/ferrule assembly (causing degraded product performance).  It is better that the stripped edge is cut cleaning and 90-degree angle to the fiber, so that the edge will butt evenly against the rear of the ferrule during potting.

Any nicks to the fiber during stripping create an EXTREMELY weak point in the fiber.  The transition point where the protective coating ends and the stripped fiber begins is especially prone to damage and is always the weakest part of the entire fiber length of a product.  This is where most fiber breaks occur, either in the production line or in the field.

Damage to the fiber during stripping may not always cause the fiber to break immediately.  It is very possible for a damaged fiber to remain unbroken during the production line processing.  Unless the weakened fiber breaks during production processing, there is no way to measure if the fiber has been damaged or not.   A product with damaged fiber may pass all standard production testing (visual inspection, IL / RL, etc), but will be extremely susceptible to future field failure.  Controlling stripper blade quality and stripping processes are essential to ensure maximum product life.

Fiber Cleaning:

For the epoxy to properly bond to the fiber, the fiber must be clean and free of oils or other contamination.   This is usually done by wiping the bare fiber with a soft, lint-free wipe and high-quality alcohol.   Proper cleaning materials and techniques must be identified and strictly adhered to in the production line because the effectiveness of the cleaning cannot be confidently verified in subsequent process steps/tests.

Epoxy Preparation and Curing:

Epoxies used in FO cable assembly production are carefully selected for their cured bonding properties—their ability to keep a strong fiber/ferrule bond during the service life of the cable assembly.   During their lifetime, connectors are exposed to a variety of environmental and mechanical stresses (changes in temperature and humidity, forces from mating against another connector, etc).  A properly cured, solid epoxy bond helps ensure the fiber will not move within the ferrule, and thus maintains a proper fiber-to-fiber connection, regardless of exposure to such stresses.

For a two-part epoxy to achieve its maximum cured bonding properties, the two parts must be sufficiently mixed and cured at the proper temperature and time.  Improperly mixed or under-cured epoxy will generally NOT continue to cure at room temperature——the epoxy may APPEAR to be fully cured, and will exhibit sufficient bonding for the product to complete the assembly process—the product may very well pass all subsequent visual and IL / RL testing.  But it will never have the long-term properties for which the epoxy was selected in the first place.

The only way to verify that epoxy processes are appropriate for the product is through destructive product testing during process development.   And since destructive testing is not possible in a production line, very robust process controls MUST be in place in the production line to ensure the accepted processes are followed precisely.   Again, proper mixing processes or proper application of curing heat or curing time CANNOT be verified after the fact.

Epoxy De-gassing: 

When a two-part epoxy is mixed, it is very likely now filled with many small air bubbles.  These bubbles MUST be removed prior to injection into a ferrule (production lines commonly use centrifuges or vacuum chambers to remove gasses/bubbles trapped within the epoxy after mixing). Any air bubbles or voids trapped within the cured epoxy may cause excessive stresses on the fiber during the product’s life cycle (most commonly due to changes in temperature, which will cause the gasses trapped in the bubbles to expand much more than the surrounding epoxy).  Such stresses can lead to significantly reduced performance or even product failure.  And once again—it is IMPOSSIBLE to detect the presence of such bubbles inside the ferrule after the product has been cured.

Epoxy Injection into Ferrule:

It is common practice to inject epoxy into the rear of a ferrule, until a small bead of epoxy is observed exiting the ferrule tip. This is an effective way to ensure that the entire ferrule hole has been filled with epoxy prior to inserting fiber.

However, often overlooked is the importance of controlling the amount of epoxy which remains BEHIND the ferrule (inside the ferrule-holder).  This is the area that holds the weakest part of the entire fiber length:  the transition area where the protective coating has been stripped from the bare fiber (see above, “Fiber Stripping”).  It is critical that this entire transition area is completely encapsulated in epoxy. The epoxy provides strength to this area which is very weak and susceptible to fiber breakage.

Too much epoxy inside the ferrule holder can lead to other issues—for example, the inserted fibers will displace the injected epoxy so that it flows out the rear of the ferrule holder.  This may cause the epoxy to flow over the spring within a connector body, locking up the spring and creating a defective connector.  However, these cases are observable/testable after curing.

Too little epoxy inside the ferrule CANNOT be observed/tested for after curing.  Therefore,   processes and controls MUST be implemented which ensure consistent and sufficient epoxy quantities inside the ferrule assembly.  Pneumatic epoxy dispensing systems are very common, and they can work very well.  However, the amount of epoxy that is dispensed with such systems will vary depending upon the epoxy viscosity, which will change through the pot-life of the epoxy.  These systems can be effective, but only if the production processes account for this change of epoxy, and have proper processes and controls in place to ensure proper epoxy quantity has been injected.

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