Application
Notesenko.com

Epoxy Injection
into Fiber Optic Connectors
MARCH 2022
Rev. 01

Issued by:Engineering
Department

Created by:Andrei Vankov

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Introduction

Securing fiber optic connectors with epoxy is a critical step of the connector termination
process. Any mistake with storing, mixing, dispensing, or curing of the epoxy, could adversely
affect a connector’s performance in the field due to premature bond failures. This applicationnote provides information on this critical process based on SENKO’s experiences gained
through years of manufacturing and reliability testing.
This document is centered around injecting epoxy into connectors featuring composite orceramic ferrules. For multifiber connectors such as MPO please refer to the Application Notecalled MT Ferrule Epoxy Injection Techniques.
It is important to note that working with epoxies requires personnel protection. It isrecommended to work in a well-ventilated area or under an exhaust hood wearing latex or nitrile
gloves to reduce any dermal exposure. In the case of epoxy spillage, acetone or IPA (IsopropylAlcohol) can be used to carefully clean it up.

2.0

Epoxy propertiesThe fiber optic termination process starts with selecting a proper epoxy type that is suitable forthe desired application. There are many varieties of epoxy that are available currently in themarket. The adhesives may have slightly different physical properties and curing schedules.
Prior to using the epoxy, it will need to be mixed with a proper ratio measured by weight. Part A
is typically a hardener and part B is a resin. Thus, a fiber optic epoxy is a two-part structural
adhesive that bonds the fiber glass silica to the zirconia ceramic ferrule. It has low outgassing
levels, a high glass transition temperature (Tg) and shrinks minimally upon curing.
It is important to understand exactly what Tg is and how it could influence connector
performance in various temperature and humidity environments. Tg should not be confused withmelting point temperature (Tm). Tg is the temperature range in which a thermosetting polymer
changes from one molecular phase such as a hard “glass” state to another molecular phase thatis a more pliable “rubbery” state. When this phase change occurs, typically in a hightemperature operation environment, the fiber could break the epoxy bond and retract or pistonback creating an undesirable air gap in the interconnect. In actuality Tg is not a discrete
thermodynamic transition, but a temperature range over which the mobility of the polymer
chains increase significantly. The ultimate Tg is determined by several factors: the chemicalstructure of the epoxy resin, the type of hardener that is used by the manufacturer, and thecuring schedule. Low temperature cures will result in the lower Tg. As an example, a SENKO
adhesive S-123 could have a Tg between 80°C and 120°C, depending on the cure temperatureand time. Therefore, it is important to cure at a maximum allowable temperature while
maintaining precise temperature control in a production setting. That is why a Technical Data
Sheet (TDS) has recommended the cure schedule as a function relating temperature and time:
the higher the temperature, the faster the epoxy will cure.
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SENKO has a proven track record with connectors passing strict GR-326 environmental
requirements that feature uncontrolled environment where temperatures vary from -40 to 85C.
To achieve these results SENKO has developed custom curing schedules for many connectors
using SENKO S-123 adhesive. Contact a SENKO sales representative for more information.
Once an appropriate adhesive is selected, it is important to note the shelf life and recommended
storage temperatures. For example, a common Epotec 353 ND or S-123 epoxy has a shelf lifeof 365 days when stored at 23°C. This time frame begins on the date the epoxy was packagedat the manufacturer. On occasion during storage at room temperatures epoxy may look foggy orgrainy. Most likely if the storage temperature was stable, a crystallization process could occur,
because the liquid resin may start a natural solidification. In this case, the epoxy is still usable.
To validate this point, the pack can be gently heated to about 65 ○C for a period of 15 minutes
and it should return to a clear liquid state.
Epoxies also have a limited working life. Typically, after mixing the workable viscosity ismaintained for a certain amount of time depending on the epoxy type and the ambient
temperature in the manufacturing facility. As the epoxy thickens, the pressure that is applied toa plunger dispenser needs to be increased in order to keep up with consistent volume dispatch.
The epoxy technical data sheets will have values for Pot Life and Working Life. These twoshould not be confused as it is not the same thing. Pot Life is defined as the amount of time ittakes for an initial mixed viscosity to double (or quadruple for lower viscosity products). Working
Life, on the other hand, is the amount of time an epoxy remains low enough in viscosity that it
can still easily be dispensed from a syringe. For these reasons, Working Life can vary due toambient air properties of the facility, the application, and even the connector type.

3.0 Epoxy types used in the Fiber Optic industry
Most fiber optic epoxies are two-part systems, which are formed through the polymerization of
two starting compounds: a resin and a curing agent. The curing process takes place when thereactive materials of the resin and curing agent combine. During initiation of the reaction, anexothermic process develops, enhancing the crosslinking of the two components. Two-partversions are stronger than one-part alternatives because of this bonding type mechanism.
That’s why it is necessary to thoroughly combine and mix the two-part adhesives to ensure thebest performance.

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Proper epoxy mixing, application and curing is necessary for a successful final result. Senko’s
recommended epoxy is S-123 which consists of Parts A and B. These parts need to bethoroughly mixed. The typical mixing ratio is 1 part of hardener to 10 parts of resin measured bymass. Proper mixing allows for the best possible performance of an adhesive. It is required touse a scale that has a resolution of one hundredth of a gram. A 3cc syringe typically filled withapproximately 2.5 grams of epoxy which would mean as an example 0.27 grams of resin and0.23 grams of hardener ration-vise when measured by weight, meaning you will need 5 times
0.27 grams of resin mixed with 5 times 0.23 grams of hardener. It is challenging to measureproperly. A minimum of two grams of material should be used each time a product is mixed.
This will ensure there is enough material for the hardener to adequately distribute in the resin.
Often epoxy can be ordered in so called bi-pax. This is slightly more expensive compared tobuying in bulk, but it prevents dealing with weighing the parts on a precision scale. Preweighted epoxy components Part A and Part B are separated with the clip or burst seal andcome in one pouch. It is recommended to use a roller for mixing the epoxy for approximately 45
seconds to ensure both epoxy components are homogenously distributed and the package
turns into one uniform color.

Figure 1 Roller used to mix epoxy in bi-pax

After mixing the epoxy, air bubbles will inevitably be present. To draw the bubbles out of a
freshly mixed liquid epoxy, commonly a centrifuge with a constant 3600-rpm speed and 6-inch
chambers is used in the industry. The centripetal acceleration of the centrifuge forces theheavier adhesive to settle to the bottom while the lighter air bubbles rise to the surface and arereleased. Bubbles and voids in fiber optic assemblies may result in serious performance andreliability problems. This is mostly due to irregular stress conditions however, assembly failure
could occur over time when a bubble or a void creates a condition that allows lateral loading
forces to a fiber. This can cause micro-crack propagation and breakage. Typically, a 7–10minute spin cycle in a centrifuge will draw the air bubbles out. Failing to degas the epoxy could
also reduce the proper fiber to ferrule bond because air expands at high curing temperatures.
This reduces epoxy bonding strength, which causes the fiber to move during environmental
temperature changes.

3Figure 2 Epoxy after mixing contains undesirable bubblesFigure 3 Epoxy after being centrifuged is bubble freeFinally, it is possible to purchase a frozen epoxy pre-packaged in syringes that are premixed
and degassed. This would be the most expensive option starting with the fact it will be shippedin dry ice to keep it frozen at -40°C. Once the package is received, the syringes should bemoved into a freezer at -40°C as soon as possible. Prior to using it, a thaw time will be requiredwith the needle end pointing down. Refreezing epoxies is conceivable, but typically is notrecommended due to the possibility of tracking moisture from exterior environment.

4.0

Applying epoxy techniques: manual versus automated dispensing

Once the selected epoxy is properly mixed, degassed and transferred to a syringe it is ready to
be used. Dispensing can be manual or automatic. We will consider manual first. Manual is aless expensive option as it requires just a 3cc syringe with a 20-22 Gage needle.
SENKO recommends the following technique in the Engineering Termination Procedures (ETP)
for ceramic ferrules:

1. Wipe clean the needle tip before applying the epoxy. Push epoxy application needle
up into the back of the flange so that it bottoms out against the back of the ferrule.
2. Carefully Inject the epoxy into the ferrule and clean the ferrule assembly with alcohol
including excess epoxy from the metal flange.
Recommended manual technique: Hold the body with the ferrule pointing up. Press thesyringe plunger gently until a very small bead of epoxy appears on the tip of the ferrule.
Slowly retract the syringe.
3. Clean the bare optical fiber with lint free wipe soaked with IPA (99% Isopropyl Alcohol),and then insert the optical fiber into the back of the connector body. The fiber is fully
inserted when the buffer tube bottoms out on the back of the ferrule.
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Note: Be careful not to break the fiber, insert the fiber slowly.
Note: Be careful not to damage bare fiber protruding from the connector ferrule.

Figure 4: Insert fiber into ferrule

SENKO instructions have additional professional tips that explain the proper epoxy
amounts required inside of the ferrule from a different perspective. For example:Ensure sliding the fiber buffer or loose tube all the way to the ferrule ceramic “well” filled withepoxy. The back-end of the ferrule (where it is pressed into the ferrule-holder) should havesufficient epoxy fill to ensure the fiber strip point can be fully encapsulated with epoxy.
Too little epoxy could lead to bubbles and voids and may cause the fiber to not be fully
encapsulated, drastically increasing risk of fiber breakage.
Too much epoxy may spill-over the ferrule holder after inserting the fiber, which may lock-up orinterfere with connector spring-action.

Figure 5 Proper ceramic "well" fill

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The “Epoxy Injection” process requires robust process controls, as it is nearly impossible toverify the effectiveness of the proper ferrule fill immediately at the termination station.
To verify the adequate epoxy amounts SENKO suggests performing periodic ferrule crosssection inspections. These can reveal how much epoxy is inside, whether the epoxy
encapsulates the fiber correctly inside the “well”, and if there are any bubbles present. Notenough epoxy, or large voids in the “well”, may cause the fiber to not be fully encapsulated. Thisdrastically increases the risk of fiber breakage. Too much epoxy in the ferrule and it may spillout the back the ferrule holder after inserting the fiber, this may undesirably lock-up theconnector spring or fiber.

Ferrule “well” filled with epoxy

Fiber properly epoxied

Figure 6 Proper "well" fill has no epoxy voids

However, the better way to ensure the epoxy dispensing process has a repeatability and control
is to use automated epoxy dispensing. Automated epoxy dispensing brings consistency toepoxy amounts inside the connectors. The automated epoxy station is a precision dispensing
system that applies accurate, consistent amounts of epoxies used in benchtop assemblyprocesses. Benefits of the systems’ consistent fluid application include: higher yields, better
process control and shorter training time for new operators. There are many AutomaticDispensing Systems (ADS) available in the market. Some of them use pneumatic air pressure
to press the syringe plunger, some of them have a lead screw for applying pressure. In general,
epoxy viscosity remains consistent for the first 30-40 minutes after mixing. The viscosity
increases in time requiring changes of pressure on the plunger in order to keep up with correct
epoxy volumes. Note, the epoxy will remain within a work life limits, however the viscosity will bedifferent. In this case the ADS that has a metered mechanical feature for applying pressure onthe plunger. This would show more consistency in epoxy dispensing amounts during viscosity
changes. For example, the method that uses an electronically controlled lead screw to push thesyringe’s piston can keep track of the amount dispensed with each shot and alert the operator
when it is time to load a new syringe. This approach achieves in theory the most accurate andrepeatable dispensed epoxy quantity, and it requires the least amount of operator training. Thismethod is well-suited for any operation, but it can be more expensive than other methods.
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It would be a manufacturing engineers’ area of expertise to develop the optimum ADS
techniques, including identifying the syringe needles shape and size, fixturing, etc. For example,a 22-gage needle might work better for 1.25mm ferrules, while for SMA connector types, a 20gage would be more suitable. Before dispensing epoxy the needle needs to be inserted all theway into the metal ferrule flange against the ceramic ferrule “well”. It is better to have a syringe
secured in the fixture with the needle pointing up and to use a foot pedal for pressure activation.
That way a technician will have both hands free for maximum control to inspect and clean theneedle before changing the connectors. Connectors or ferrules can be pointed up for easiervisual epoxy bead assurance as shown in Figure 7. Holding the connector firm with the fingers
and pressing the pedal while holding it makes it possible to prepare connectors in volumes forthe production line. After epoxy injection an operator should see a uniform bead atapproximately 0.2mm in diameter. It is advisable to store epoxy injected ferrules in trays with
end-faces down as in Figure 8. This way epoxy will not leak out and it will settle in the ferrule
well properly.

Figure 8 SENKO recommends using verticaltrays where the ferrules with dispensed
epoxy can be stored with flanges up.
Figure 7 Example of syringe holder for ADS

Using ADS helps in consistency of epoxy bead sizes. After cleaving, the fibers remain
encapsulated in a greatly reduced, consistent, and supportive epoxy bead. This means that they
are sufficiently protected for the initial polishing stage. This results in increased process speed
and higher output. Automation should be considered whenever production volumes justify
upfront investments through Return on Investment (ROI) calculations. Many military anddefense customers require automation in epoxy dispensing processes as this assures quality
and reliability.

5.0

Conclusion

Epoxying is an important part of the termination production process, which should result inquality, yields, and efficiency. SENKO can help the manufacturers select proper epoxy for an7

application, the dispensing method and level of automation to meet their needs, depending on
their daily volumes, connector types, and workflow.
There are two epoxy dispencing approaches most commonly used - manual and automated:
1. Manual approach relies on the skill of an operator to gage the epoxy amount after pressing
the syringe’s plunger, inspecting the end-face epoxy bead, and determining if the ferrule iscorrectly filled. Manual dispensing is inexpensive and practical for small manufacturers withlow production volumes.
2. Automated approach allows an operator to connect the epoxy filled syringe to a machinethat uses set air pressure to press the plunger and dispense a controlled amount ofadhesive. More sofisticated ADS have additional controls that regulate the applied pressure
to compensate for the reduced amount of adhesive due to viscosity changes in time. With
automatic dispensing, the process is less dependent on operator’s skills, but there isadditional capital expense for the equipment. The advantage is better control over thequantity of adhesive being dispensed with each shot versus a manual injection.

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