Adhesives for Fiber Optics Assembly: Making the Right Choice
Using the proper adhesive not only saves time and expense, but also can improve reliability and performance.
Edward A.Y. Fisher
Fast becoming pivotal to the assembly of fiber optic components, adhesives perform well
on glass, metal, ceramic and most plastic substrates, provide excellent chemical
and solvent resistance, act as an electrical insulator, and may be used in high-strength
optical alignment applications. Adhesives can bond dissimilar materials quickly
and efficiently, enabling the production of many optical components that would be
impossible to make using mechanical fastening methods.
Adhesive technology has always played a role in
fiber optics assembly, bonding optical connectors, fibers, lenses, prisms and other
components. Initially, epoxy technology was the method of choice, primarily in the
connector market, but today’s adhesives are highly engineered products available
in a wide variety of choices to help fabricate fiber optic assemblies.
Benefits of adhesives
By choosing the correct adhesive, manufacturers
can speed fabrication, lower costs and even improve reliability and performance.
By their very nature, adhesives allow devices to be made stronger, faster and more
cost-effectively, fulfilling the most basic demands of the marketplace.
Adhesives offer several major benefits
over other assembly methods, such as using nuts and bolts, rivets, welding and soldering,
any of which can result in localized stress points. One problem with welding and
soldering is that both can introduce unwanted heat to the device, and they usually
are not effective on dissimilar substrates. They also require highly skilled assemblers.
Adhesives, on the other hand, distribute
stress load evenly over a broad area, thereby reducing strain on the joint. Because
adhesives are applied inside the joint, they are invisible within the assembly.
They resist flex and vibration stresses, and form a seal as well as a bond, to protect
the internal components from harsh environments. They join irregularly shaped surfaces
more easily than mechanical or thermal fastening do, increase the weight of an assembly
negligibly, create virtually no change in part dimensions or geometry, and quickly
and easily bond dissimilar substrates and heat-sensitive materials. Adhesives are
“one size fits all,” and assembly can be easily automated.
One limitation is the curing time,
which is the amount of time it takes for the adhesive to reach its maximum strength
and fixture. Other drawbacks include surface preparation requirements and the potential
need for joint disassembly.
When selecting adhesives, manufacturers should
consider the three aspects of a bonded assembly — the parts that make up the
component, the assembly process and the adhesive — and how they will affect
device production (Figure 1).
Figure 1. The three variables that will affect device production
include the component, the assembly process and the adhesive.
To select an appropriate adhesive for
an application, a designer should consider how the device will be assembled and
what substrates will be bonded. It is also critical that the adhesive specified
during the design phase is appropriate for the production process.
Three properties are key to the application
of an adhesive: glass transition temperature, outgassing and testing that involves
placing samples in an environmental chamber set at 85 percent relative humidity
and 85 °C. Outgassing is the evolution of unreacted or unbound product when
the adhesive is subjected to high heat. These properties are called out in Telcordia
Glass transition temperature is the
temperature at which an adhesive transforms from hard and glassy to soft and rubbery.
It can be measured using dynamic mechanical analysis, thermal mechanical analysis
or differential scanning calorimetry. All three techniques are accepted in the industry
and provide different results based on spectral interpretation.
Dynamic mechanical analysis is generally
accepted as having excellent repeatability with good dependability. Regardless of
technique, the Telcordia specification calls for a minimum glass transition temperature
of 95 °C. A high transition temperature does not guarantee a “pass”
in heat/humidity aging. Cyanoacrylate adhesives can have temperatures much higher
than 95 °C, yet will fail in a hot, moist environment. For cyanoacrylates,
a high temperature does not equal high performance. There are low-temperature (60
°C) UV acrylics that have outstanding adhesion to glass and that will not lose
adhesion after 2000-plus hours of damp heat testing.
Without industry guidance, most engineers
gravitate toward the NASA outgassing specification for total mass loss and collected
volatile condensable materials. However, it is up to the end user to determine the
acceptable outgassing levels for an adhesive. A low-outgassing adhesive tested at
85 °C for a couple of hours seems more realistic than the NASA test of 24 hours
at 120 °C in a vacuum. Those with hermetically sealed packaging may have to
use low-outgassing products that meet the NASA specification to avoid adhesive condensation
on sensitive optics.
The aggressive 85 percent humidity,
85 °C test, or 85%RH/85C, is the final and most important criterion for obtaining
Telcordia certification. The high test temperature will cause materials with a glass
transition temperature at or below 85 °C to become soft and rubbery, allowing
moisture to penetrate the adhesive bond line and causing the assembly to delaminate
at the adhesive/substrate interface. This test, considered one of the toughest in
the industry, is used in the fiber optics market and other communications markets
as the benchmark for determining the life of a device.
Other adhesive performance criteria
that may affect an assembly are product shrinkage upon cure, optical clarity and
transmission (if the adhesive is in the light path), viscosity (for processing),
thermal expansion and hardness.
If properly selected, adhesives can
deliver easily assembled, high-performance fiber optic devices that will provide
years of predictable service and reliability.
Available adhesive technologies
Of the multitude of adhesives currently available,
five families are most commonly used in fiber optics assembly. Each offers a unique
combination of performance and processing benefits. Manufacturers who dedicate significant
up-front time to researching and selecting the proper adhesive for an application
will save significant time and expense later in manufacturing and reliability.
Epoxies: Long the workhorse of the
fiber optics industry, epoxies are common one- or two-part structural adhesives
that bond well to a wide variety of substrates, have low outgassing levels and shrink
minimally upon cure. Their major disadvantage is that they tend to cure much more
slowly than other adhesives, with typical fixture times from 15 minutes to two hours.
Two-part epoxies also must be thoroughly mixed to ensure best performance.
Despite their cure time, epoxies are
a favorite in fiber optics assembly because of their high glass transition temperature
and low shrinkage properties. Available in small package sizes, easy-to-mix two-part
“bipack” epoxies are extremely popular because they minimize adhesive
waste and accommodate small production runs. One-part, premixed heat-cure epoxies
also are popular, although work life becomes short once the adhesive is removed
from cold storage. Dual-cure (UV and/or heat) epoxies are a recently introduced
option that increases processing speeds. As production volumes increase, assemblers
can use larger volume, side-by-side static mix products.
Two-part epoxies are used extensively
in bonding fiber to ferrule. They also are used for potting electronic components,
bonding dissimilar materials such as ceramic or glass to aluminum. Epoxies also
can bond secure strain relief boots and can secure fiber onto packages.
UV-curable acrylics: One-part, solvent-free,
UV-curable acrylics offer performance benefits comparable to those of epoxies. These
adhesives contain a photo-initiator that starts to cure when exposed to UV light.
Although early UV-curable acrylic adhesives had low glass transition temperature,
high outgassing and high shrinkage values that made them unsuitable for use in critical
fiber optic devices, today’s UV technology provides glass transition temperatures
greater than 100 °C, shrinkage of less than 1 percent and very low outgassing
values. Because cured acrylic adhesives are thermoset plastics, they offer superior
thermal, chemical and environmental resistance.
Traditional UV adhesives must be in
direct UV light to cure — any adhesive in dark or shadowed areas remains uncured.
However, new light-cure acrylic formulations are available with a secondary cure
mechanism (such as exposure to heat or chemical activators) that allows the adhesive
to cure completely in shadowed areas. Because cure is on demand, light-cure acrylics
offer extended open times for positioning and repositioning parts. These adhesives
have high bond strength with a wide variety of substrates and come in varying degrees
of flexibility, from soft elastomers to glassy plastics. All this, coupled with
cure times of just two to 60 seconds, makes UV-curable acrylics an attractive alternative
to epoxy technology.
Figure 2. A light-cure adhesive was previously applied to a clear plastic part and here
is undergoing manual spot curing that takes mere seconds.
The clarity of UV acrylics enables
their use in the light path for bonding glass ferrules and for attaching direct
fiber. They also are used in V-groove and lens bonding, critical laser alignment,
potting fiber bundles and leads, coating ribbon cables and block bonding/sealing.
Silicone: For bonding dissimilar substrates
such as glass to metal, the best option to ensure a robust assembly is silicone
technology. Silicones are flexible, rubberlike materials that cure at room temperature,
exhibit excellent resistance to heat and moisture, and bond a wide variety of substrates.
Their pliability over a broad temperature range (240 to 250 °C) makes them
ideal stress absorbers. Today UV-cure, UV/moisture-cure, heat-cure and two-part
silicone technologies complement the older moisture-cure chemistry. Moisture-cure
silicones require a minimum amount of ambient moisture to ensure maximum performance.
Silicones are used to bond ceramic
and epoxy glass boards to metal packages, to toughen tall components, gasket and
seal packages, and to pot components exposed to extreme temperature swings.
Cyanoacrylates: High-strength, one-part
cyanoacrylates are commonly used as processing aids in fiber optics assembly. They’re
instant adhesives, also known as “super glue.” Although their structural
bonding properties are inadequate for most fiber optic assemblies (because they
do not adhere well to glass and have poor high-temperature resistance), they excel
at temporarily tacking down fiber, components and boards while the permanent adhesive
is curing. They achieve fixture strength in just seconds and full strength within
24 hours. Cyanoacrylates are also used for locking screws, making setscrews
tamperproof and bonding boots to ferrules.
Anaerobics: Traditional anaerobic adhesives,
which are also known as threadlockers, are single-component substances that remain
liquid when they are exposed to air. Once confined between metal substrates in the
absence of oxygen, anaerobic adhesives cure or harden into tough thermoset plastics
that provide excellent environmental and temperature resistance. These adhesives
are ideal candidates for bonding fibers to ferrules because of their fast fixture
time (minutes) and high ultimate strength. Anaerobics also are used in the traditional
way to “lock” down lids in devices and to make setscrews tamperproof,
ensuring that factory settings can’t be changed and helping to void any warranty
claims for products that have been opened by the user.
Meet the author
Edward A.Y. Fisher joined Henkel Loctite in 1994
as a chemist developing cyanoacrylate primers, UV-curable adhesives and maintenance
products. He recently joined the company’s industrial group specializing in
fiber optics adhesives after two years as an applications chemist in the electronics
group specializing in surface-mount and thermally conductive adhesives.
Questions to Ask When Choosing an Adhesive
• Does the design include difficult-to-bond substrates such as gold plate, polypropylene
• Are there dissimilar metals that may cause thermal expansion problems when heated?
• Are any of the parts UV-absorbing, making a UV-curable adhesive inappropriate?
• Are there shadowed areas that will not be directly exposed to UV light?
• Will surface treatments (plasma, corona treatment) enhance bonding?
• Will the substrates and adhesive perform properly in the end-use environment?
• Are there temperature-sensitive substrates that can’t tolerate heat, making the selection of a heat-cure or even a UV-cure adhesive inappropriate?
• What kind of joint stress will the assembly see? Tensile? Compressive? Peel?
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