Adhesives deliver excellent performance on a range of materials used to manufacture optical components, including glass, metal, ceramic and most plastics. By their very nature, adhesives allow devices to be made stronger, faster, and more cost-effectively, living up to the most basic demands of the marketplace.
The three critical adhesive performance properties for optical applications are glass transition temperature (Tg), outgassing, and 85%RH/85C performance.
Tg is the temperature at which an adhesive polymer transforms from hard and glassy to soft and rubbery. A high Tg is desirable but does not guarantee a “pass” in heat/humidity aging. At temperatures above the Tg, the adhesive’s coefficient of thermal expansion(CTE) rises rapidly as the polymer matrix expands. This increase in space can result in unacceptable part movement or even open up the polymer matrix to chemical or environmental attack. While Tg is a good criteria for initial product selection, other characteristics such as the adhesive’s affinity to the substrate and its chemical and environmental resistance are also important.
High Tg is important to the very aggressive damp heat or 85%RH/85C test that is used widely as the benchmark for determining the life of a device. The high test temperature causes materials with a Tg at or below 85°C to become soft and rubbery, allowing moisture to penetrate the adhesive bond line, and causing possible delamination of the assembly at the adhesive/substrate interface.
Low outgassing is required by most device manufacturers for optical components. When selecting adhesives, most engineers rely upon NASA outgassing specifications for total mass loss and collected volatile condensable materials. For many optical assembly applications, a low outgassing adhesive tested at 85°C for a couple of hours may provide acceptable performance replacing adhesives that meet NASA requirements of 24 hours at 120°C in a vacuum. Optical manufacturers with hermetically sealed packaging, however, may need to meet the NASA specification in order to avoid adhesive condensation on sensitive optics.
Other adhesive performance criteria that may affect optical assemblies are product shrinkage, optical clarity, optical transmission, processing viscosity, thermal expansion and hardness. If properly selected, adhesives can deliver easily assembled, high performance fiber optic devices that will offer years of predicable service and reliability.
High Performance Adhesives
Adhesives provide excellent chemical and solvent resistance, act as electrical insulators, and may be used for optical alignment, potting, sealing, and structural applications where they can bond dissimilar and heat-sensitive materials quickly, efficiently, and cost-effectively.
Adhesives can speed the manufacturing process, lower costs and even improve and enhance reliability and performance. They distribute stress load evenly over a broad area, reducing stress on the joint. Since adhesives are applied inside the joint, they are invisible within the assembly. Adhesives resist flex and vibration stresses, and form a seal as well as a bond, protecting the internal components from harsh environments. They join irregularly shaped surfaces, negligibly increase the weight of an assembly, create virtually no change in part dimensions and are easy to automate.
Available Adhesive Technologies
There are six adhesive families that are most commonly used in optical assemblies. Each offers a unique combination of performance and processing benefits.
Epoxies — Long the workhorse of the optics industry, epoxies are one or two-part structural adhesives that bond very well to a wide variety of substrates and are low outgassing. Epoxies tend to cure slower than other adhesive families, with typical fixture times between 15 minutes and two hours.
Epoxies offer high Tg and shrink minimally upon cure. They are commonly used for bonding fiber to ferrule, potting, joining dissimilar materials, and underfilling. Epoxies can also be used to bond/secure strain relief boots, secure fiber onto packages, and for structural reinforcement.
Light Curable Acrylics — One-part, solvent free UV cure acrylics offer performance comparable to epoxies. Today’s UV technology delivers Tg greater than 100°C, shrinkage of less than one percent, and very low outgassing values. To enhance worker safety, optically clear and non-yellowing UV acrylics cure well into the visible light spectrum and allow use of UV blocking substrates. Cured acrylic adhesives deliver superior thermal, chemical, and environmental resistance.
Light cure acrylics cure in seconds and often feature a secondary heat or chemical cure mechanism that allows complete cure in shadowed areas. As cure is on-demand, these adhesives offer extended open times for part positioning. These high strength materials are available in ranging degrees of flexibility from soft elastomers to glassy plastics.
High clarity UV acrylics can be used in the light path to bond glass ferrules, for direct fiber attach in component tacking, lens bonding, critical laser alignment, potting fiber bundles and leads,
and edge sealing of flat panels and touch screen displays.
Elastomers — Elastomers, including silicone and silicone-free modified silane products, are the best option to ensure a robust assembly when bonding dissimilar substrates like glass to metal. These flexible, rubber-like materials cure at room temperature, exhibit excellent resistance to heat and moisture, and bond a wide variety of substrates. Their pliability over temperatures from -40 to 250°C make them ideal stress absorbers.
UV cure, dual UV/moisture cure, heat cure, and two-part silicone technologies complement older RTV chemistry. Elastomers are used to ruggedize components, gasket and seal packages, and pot components exposed to extreme temperature swings.
Cyanoacrylates — While their structural bonding properties are inadequate for most optical assemblies, instant adhesives excel at temporarily tacking down fiber, components and boards while the permanent adhesive cures. CAs achieve fixture strength in just seconds and full strength within 24 hours. Recent advances include the introduction of light cure, two-part and flexible formulations. CAs are used for general bonding, threadlocking plastic screws, tamper proofing set screws, and bonding boots to ferrules.
Anaerobics — These single-component adhesives remain liquid when exposed to air, but once confined between metal substrates, they cure or harden into tough thermoset plastics that provide excellent environmental and temperature resistance. In applications where cleanliness is critical, semi-solid gels and tapes are available. Their fast fixture time and high ultimate strength make them ideal candidates for bonding fibers to ferrules. Anaerobics are also used to lock screws and bond bearings to shafts.
Structural Adhesives — Two part structural adhesives typically consist of a resin and an activator/hardener, but are also available in one-component “no-mix” formulations where the activator is applied before the adhesive. Halogen-free versions are available to meet local health and safety requirements. These systems can develop strength in as little as two minutes and have outstanding environmental and impact resistance, making them ideal for weld replacement or structural bonding of both plastics and metals.
To select the appropriate adhesive for optical applications, designers should consider how the device will be assembled, what substrates will be bonded, and the actual production process.
Answers to the following questions will help when specifying the appropriate adhesive:
- Is the device made with difficult-to-bond substrates like gold plate, polypropylene or nylon?
- 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 see UV light?
- Will surface treatments (plasma, corona treatment) enhance bonding?
- Will the substrates and adhesive perform properly in the end-use environment?
- Will temperature-sensitive substrates make heat cure or UV cure adhesives inappropriate?
- What kind of joint stress will the assembly see? Tensile? Compressive? Peel?