Adhesives for Optical Assembly
- Friday, 01 April 2011
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.