Adhesives for Optical Assembly
- Created: Friday, 01 April 2011
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?