Light cures sticky problems in fiberoptics
Spot-curing technology is the process by which a dose of energy of a specific wavelength, bandwidth, and irradiance is used to cause an adhesive, encapsulant, or sealant to change from a liquid to a solid in a small area. Because fiberoptic devices are subject to long-term exposure to the environment, the performance and durability of adhesives in their cured state becomes an important consideration.
Typical light curing uses the ultraviolet (UV) and visible spectra emitted by vaporized mercury within a pressurized quartz envelope. These lamp assemblies are known as mercury lamp systems, or short-arc lamps, for applications where the area of cure typically is less than one inch in diameter. The mercury lamp is best for UV/visible light, emitting from 300 nm to 3 µm (see Fig. 1). For infrared (IR) curing, special tungsten halogen lamps are utilized, emitting in the 0.7- to 4.5-µm range.
Adhesives, sealants, and coatings used in the optoelectronics and fiberoptics industries have moved away from solvent-based formulations toward reactive systems such as photopolymerizing curing formulations (also known as UV/visible curing), heat-curing formulations, and combinations of both. The fastest polymerizing systems belong to the UV/visible family in which specialty chemical manufacturers have formulated a wide range of products to address the evolving needs of the marketplace. Light is used in the assembly process to cause photopolymerization of adhesives. Now, IR is used to initiate the curing process in thermal-cure adhesives. Whereas UV/visible light reacts with a photoinitiator contained in a typical light-cure adhesive, IR has been proven to excite the adhesive to initiate cure in thermal-cure epoxies typically used in the industry.
The use of radiation technology in the cure of materials utilized in component assembly is relatively new to fiberoptics manufacturing. While subcomponents and materials are manufactured to exact tolerances, variables in the assembly process are seen to have an impact on yields and throughput. High precision is necessary in the assembly process of active and passive components to increase productivity and quality. The use of UV/IR spot-curing has significantly improved the quality of the cure process.
Spot-curing with UV/IR light, however, is just one piece of the puzzle. In determining which adhesive to use, manufacturers must consider the materials to be bonded, the performance demands of the resulting component, and the assembly process itself.
Because of the rigorous performance requirements required of components submitted for industry validation, all parts of the assembly including the cured adhesive must be optimized (see Fig. 2).
Early testing indicates that UV/IR spot-curing reduces moisture absorption and shrinkage of adhesives, two important properties that measure component validity. Performance of an adhesive greatly depends on its ability to resist shrinking and absorbing moisture. The Bellcore 1216 test calls for 500 hours of exposure to 85% relative humidity at 85°F. Applying a multistep curing profile to the adhesive during assembly results in a marked improvement of the adhesive in withstanding this rigorous evaluation. Similar effects have been shown to reduce shrinkage of the material upon cure.
Hard and fast rules
In using UV and IR technology a sequential process is followed in determining the correct adhesive and curing profile for the application. The first step is to determine the materials to be bonded. The substrate(s) selected for any given application have a significant impact on the adhesive selected to create any bonded assembly. Issues such as the coefficient of thermal expansion and adhesion properties become important variables in the selection process. With the wide variety of commercially available glass and plastic products, it is critical to be knowledgeable about the materials being joined.
Components transparent to light are very conducive to UV curing. Temperature can occasionally become an issue and should be monitored during the evaluation phase of any application to ensure all components maintain their integrity. The fiberoptics industry makes use of some components that are not transparent, leading to common use of thermal-cured adhesives. Infrared can be used to cure this type of adhesive in the assembly line, typically with greater speed and efficiency than batch processes in convection ovens.
Once an adhesive has been selected, the optimal cure profile should be based on the adhesive manufacturer`s guidelines. The leading formulators provide comprehensive information regarding the best wavelength to use for their adhesive. Formulators determine guidelines for cure using a specific intensity meter and light source suitable for their adhesive. In many cases the light source used is a medium-pressure mercury or metal halide bulb. Spot-curing systems use high-pressure mercury bulbs that shift the wavelength toward the upper end of the energy spectrum. Results may vary depending on the nature of light source selected for a given application.
Irradiance and dosage are the next elements to be determined. Dosage is defined as the product of irradiance with time, and varies with wavelength (see Fig. 3). Manufacturers usually recommend a dosage to start with. Spot-curing systems typically use a light guide to deliver the light to the cure site. Because irradiance decreases with distance from the source, many users measure the light at the end of the light guide to determine the cure profile, although the ideal method is to measure the irradiance directly at the cure site.
The distance between the light guide and the adhesive should be strictly controlled to ensure that the correct dosage of light is delivered to the adhesive. An additional benefit of UV/IR spot-curing is the prevention of collateral heating of components. Because the light is focused on a defined area the surrounding substrates are not affected (see Fig. 4).
If possible, it is always recommended that competitive adhesives and equipment be evaluated concurrently through the process. Although the up-front engineering effort may seem tedious, long-term benefits will be realized through ease of manufacturing. Some companies offering light-based solutions will include this evaluation as part of their engineering services.
When choosing UV/IR curing equipment, one consideration is the amount of control it provides the user. Some systems offer adjustable power supplies, which enable control of the amount of irradiance, whereas spot-curing systems use an iris to control the amount of light delivered through the light guide. Spot-curing systems typically use filters to select the appropriate bandwidth to further define the control (see Fig. 5). Some state-of-the-art spot systems also allow the user to program curing parameters into the system.
Closed-loop control-which incorporates the critical elements of hardware, software, optics, and radiometry-can assist in the optimization of UV/IR spot-curing of adhesives. A total solutions approach to dealing with applications in the fiberoptics and optoelectronics industry will stand a much greater likelihood of success.
The use of UV/IR light to polymerize UV/visible and thermally cured adhesives in the fiberoptics industry has moved from the evaluation stage to the point where high-speed manufacturing processes are being optimized. When one considers the demand placed on WDM components, radiation curing is certainly a viable option to consider for improving productivity and yields.
Kevin Davis is market development manager at Efos, 2260 Argentia Rd., Mississauga, Ont., Canada, L5N 6H7. He can be reached at firstname.lastname@example.org.
FIGURE 2. As part of the validation process, manufacturers of light-based spot-curing systems use an integrating sphere to determine the desired lamp output and beam profile.
FIGURE 5. Unique spectral requirements can be met with the right bandpass filter. Filters can improve cure characteristics and output refinements for specific conditions.
FIGURE 1. Spectral output from a high-pressure mercury lamp is optimal from the near UV through the near IR.
FIGURE 3. Irradiance is the concentration of power, or energy per unit area per unit time. The irradiance of a high-pressure mercury bulb peaks at 335 nm.
FIGURE 4. Adhesives change form during UV curing. Before being exposed to UV light, the free radicals in the adhesive are randomly distributed. After exposure, the free radicals align, causing hardening and bonding of the components.