Although the two most widely used curing technologies today - UV/visible radiation and thermal curing - offer advantages, neither offers ideal bond quality and processing speed. Bond quality refers not just to the strength of the bond but to the final condition of the substrate. Processing speed is influenced by two factors: curing time of the adhesive and whether the curing step is to be performed on or off the production line.
Thermal curing with ovens or infrared lamps is a process in which heat is applied to the entire product or subsystem. Thermal curing uses much more energy than is needed and is characterized by cure times in the tens of minutes. Because it is often an off-line process, thermal curing also interrupts process flow and slows the production line. Thermal-cure adhesives generally tend to be formulated with epoxy chemistry and, consequently, can achieve tack-free surfaces and much greater glass-transition temperature (Tg) than polyacrylate UV-cured adhesives.
Spot curing with UV/visible radiation is a process in which energy is applied only to the adhesive area. It offers a rapid and on-the-spot cure and can frequently be accomplished on a production line. UV-curing adhesives are polyacrylate formulations that generally offer good adhesion and fast reaction to light energy in the targeted absorption bandwidth of their photoinitiators. However, they can suffer from tacky surfaces and inconsistent physical properties when they are improperly irradiated.
Escalating energy costs and environmental concerns have become an important consideration for adhesive curing. While thermal curing is still dominant, it is not energy efficient. The advantages of in-line curing and the need for improved energy efficiency have encouraged process engineers to look for spot-curing alternatives that do not have the limitations of UV/visible-light curing.
Curing adhesives with infrared radiation (IR) or heat energy provides a fast, high-quality bond that can be accomplished without taking the process off line. In addition, it is energy efficient. While technical limitations have kept it from being commercialized in the past, recent technology innovations have solved the setbacks. Products are becoming available that finally make good on the best-of-class promise of IR spot curing. To fully appreciate these innovations, a quick review of the qualities that add up to a perfect cure is needed.
The Perfect CureCuring procedures and goals vary as widely as the products being manufactured. Medical devices, for example, are deployed in a highly regulated operating environment and, therefore, require the most-durable, highest-quality bond possible. In contrast, some electronic assembly applications require the bond to hold components in place only as long as is necessary for them to be soldered into place.
There are seven specific characteristics that all applications and industries require.
- The ability to create a high-quality bond in seconds.
- The flexibility to handle any curing profile (e.g., apply just the right amount of heat at just the right time to minimize cure time and maximize bond quality).
- Easy integration into a production line, whether it is manual, semi-automatic or automatic.
- Self-containment and self-calibration.
- Minimum energy use when creating the bond.
- Delivery of energy to a “glue spot” no matter its location within the product.
- Provide an economically attractive solution for the manufacturer.
Spot curing with UV radiation has significant constraints and limitations as well. For example, it requires specially formulated adhesives that are highly dependent on the application. Another drawback of UV curing, particularly in a production environment, is that UV spot curing requires UV-transparent substrates - which excludes silicon and many other visibly opaque substrates. Although spot curing with UV radiation can be an effective choice in heat-sensitive applications, UV’s limitations are important and have left users looking for alternative in-line curing methods.
IR Spot CuringIR spot curing is a superior and more flexible bonding technology than UV and conventional thermal curing. Unlike UV-curing systems that require UV-transparent substrates, IR wavelengths penetrate silicon and other visibly opaque substrates. In addition, the absorption of IR energy by the adherend (the material being bonded) creates a less thermally stressed bond than only heating the adhesive.
Because epoxy adhesives absorb infrared energy very effectively in the mid-IR region of the optical spectrum, IR spot curing creates internal molecular agitation that manifests itself as heat. Continued and controlled application of more IR energy accelerates and completes the heat-curing reaction of these epoxies, thus hardening the epoxy.
To be effective in a production environment, an IR spot-curing system must provide a quick cure. Empirical testing indicates that it is possible to fully cure a 2-mm-diameter spot of heat-curing epoxy in less than 60 seconds by concentrating light energy in the mid IR bandwidth.
Figure 1 shows a simplified drawing of an IR spot system, including the lamp and reflector that create the energy source; a shutter to control the amount of energy being transmitted; a light guide (optical fiber) that transports the IR energy; and the lens that focuses energy on the adhesive spot.
IR Spot-Curing ChallengesFor years, two basic challenges have prevented deployment of an efficient, reasonably priced IR spot-curing system: finding a way to transport large amounts of IR energy to the adhesive spot and engineering a cost-effective light source.
Traditional silica fiber was the first idea that came to mind to transport large amounts of energy to the adhesive spot, but the problem is that silica fiber simply does not transmit light above 2,100 nm. The fiber used to deliver the IR energy would have to be highly transparent in the mid-infrared range (2-4 µm), capable of resisting high optical fluences, and cost-effective. Such a fiber is now commercially available, thanks to years of R&D efforts from infrared specialists. Figure 2 shows typical transmission of a mid-IR optical fiber.
The other challenge involved the light source - it had to produce sufficient power in the right wavelengths yet be inexpensive enough to fit into a restrictive bill-of-materials. A custom-designed mercury arc lamp designed to specifications fit the bill.
Engineers also had to find a way to couple the light source to the fiber cable without excessive energy losses. The solution required precise alignment of the optical-fiber bundle so it could effectively transmit all of the useful optical energy. A mechanical adapter design allowed the thermal and light management to be accomplished seamlessly.
With these improvements integrated into a single in-line system, IR spot-curing systems are now being commercialized. For example, a bonding time of 15 minutes for some epoxies using conventional thermal curing can be reduced to less than 60 seconds with IR spot-curing system. In an important development within the technology infrastructure, epoxy manufacturers are starting to work with IR spot-curing system manufacturers to develop formulations that can more efficiently absorb the IR energy, thus improving the value of their epoxy product lines.
A summary of the characteristics of thermal ovens, UV spot curing and infrared spot curing is shown in the Table.
System RequirementsIn addition to providing a fast, high-quality, in-line solution, a successful infrared spot-curing system should offer a small footprint and versatility. It should also be operator friendly and accommodate integration into production environments where curing can be automatic, semiautomatic, or manual.
Ease of use is important on production lines. Technicians should be able to read and adjust display parameters (such as relative radiation intensity and exposure times) easily and accurately. Ideally, the system should be programmable using a touch-screen display to monitor system performance and provide a flexible user interface with a range of reporting features.
Consistently producing high-quality bonds in minimal time requires precise curing profiles. The ideal unit should also have programmable software that can save curing profiles in a non-volatile memory within the unit.
ConclusionAlthough the advantages of IR spot curing have been well known for years, technical problems prevented a truly competitive system from entering the market. Innovative technologies pioneered by IRphotonics solved the problems and have paved the way to a new generation of small, lightweight, infrared spot-curing systems that operate as an in-line process and can produce high-quality epoxy bonds in a fraction of the time previously required.
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