UV LED curing refers to a technique that utilizes “energy” output from the LEDs in the UV spectrum to treat inks, coatings, adhesives, and other UV curable materials. The energy generated by the ultraviolet light triggers a chain reaction, resulting in polymerization of the wet raw material, thus hardening (or curing) the material. This article looks at the technology.

Figure 1: Semiconductor based light emitting diodes (LEDs)

LED curing technology uses semiconductor-based light emitting diodes (LEDs) to project ultraviolet (UV) light when an electric current is passed through them. When an LED is forward-biased, electrons are able to recombine with electron holes within the device, thus releasing energy in the form of photons. The color of the light emitted or corresponding energy of the photon is determined by the energy gap of the semiconductor material.

Impact of Curing Technology
Traditionally, mercury-based UV lamps had been utilized for curing. Now, more energy efficient and environmentally friendly LED-based UV technology is becoming a viable solution in the medical device industry. UV LED curing technology could potentially replace all existing traditional mercury arc lamps. This transition from traditional lamps to UV LED lamps is beginning to make a huge impact on adhesive curing in medical devices. Small form factor coupled with low heat generation are the two main driving factors in medical applications, leading transition from Hg lamps to UV LED curing solutions. Additionally, consistent energy output for curing adhesives reduces process variations when using UV LED curing lamps, thus providing improved yields and throughput with tighter process controls as required in the medical industry.

Technology Building Blocks
UV LED curing lamps consist of multiple subcomponents that, when optimally combined, can drive the system performance. Key components of the UV LED light source can be grouped into four categories:

LEDs
LEDs consist of semiconducting material that is doped with impurities to create a p-n junction. Charge carriers, both electrons and holes, flow into the junction from electrodes (anode and cathode) with different voltages. When an electron meets a hole, it falls into a lower energy state, resulting in the release of energy in the form of a photon. The wavelength of the light emitted depends on the band gap energy of the materials (dopants) forming the p-n junction. The right combination of LEDs maximizes the total UV energy.

Arrays
Arrays are a grouping or clustering of individual LEDs. The number, type, and size of LEDs, including the shape of the array and the method of electrically connecting the LEDs, impact the array. The array architecture is targeted for both air- and/or water-cooled applications. The target application ensures the optimal performance and reliability for the system manufacturer.

Figure 2: UV LED curing lamp

Optics
Photons coming out of the light source are optimized by using various optical layouts. Optics are used in reflecting, molding, guiding, and shaping the UV LED light to maximize the energy reaching the media to optimally cure the UV material. The use of optics has three benefits to the user:

• Increases the efficiency of the UV energy irradiated onto the material
• Lowers the heat generated by an array
• Provides for optimal system pricing

Thermal
UV LEDs can last up to 20,000 hours and beyond if they are maintained at a proper operating temperature. As LEDs emit more energy, they also generate more heat, which needs to be managed. Thermal management techniques remove excess heat from the system while providing a consistent operating temperature for the diodes to function at maximum performance, providing longevity to the lamps.

Materials and Formulation
With the advancements in the availability of UV LED optimized adhesive and ink chemistry, UV LED sources have become a very viable curing solution for various applications in the medical device industry.

Material suppliers have responded to the demand and challenge in the adhesive curing world to formulate raw materials that absorb energy corresponding to the output wavelength of UV LED curing units. One of the key ingredients in the chemical formulation is a photoinitiator that serves as a catalyst to initiate the polymerization process when exposed to a narrow spectrum of UV LED energy. Further, with the continued widespread acceptance of UV LED systems, availability of suitable base materials continues to grow. The driving factors in the advancement of chemistry of raw materials is increased capability and cost effectiveness of commercially available UV LED curing lamps, as well as end users driving the acceptance of the UV LED curing technology.

Figure 3: Monochromatic distribution (wavelength)

Wavelength
UV LED lights have a narrow spectral output centered on a specific wavelength, ±5.0 nm. LEDs are a solid state device; they can be built with various wavelength diodes including, but not limited to, 365 nm, 385 nm, 395 nm, 405 nm, and 410 nm, unlike the broad spectrum of wavelength ranges output by Hg-based lamps. This monochromatic distribution (Figure 3) requires new chemical formulations to ensure proper curing of UV materials. In the medical industry, a 365 nm wavelength is better suited for some applications over 395 nm since the heat generated with the 395 nm wavelength is too high and can cause warping of the peripheral adjacent components. Also, in medical device manufacturing, adhesives and inks are used in small quantities, thus requiring a small area cure. As such, damage caused by heat generation to the adjacent components is one of the primary concerns when curing inks and adhesives.

Applications in Medical Design & Manufacturing
UV curing is gaining traction in the medical field for curing inks and adhesives on various devices, accessories, and other medical equipment. LED curing technology is a good fit for many medical device assembly applications providing multiple benefits, including higher throughputs, controlled process repeatability, and operator safety. The medical industry is driving toward various new challenging applications to use the latest and greatest UV LED curing lamps.

Figure 4: Adhesive application example

Syringes, Needle Bonding, and Catheters
UV LED curing of adhesives has shown benefits in these applications because of small form factor, ease of setup, and minimized heat generation. Since the parts are very small, the challenge lies in using the UV LED light to cure adhesive in a small area to mechanically bond the components together. However, medical devices that come in direct contact with skin require FDA approvals. As a result, end customers are not driving the change from existing technology to new UV LED curing, leading to slow adoption of UV LED curing in some cases. Oxygen inhibition during curing is another factor for some of the uncured adhesives and the use of an inert environment provides different challenges.

Medical Equipment
UV LED curing is making great headway in certain medical device areas. Various applications are being examined to determine how to best utilize the advantages of the small form factor and specific wavelengths that UV LED curing offers. For example, in the manufacture of defibrillator machines, engineers are trying the UV LED technology to cure ink on a piece of foil to act as a mask for electrical contact points. Later in the process, the ink would be etched away.

Customers are also considering UV LED solutions for curing adhesives on sensors that go into portable blood pressure machines. Besides the small form factor, the advantages achieved in overall throughput and operator safety during assembly are driving the transition away from standard Hg-lamp technology. UV LED curing is also being used in analysis machines, which analyze chemical formulations in biological fluids, to tandem bond a piece of thin membrane to the printed circuit board (electronic assembly).

Conclusion
UV LED curing is fast becoming an accepted, user-friendly technology in the medical device industry, as it provides good process control with higher throughput. Compact size with consistent output energy enables curing of adhesives with minimal heat damage to the adjacent components. This solution provides the end user with repeatability in the manufacturing process and promotes a safe working environment for the operators in the assembly of medical equipment. Both the 365 and 395 nm wavelengths are used equally in various applications with the 365 nm option preferred for applications where heat is a major concern.

Various applications in medical equipment manufacturing are fast adopting the latest and greatest UV LED curing technology but it faces some serious challenges for applications that are used in direct contact with skin. However, continued improvements in UV LED curing lamps will continue to drive wider acceptance of this technology.

Richa Anand, Ph.D. is a product marketing manager at Phoseon Technology She is responsible for leading various functions at Phoseon to execute product strategy for Phoseon LED UV curing lamps. Dr. Anand can be reached at 503-619-2342 or Richa_Anand@Phoseon.com. (Author would like to thank her colleagues at Phoseon who have provided guidance in writing this article.)