Below the visible spectrum lies a band of wavelengths called ultraviolet (UV). Ranging from 100 to 400 nm, the radiation can effectively be used to sterilize cosmetics, perform forensic analysis, disinfect water and cure materials. More specific to manufacturing, UV rays take on roles designed to speed up the entire fabrication process.
Uses in Manufacturing
Ultraviolet curing is a process in which UV energy produced by a mercury discharge lamp is absorbed by a sensitizer, causing a reaction in the monomer which makes it hard and dry. This UV “curable” monomer includes a sensitizer which absorbs UV energy and initiates a polymerizing reaction. Manufacturers use this process to instantly cure or “dry” inks, coatings or adhesives throughout the engineering process.
At Norlux, for example, UV acrylate encapsulating resins are employed for achieving brighter chip-on-board (COB) LED assemblies. When cured with a DYMAX 2000-EC moderate intensity flood curing system, the Light-Cap family of encapsulates offer superior optical transmission during the life of the LED assembly. Their resistance to elevated operating temperatures results in reduced yellowing (a common phenomenon in brighter COB applications) compared to traditional encapsulating resins. UV/visible light curing coatings offer a wide range of high performance properties including lower stress and more durable wire bonds, superior adhesion, multiple viscosities, minimal shrinkage and low outgassing. Other characteristics described include good moisture resistance, optical clarity and excellent environmental resistance.
UV cured materials are widely used in circuit protection and electronic assembly applications, where products are electrically insulated and designed for various operations including: conformal coating, encapsulation, bonding, keypad coatings, thermal management, and masking. These materials are IPC approved, MIL-I-46058C and UL listed self-extinguishing graded, and are available in multiple viscosity grades depending on application method.
Advantages
UV curable materials provide manufacturers of electronics ease and flexibility in their conformal coating process. Because UV curable materials can be applied in a variety of ways including manual and automatic spray and dispensing, manufacturers are provided with different processing options while allowing them to meet cycle time and product specification requirements. For example, some Printed Circuit Board Assemblies (PCBAs) require environmental or dielectric protection or both. A PCBA manufacturer may set up a conformal coating line that has an automated spray/dispense system and a UV curing chamber. This curing methodology allows manufactures to eliminate thermal and room temperature curing methods, and also ensures that the material has reached full cure upon exit from the UV curing chamber. Other coating materials do not reach full cure for 48–72 hours.
LEDs as Alternatives to mercury arc lamps
UV LED technology is rapidly replacing traditional mercury or arc-based lights. In production curing operations, mercury-vapor lamps are hindered by short lifetime (2000-10,000 hrs.), slow warm-up and cool-down times, and wide spectral power distribution. The UV output of a mercury vapor lamp drops off rapidly over its operational life because some of its electrode material vaporizes, depositing a film on the inside of the quartz tube, which the UV cannot penetrate. This results in an unpredictable amount of UV output over time, which is a critical process parameter.
The mercury lamp has a main peak at 365 nm but several smaller peaks in the visible and infrared regions. The results are peaks in heat output, making working with plastics and other heat-sensitive materials a challenge. Due to the high voltage heat and environmental issues surrounding disposal of mercury bulbs, manufacturers now consider LED based UV curing. There seem to be too many variables to consider when dealing with traditional mercury or arc-based lights in standard curing processes.
UV LEDs bring such benefits as knowing precisely what power level is being delivered to the curing medium, as well as the other advantages LEDs are known for, such as the ability to pulse-width-modulate the output. The change from conventional curing technologies to UV LED allows system manufacturers to offer their users lower operating costs and higher quality products while enabling new capabilities and even reduced shipping and inventory storage due to the ability to print on thinner, heat-sensitive substrates.
Source: (http://ledsmagazine.com/features/9/2/5), (http://ledsmagazine.com/press/8554)
