UV LED Curing System Lifetime
Light Emitting Diodes (LEDs) are solid-state devices that produce light when an electrical current is allowed to flow from the positive (anode) side to the negative (cathode) side. Like other solid-state devices, they have a very long lifetime if used properly. If engineered correctly, these semiconductor devices last beyond 40,000 hours of operating time unlike traditional UV lamps.
Traditional lamps produce light by generating an electric arc inside an ionised gas (typically mercury) chamber to excite atoms, which then decay, emitting photons. Additionally, the European Commission (EC) through its Restriction on Hazardous Substances (RoHS) guidelines has continually monitored the rapid progress of UV LED’s capability.
Many in the industry predict the EC will begin limiting traditional lamp usage, starting first with smaller systems then working towards larger systems in ensuing years. Combining energy generation of today’s UV LED light sources and the right formulation of UV curable resins, photoinitiators, and additives, it is possible to surpass current process capabilities for a variety of applications.
LED End of Life Characteristics
The normal ‘failure mode’ of an LED is gradual degradation of the light output. Many commercial LED lighting systems define failure at 70% of the original operating output. Phoseon light sources are designed to ensure a lifetime beyond 40,000 hours with proper care and service, and have shown to provide approximately 90% of the original output at 60,000 hours.
Figure 1: UV LED Intensity Over Time vs. Mercury UV
Factors Affecting LED Lifetime
Two major factors that affect the lifetime of LEDs are temperature and current. As LEDs convert electricity into light, heat is created within the p-n junction, known as the junction temperature. For a diode to achieve maximum life expectancy, the junction temperature has to remain in a safe operating zone. The UV Power output of a diode increases with input current but decreases with junction temperature. At any fixed input current, the cooler the junction temperature remains, the more UV output the diode will provide.
Causes for premature LED failure include diode defects, diode packaging issues, or subjecting the LED to operating conditions outside of its specifications, in which case, it may show other failure mechanisms including rapid degradation, burned or broken metallization, or complete failure. Some lamp suppliers achieve their stated high-output irradiance by over-charging their diodes, which reduces lifetime. Unfortunately for the customer, this failure will only show up after they have purchased the inferior product and then experience curing issues.
If a diode has an inherent defect due to contamination or other manufacturing issues, it will typically surface within the first several hours of operation. Phoseon has strategic partnerships with LED manufacturers in order to ensure the LEDs meet Phoseon’s tight specifications for light output efficiency, wavelength, and reliability.
Phoseon Technology invented the patented Semiconductor Light Matrix™ (SLM™) which is modularized LEDs for easier performance control. In order to ensure only the most reliable LEDs are used, the manufacturing process includes testing and characterization of each LED plus a burn-in period for each SLM.
Since 2014, Phoseon has implemented stress screening for SLMs to screen out SLMs with the potential to fail prematurely. This has reduced the failure rate of SLMs by over 50%. Stress screening (high current/high temperature applied) is carried out on all individual SLMs prior to assembly, in order to screen out any SLM which may encounter early life failure (latent defect).
Other failure modes may not be due to the diode itself, but rather the diode packaging. This can include bond wire failures or thermal/electrical failures due to poor attachment of the diode to a heat sink. Bond wire failures can be caused by excess current or manufacturing defects during the bond wire attachment.
The packaging of the diode also directly impacts the ability to cool the LED. Pre-packaged diodes are typically intended for use with limited current flow and are not intended for use in high power UV light sources. A well-engineered UV light source incorporates custom LED packaging with efficient cooling to maximize UV light output without having to increase the power input past the maximum specification.
System Lifetime
UV LEDs are one small, though important, part of a UV light source, but the rest of the system must also be taken into account when discussing the lifetime. Failure of the LED itself is a rare occasion, and early failure modes tend to be design or environment related.
Therefore, the light source as a whole must be designed to be as rugged as possible, in order for it to survive as long as the LED itself. This includes the internal control circuitry plus the cooling system to ensure the LEDs remain within their operating specifications, as well as the housing to protect the LEDs from environmental factors. Phoseon’s systems use patented and proprietary thermal management, sealing techniques, optical enhancements, and control circuitry to maximize performance without sacrificing lifetime.
Power and control of the LED system must ensure that user error does not immediately damage the light source; for example, wiring the +/- DC connections backwards or momentarily applying DC voltage outside of the light source specification. Phoseon light sources are designed to protect the internal circuitry from incorrect voltage levels and voltage spikes for both power and control signals.
The light source must also monitor the internal operating temperature to ensure the LED does not overheat in the event of a failure of the cooling system. The control circuitry must include protection against the possibility of pushing too much power through the LED array; although this may increase the light output for a short amount of time, even with efficient cooling, prolonged use in this manner will shorten the lifetime. All of Phoseon light sources include an automatic thermal shutoff feature, so that in the event of a cooling system failure, the lamp is protected from thermal runaway conditions.
An additional variable that can cause premature failure is contamination of the LED array. If an LED comes in contact with any form of liquid or conductive material, it can short out one or more LEDs causing an overall drop in UV output. Any foreign material on the surface of the LED will also retain heat, causing the LED temperature to rise and reduce its lifetime. It is important when choosing a UV LED system supplier, to understand the construction of the unit to ensure the LED cavity is properly sealed to keep out all contaminates. For air cooled systems that require the use of filters, it is important to change these filters often in environments that generate debris or ink mist. If these filters were to become clogged, it can reduce air flow to the point of over-heating the light source.
The housing of an LED system should be sealed against potential fluid ingress which could also harm the internal circuitry. For water cooled products, an IP rating of IP54 ensures it is protected against dust (no harmful deposits), completely protected against solid bodies, and also protected from splashing liquid from any direction. For air-cooled products, this not likely feasible due to air flow requirements, but should at least include a sealed chamber around the LED arrays.
It is important to understand the cooling requirements for water cooled systems including flow rate, temperature, and water quality. If an LED system is designed to use water colder than the ambient air temperature, it can cause condensation inside the light source which results in shorting out power and control circuitry with water. Phoseon recommends the use of 30-35°C cooling water to reduce the risk of condensation. The use of a flow switch in-line with the cooling water ensures the minimum flow rate is met prior to allowing the light source to be enabled. If the water flow is below the specification, it can lead to over-heating of the light source and a thermal shutdown event.
Water quality is important to maintain for both the light source and the cooling system. Phoseon’s water-cooled light sources are designed to use similar metals within the water cooling path to reduce the risk of corrosion, but if the water quality is not properly maintained, it can cause the cooling channels within the heat sink to become clogged with mineral deposits or organic particles like algae.
In order to fully test the performance of all of these components throughout the design process, Phoseon light sources are subjected to a HALT procedure (Highly Accelerated Life Test), as well as testing in an environmental chamber, and long-term life testing. A HALT test involves stressing the product with rapid thermal changes and vibration in a variety of combinations. Any failures are then analyzed, improved, and then verified. Environmental chamber testing involves subjecting the lamp to varying ambient temperatures and humidity. Long-term life testing involves continuous operation of the lamp in standard operating conditions, as well as stressed conditions that mimic harsh working conditions.
Conclusion
When choosing a UV LED System supplier, it is important to not just take into account their reported lifetime values, but to evaluate the whole system’s ability to protect the LEDs. Phoseon Technology is 100% focused on UV LED systems with an engineering team dedicated to building the most rugged and reliable systems available.
For more information visit phoseon.com.
