As a result of new legislation, ongoing cost saving requirements and attempts to reduce energy consumption, the use of LEDs (Light Emitting Diodes) will continue to replace incandescent bulbs in public areas such as street lighting and traffic lights, where despite the initial high investment, authorities will derive large savings in power consumption and reduced service costs (LEDs have significantly longer life-times).
In fact, the use of LEDs in advertising signage, sports arena displays, passenger information panels, traffic signs, car headlights, warning systems, under-water lighting, and other applications continues to increase rapidly. One big difference with this new technology is that nearly all applications require drive/control circuitry, in the form of a printed circuit board, to control the functionality of these devices as the LED’s require low voltage Direct Current, as opposed to AC.
Most, if not all of these applications, will end up in exterior locations, where the units are likely to be exposed to a combination of temperature change, moisture (rain as well as humidity changes and condensation), corrosive gases, salt-spray etc.
For these exterior applications, the visual impression of the display is the key output, and importantly ensuring the output is consistent throughout the life of the product. A key requirement in ensuring consistent output throughout the lifetime, depends heavily on protecting the display and controls from the weather, without affecting the optical output. Further information on LEDs can be found here.
When the lights go out…
Most LEDs have a natural lifespan that ends in a wear out mechanism. Defects in construction in the active area can reduce the lifetime significantly and are particularly affected by current and temperature. Although these degradation failures are unavoidable, careful selection can ensure it will not be during the lifetime of your product.
Of more concern, are premature failures in-service, and for the purpose of this brief piece, we will not consider in-package failures only those at the assembly level.
LED Failure mechanisms: Opens and Shorts
There are basically two ways that corrosion can cause issues with the functionality of an LED device. At the LED package itself, or at a remote site that changes the current flow around the circuit.
At the package level, as shown in the following examples (both Through-hole and SMT design), the metallic anode and cathode leads are exposed metal to enable soldering to the drive assembly. It is these exposed metal leads that are most susceptible to corrosion from the aggressive end use operating environment.
Electro-chemical migration, or dendritic growth from the anode to the cathode would result in an electrical short, resulting in failure of the LED. Corrosion of either lead-frame would result in an open failure, and a failure in the LED.
Often, for reasons of solderability, the leadframe is of tin construction, and so tin whiskering, and potential shorting is as likely with an LED as any other component, although not strictly speaking a corrosion product.
Corrosion products in remote areas of the drive assembly can also cause issues with the functionality of the LED device. The presence of corrosion can alter the effective current or voltage flowing through the LED device, resulting in increased junction temperature. Prolonged temperature creep can lead to the emrgence of thermal runaway.
Runaway occurs when the junction resistance continues to increase with temperature, added further heat to the package and thus raising the junction temperature in a self-propagating vicious circle.
Keeping the lights on…. The correct conformal coating can help
The use of the correct conformal coating on the drive circuitry and LED lead-frames can mitigate against the corrosive effects of the operating environment. This is achieved by the coating to behave as an effective barrier to extraneous contaminants, allowing for preservation of the circuit’s integrity, as well as mitigating against the potential effects of tin whiskers.
One of the unique challenges with coating SMT LED assemblies is the sensitivity of these devices to solvents and certain organic compounds present in certain coating materials. This incompatibility could result in discoloration and deterioration of the LEDs. More information on the effect, test & mitigation strategies and compatible materials can be found here.
HumiSeal is a leading supplier of conformal coatings to the LED signage and display electronics, and has worked closely with most major LED manufacturers to ensure chemical compatibility.
With the widest range of high performance conformal coatings, from every major type of protective chemistry (including Acrylic, Urethane and Silicone), you can be certain that HumiSeal has a high-performance solution for your specific application. HumiSeal has a wide range of environmentally compliant, low-outgassing, solvent-free materials, in addition to a wide range of traditional solvent-borne chemistries. Whatever your requirements, HumiSeal has the solution.