Nowadays LED components are used in a wide variety of application areas and are therefore exposed to a barrage of influences and environmental conditions. In the automotive industry, for example, LEDs in lighting systems are
exposed to extreme conditions such as vibration, variations in temperature, humidity and others.
In order to evaluate possible consequences, this article examines the effects of some standard and user-specific criteria and operating conditions on the lifetime of LED components.
Design of SMT LEDs
Most surface mounted technology (SMT) LEDs consist of a lead-frame with a moulded housing. A die is glued or soldered to the lead-frame to create one contact of the LED. The die is electrically connected to the second contact pin of the LED’s lead-frame with a bond wire. Finally the LED is casted with a transparent encapsulation to protect the die and wire bond.
Figure 1 shows the basic structure of an SMT LED package mounted on a printed circuit board. Silicones are frequently used for the transparent encapsulation as they possess high optical transparency, favourable mechanical properties and superior thermal and radiation stability. For example, their maximum operating temperature can be extended to more than +125°C. Special care has to be taken because silicone is permeable to gases, which in turn can vary from being harmless to highly aggressive.
LED in a system environment
In general, the LED is mounted together with mechanical, electrical, optical and thermal conductive components in a system housing required to protect the array against outside environmental conditions and to allow the specified function.
The LED is part of this system and can now be exposed to new differing environmental conditions compared to its original qualification.
Due to the proximity of various components and materials in such a system, those environmental conditions are mainly originating from the system itself and may vice versa affect the function and reliability of individual components of the system, including the LED.
Especially at higher temperatures, aggressive substances from materials like foam pads, rubber sealing, anti vibration pads or thermal conductive pads – as well as others – can evaporate. These substances may not only get into contact with the surface of the LED but can also diffuse through the silicone encapsulation and could finally contaminate the die, bond wire and lead-frame.
However, the critical materials are not only confined to a final system. As it has been confirmed, such kinds of materials can also be found in machines or stopovers of the production line. In this case damage to the LED components can be observed prior to the real system setup.
Even though any harmful substances should be avoided in proximity of electronic components in general, some in the industry have experienced corrosion issues linked to a very high concentration of sulphur in proximity to LEDs.
It has been reported that H2S, for example, can evaporate especially from rubber-like materials, if no special focus is put on low sulphur content or low evaporation properties of these materials. These sulphur compounds can corrode the silver plating of the lead-frame which may result in destruction of the electrical contact between lead-frame and wire bond or die bond.
The permanent presence of such micro climates causes a diffusion of the sulphur-containing gases through the silicone and a reaction with the lead-frame plating. The plating turns black due to formation of silver sulphide, which can easily be noticed by visual inspection.
In general heat, humidity and light, among others, are able to accelerate such a corrosive process. However, if the H2S concentration is very high (e.g. due to a high evaporation rate and restricted volume of the containing system), the main influencing factors can be limited to concentration level and temperature. Such high concentrations can result in growing corrosion.
Since OSRAM Opto Semiconductors’ LEDs withstand the corrosive gas test according to EN 60068-2-60 method 4 with 10 ppb (parts per billion) H2S without any damage or visual change, it follows that the concentrations leading to corrosion inside the LED must be magnitudes higher. In fact, studies show that corrosion becomes visible at concentrations greater than 100 ppb.
Based on experiments, it can be concluded that the concentration present if contaminated rubber is used can be significantly more than 10 ppm (parts per million).
The verification via SEM and EDX analysis illustrates the effect of the corrosion: the sulphur-containing compound reacts with the lead-frame surface to form silver sulphide. In extreme cases the corrosion can damage the connection between the wire and the lead-frame and can result in a lifted wire or open contact.
The first and most obvious solution to avoid corrosion of electronic components is to avoid corrosive or harmful atmospheres in their vicinity. Not only the silver lead-frame of LEDs can be affected, also any copper, even coated e.g. with a NiAu layer, on printed circuit boards could be affected.
Figure 2 shows an IMS-PCB which was exposed to a corrosive sulphur atmosphere. During the test (1500 h @15 ppm H2S) the red highlighted area of the surface was covered with an adhesive tape. As can be seen, the unprotected contacts – especially the copper layer – are affected by the sulphur (layer construction 35 μm Cu, 6 μm Ni, 0,1 μm Au). In the case of rubber, peroxide cross-linked materials are available on the market as an alternative to sulphur cross-linked versions.
If materials evaporating corrosive substances cannot be avoided in the vicinity of the LEDs, OSRAM offers LEDs with improved robustness.
If the silicone properties – stability against temperatures >125°C and high stability against radiation of short wavelength light – are not required, epoxy casted LEDs can be used. Epoxy is not permeable to gases such as H2S and therefore provides protection of the LED. Epoxy casted LEDs are also fully automotive qualified (AEC-Q101), are in use even longer than silicone casted LEDs and are thus a useful alternative in many cases.
For applications in which the specific silicone properties are essential, OSRAM offers a variety of silicone casted LEDs with the lead-frame plated with gold instead of silver. The gold plating is more stable against corrosion by H2S.
However, it must be pointed out that any electronic device including LEDs with high corrosion stability may be destroyed by the long-term influence of H2S at high concentration levels.
Using an LED with silicone encapsulation and silver plated lead-frame requires a careful analysis of all materials used in the application or in the vicinity of the application for any harmful, gaseous and corrosive materials or substances.
Known and theoretical sources of corrosive substances, especially H2S, include elastomers vulcanised with sulphur, contaminated PCB material, solder resist, stop-off lacquer, paper and paperboards, or an industrial environment with high sulphur or sulphide concentration.
OSRAM highly recommends using sulphur-free materials in the proximity of electronic components, including LEDs. In order to avoid evaporation from rubber seals, peroxide cross-linked elastomers could be applied instead of the sulphur cross-linked types.