Datasheet

LUXEON LED Assembly and Handling Application Brief AB32 20121217 37
7. Product Packaging Considerations – Chemical Compatibility
LUXEON LEDs contain a silicone overcoat and silicone lens to protect the LED chip and extract the maximum amount of light. As with most
silicones used in LED optics, care must be taken to prevent any incompatible chemicals from directly or indirectly reacting with the silicone.
The silicone overcoat in the LUXEON LED is gas permeable. Consequently, oxygen and volatile organic compound (VOC) gas molecules can diffuse
into the silicone overcoat. VOCs may originate from adhesives, solder fluxes, conformal coating materials, potting materials and even some of the inks
that are used to print the PCBs.
Some VOCs and chemicals react with silicone and produce discoloration and surface damage. Other VOCs do not chemically react with the silicone
material directly but diffuse into the silicone and oxidize during the presence of heat or light. Regardless of the physical mechanism, both cases may
affect the total LED light output. Since silicone permeability increases with temperature, more VOCs may diffuse into and/or evaporate out from the
silicone.
Careful consideration must be given to whether LUXEON LEDs are enclosed in an “air tight” environment or not. In an “air tight” environment,
some VOCs that were introduced during assembly may permeate and remain in the silicone lens. Under heat and “blue” light, the VOCs inside the
silicone lens may partially oxidize and create a silicone discoloration, particularly on the surface of the LED where the flux energy is the highest. In
an air rich or “open” air environment, VOCs have a chance to leave the area (driven by the normal air flow). Transferring the devices which were
discolored in the enclosed environment back to “open” air may allow the oxidized VOCs to diffuse out of the silicone lens and may restore the
original optical properties of the LED.
Determining suitable threshold limits for the presence of VOCs is very difficult since these limits depend on the type of enclosure used to house the
LEDs and the operating temperatures. Also, some VOCs can photo-degrade over time.
Table 4 provides a list of commonly used chemicals that should be avoided as they may react with the silicone material. Note that Philips Lumileds
does not warrant that this list is exhaustive since it is impossible to determine all chemicals which may affect the performance of LUXEON LEDs.