AB190 LUXEON IR Compact Line Application Brief 20170725 ©2017 Lumileds Holding B.V. All rights reserved.

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Figure 11. An example of Juki KE2080L feeder.

3.6 Reflow Accuracy

Using the solder pad footprint and stencil pattern layout as shown in Figure 5 with solder paste print as shown in Figure

6 and without any obstruction from nearby silkscreen (ink) print feature, LUXEON IR Compact parts can self-align to the

nominal position during reflow process.

4 Packaging Considerations—Chemical Compatibility

The LUXEON IR Compact package contains a silicone overcoat to protect the LED chip. 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 LUXEON IR Compact 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 IR Compact 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

overcoat. Under high temperature operation, the VOCs inside the silicone overcoat may partially oxidize and create a

silicone discoloration. 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 overcoat 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.