LED assembly includes dispensing of various materials, such as yellow phosphor filled silicone. Typical dispensing applications are:
- Cavity Encapsulation
- Phosphor Plate Attachment
- Phosphor Coating
- Remote Phosphor
The common requirements for dispensing LED optical materials include:
- Accuracy and consistency
- Faster throughput—high unit per hour (UPH)
- Sustaining suspension of phosphors in fluid
- Conformal spraying or coating
The correct amount of phosphor must be distributed evenly over the LED die. A white LED is typically made by combining a blue LED with yellow phosphor (other combinations of red and green phosphors are also possible). It is the secondary emission from the yellow phosphor combined with the correct blue light mixture that makes white light. Color quality is determined by the amount of phosphor and its distribution over the LED, and may be specified and measured by the x- and y-coordinate range of the emitted light’s CIE (International Commission on Illumination). Find out more about each application below.
Cavity encapsulation is the most common method of covering the LED die. A mixture of phosphor and a binder (e.g., silicone or epoxy) is used. Since phosphor particles tend to settle out, one of the technical challenges is to ensure uniform mixing and dispersion of the binder and phosphor. Less uniformity can adversely affect the color quality.
After LED chips are placed in the cavity bottom, the silicone phosphor is jetted to encapsulate the cavity. Nordson ASYMTEK’s patented Calibrated Process Jetting (CPJ) and active nozzle technology ensure dispense weight accuracy and consistency, which contribute to a tight CIE.
Phosphor Plate Attachment
Phosphor plate attachment requires an exact amount of clear silicone dispensed on the die, similar to die attach applications. Excess silicone causes side-drop and may result in plate tilting on the die.
Phosphor coating and remote phosphor technologies are aiming for less binder and more uniformity by spraying phosphor with minimal binder on die surface or optics surface. The thin phosphor layer thus is formulated on die or optics. Achieving uniform spray thickness is a key challenge.
In spraying processes the silicone/binder/solvent/phosphor combination is typically a low viscosity (<100 cps) mixture. The chemistry challenge is to keep the phosphor dispersion in suspension and in a uniform mixed. Special conformal coating spray applicators with a combination of process techniques and masks provide the uniform coatings. Also, in many applications a mask is used to prevent the phosphor spray from coating all substrate areas.
In remote conformal coating applications the phosphor or diffuser materials are deposited onto a lens. The cavity may be filled with a clear silicone to provide a good optical connection between the LED and phosphor. In the case of diffuser applications the LED is phosphor coated or the encapsulation silicone is phosphor filled. The purpose of the diffuser material is to make the emitted light more uniform. In these applications the coatings may be applied with special conformal coating valves.