LED lighting has become an indispensable presence that enriches our living environments. Its vivid and stable light output is supported by precise phosphor coating technology applied to LED chips. However, even slight nonuniformity in this coating can cause unintended color unevenness or luminance variation, potentially degrading the inherent light quality of the LED package.
This article examines how uniform phosphor coating on LED substrates affects emission quality and explains the technical mechanisms required to achieve uniformity.
Uniformity of Phosphor Coating and Emission Quality
Occurrence of Color and Luminance Nonuniformity
When phosphor particles are not evenly distributed on the LED chip, they may accumulate in certain areas or be insufficient in others. In regions where particles are concentrated, the density of phosphor that absorbs and converts ultraviolet light emitted from the LED chip becomes higher. As a result, the emitted color may appear stronger and the local luminance relatively higher.
Conversely, in areas with fewer or no phosphor particles, color conversion is insufficient. This can cause the emitted light to deviate from the intended color or result in a significant reduction in luminance. Such uneven particle distribution leads to unintended color variation and brightness nonuniformity across the LED substrate, compromising lighting uniformity.
Degradation of Emission Color Stability and Uniformity
Light emitted from an LED chip is typically converted into white or other target colors by passing through or exciting the phosphor layer. When phosphor particle distribution is nonuniform, the thickness and density of the phosphor layer vary depending on location.
As a result, the optical conversion behavior differs across the substrate, causing instability in the chromaticity coordinates of the emitted light. This becomes a factor that degrades reliability as a light source and can lead to color differences between individual LEDs in multi LED lighting systems, ultimately reducing overall color consistency.
Phosphor Coating by Dip Coating
What Is Dip Coating
Dip coating is a technique in which the entire LED chip or COB substrate is immersed in a phosphor slurry and then withdrawn at a constant speed to form a uniform coating layer on the surface.
Although the process itself is structurally simple, precise control of parameters such as withdrawal speed, liquid viscosity, surface tension, and solvent evaporation rate makes it possible to achieve highly uniform coatings.
Applicability to High-Density Structures Such as COB Substrates
When LED chips are mounted at high density, as in COB substrates, achieving uniform phosphor coating on each individual chip requires a higher level of process control.
In COB substrates, coating is often applied simultaneously to the entire substrate or to multiple chip regions, which introduces challenges such as coating runoff into narrow gaps between chips or uneven deposition on chip top surfaces.
Dip coating addresses these challenges by controlling the timing of solvent evaporation. This helps prevent coating runoff and step formation while allowing the slurry to naturally flow into narrow gaps between LED chips.
For this reason, dip coating is increasingly being adopted for high-density structures such as COB substrates.
Summary
High-quality light emission from LED lighting strongly depends on the uniformity of the phosphor coating. Non-uniform coatings cause color variation and luminance unevenness, which degrade overall optical quality.
To address these issues, advanced coating techniques such as dip coating are applied to achieve uniform phosphor layers, even on densely structured substrates like COB boards.
Establishing these coating mechanisms is essential for maximizing the inherent performance of LEDs and forms the foundation for stable, high-quality lighting in everyday applications.