SEP Epoxy Color Pastes in LED Packaging and Reflector Layers

LED packaging is where a colour paste is asked to do something it does nowhere else: return light. A white reflector layer in an LED package is not a white coating in the decorative sense. It is an optical component whose reflectance decides how much of the emitted light leaves the package and how much is absorbed as heat inside it.

1. Where a colour paste sits in an LED package

The package chain runs from the lead frame or substrate through a reflector cup or a moulded body, to the die, the phosphor layer and the lens. Two positions are pigment dominated: the white moulding compound that forms the package body, and the white solder mask on the substrate or lead frame. In both, the pigment is titanium dioxide, and the performance metric is reflectance rather than whiteness. A visually satisfactory white that reflects only 85 percent of the incident light is doing worse than a slightly less brilliant white that reflects 92 percent.

2. Reflectance is the specification

Initial reflectance for a white LED body compound is commonly specified above 90 percent across the visible range. The more demanding figure is the reflectance retained after ageing, because the package is judged on its light output at end of life rather than at the moment of assembly. A titanium dioxide grade is selected on three properties. The crystal form matters, because rutile has the higher refractive index and therefore the higher scattering power per unit volume. The surface treatment matters twice, once for dispersion and once for resistance to yellowing under blue light. The particle size distribution determines whether the scattering is optimised for the wavelength of interest, and an optimum exists rather than a monotonic improvement with finer particles.

3. The ageing mechanisms that matter

Three mechanisms reduce reflectance in service, and each has a different countermeasure. Blue-light and thermal yellowing. The package body sits close to a die emitting at high flux density. Organic components in the binder, and any organic pigment in the compound, can yellow under that combination of blue light and heat, and yellowing absorbs exactly the short wavelengths the package is meant to deliver. Sulphur corrosion. Silver-plated lead frames corrode in the presence of sulphur, and the corrosion products darken the reflector surface. The pigment and its carrier must not introduce sulphur or species that accelerate the reaction. Ionic contamination. Soluble ionic species at the die interface raise leakage and can attack the silver or the bond pads. For an electronic grade this is a specification item, and it is one of the reasons SEP is specified for low ionic contribution and migration resistance.

4. Where SEP fits, and where it does not

This boundary has to be stated rather than implied. SEP is an epoxy-based electronic-grade colorant system containing no organic solvents. It is a natural fit for epoxy-based LED body compounds, for white solder mask on the substrate or lead frame, and for identification colour in epoxy packages. It is not a colorant for silicone encapsulation, because the carrier chemistry and the cure mechanism are different, and a supplier claiming otherwise would not be describing the product accurately. Within the epoxy family, SEP-White is the grade that carries the optical requirement.

5. SEP grade selection for LED packaging

Table 1 — SEP grades assessed for LED packaging

Selection is narrow here, and that is the honest answer. Only SEP-White carries the optical requirement. The remaining grades serve identification and black package functions, and the iron oxide grades belong to thick-section applications where their opacity is an asset rather than a defect.

6. Process practice

Pigment loading and optical path. Because reflectance depends on scattering per unit volume, a paste that reaches the target shade at lower addition reduces the volume of non-scattering material in the compound. The relevant comparison is pigment volume, not mass. Dispersion quality. An undispersed agglomerate in the optical path both scatters incorrectly and creates a visible defect on the package surface, which in a small package is a reject at inspection. Reflow compatibility. Packages pass through reflow, and the white compound must not yellow during it. Reflow resistance should be verified on the actual compound. Sulphur exposure. Where the end product faces a sulphur-containing environment, a sulphur exposure test on the cured compound is worth more than a general chemical resistance claim.

7. Verification checklist

Table 2 — Release criteria for LED packaging grades

8. Frequently asked questions

Q1. Why is reflectance specified instead of whiteness?

Because a package is judged on light output. Whiteness is a perceptual description, while reflectance is the measurable quantity that determines how much light leaves the package and how much becomes heat.

Q2. Is finer titanium dioxide always better for reflectance?

No. Scattering is optimised at a particular particle size for a given wavelength, and particles much finer than that reduce scattering rather than increase it. The distribution is what matters, not simply the median.

Q3. Can SEP be used in silicone LED encapsulation?

No. SEP is an epoxy-based system, and silicone encapsulation differs in carrier chemistry and cure mechanism. SEP applies to epoxy-based body compounds, white mask on the substrate, and identification colour in epoxy packages.

Q4. Why does sulphur resistance matter for a white reflector?

Because silver-plated lead frames corrode in the presence of sulphur and the corrosion products darken the surface, which reduces reflectance permanently. The pigment and carrier must not introduce sulphur or accelerate the reaction.

Key conclusions

1. In an LED package a white pigment is an optical component. The specification is reflectance and its retention after ageing, not visual whiteness, and the titanium dioxide grade is selected on crystal form, surface treatment and particle size distribution.
2. Three ageing mechanisms govern service life: blue-light and thermal yellowing, sulphur corrosion of silver, and ionic contamination at the die interface. Each has a distinct countermeasure and each belongs in the qualification.
3. SEP-White is the grade that carries the optical requirement, and the boundary of the product should be stated plainly: SEP serves epoxy-based LED body compounds and white mask, not silicone encapsulation.