SEP Epoxy Color Pastes in Solder Mask and Legend Ink
Solder mask is the one coating on a circuit board that must survive everything the board will ever see: reflow at 260 °C, flux, aqueous cleaning, thermal cycling and the electrical stress of the circuit underneath.
Solder mask is the one coating on a circuit board that must survive everything the board will ever see: reflow at 260 °C, flux, aqueous cleaning, thermal cycling and the electrical stress of the circuit underneath. Colour in that coating is not decoration. The mask defines where solder may and may not go, and it carries the board's identification. This article examines where a solvent-free epoxy colour paste fits in solder mask and legend ink, and the one constraint that dominates all others: UV exposure.
1. Where the colour paste enters the formulation
A liquid photoimageable solder mask is a filled, pigmented coating. The binder is typically an epoxy blended with epoxy acrylate so that the film is both UV-reactive and thermally curable. A photoinitiator absorbs UV and starts the reaction. A thermal hardener such as dicyandiamide completes the epoxy cure in the final bake. Fillers, mostly barium sulfate, commonly make up roughly 10 to 30 percent by weight. Pigments set the colour, while solvents and additives set viscosity and levelling. The colour paste occupies the pigment position in that list, and two consequences follow. First, it competes with the filler for volume, so the pigment content per unit of colour determines how much filler the formulator must give up. Second, it must be compatible with a UV-curable, solvent-containing, filler-loaded system, which is a different requirement from the solvent-free potting compounds discussed elsewhere in this series.
2. The fineness requirement is the first screen
Solder mask is a thin film, typically tens of micrometres after cure, printed through a screen or applied by spray or curtain. Any particle larger than the film becomes a surface defect, a pinhole, or a point where the coating does not develop cleanly. Industry requirements reflect this. Scraper fineness for production solder mask is commonly specified at 8 micrometres or below, with demanding grades at 5 micrometres or below. Advanced formulations specify a colorant average particle diameter below 1 micrometre measured by laser diffraction, with a distribution standard deviation in the range of 0.1 to 0.2 micrometres. A narrow distribution matters more than a small median for the same reason as in every thin-film coating: the largest particle in the distribution sets the worst-case defect. A paste with a good median and a coarse upper tail will still produce pinholes at the film thickness it is supposed to coat.
3. UV exposure is the constraint that dominates everything
This is where solder mask diverges from every other epoxy colour application. In a photoimageable mask, the film is exposed through a photomask and then developed. Light has to reach the bottom of the film to crosslink the binder there. If it does not, the base of the coating stays weak while the surface cures. The result is undercut sidewalls, poor adhesion after development, and a coating that passes visual inspection but fails adhesion or chemical resistance after reflow. Pigments interfere with that light path in two distinct ways. White depends on titanium dioxide, which has a high refractive index and scatters UV strongly. That scattering is what gives white its hiding power, and it is also what limits how deep the UV penetrates. A white mask therefore needs either a higher exposure dose or a thinner film, and both the titanium dioxide grade and its surface treatment affect the outcome. Black is the harder case. Carbon black absorbs across the UV range rather than scattering it, so very little energy reaches the bottom of the film. Dark masks are consequently the most difficult to cure and the most likely to show adhesion failure at the developer stage. Coloured organic pigments sit between the two. Their absorption is wavelength dependent, so the photoinitiator can be matched to the transmission window of the pigment. A blue or green mask built on a phthalocyanine pigment behaves very differently from a red mask built on a DPP pigment, and the exposure energy has to be set for the specific colour rather than for the mask system in general. The practical consequence for a colour paste supplier is that the paste cannot be evaluated on shade alone. It has to be evaluated on the exposure dose needed to reach full cure at the specified film thickness, and that figure becomes part of the specification.
4. Thermal and electrical requirements
Solder mask must survive lead-free reflow. Standard masks are designed for a peak of 260 °C, and high-reliability masks are tested at 288 °C for short periods. Multiple reflow passes accumulate that exposure. A colour paste whose heat resistance ceiling falls below the reflow peak will drift in shade or degrade at the surface. Electrical requirements are equally specific. Volume resistivity is commonly specified above 1 × 10 to the power of 14 ohm centimetres, surface insulation resistance at or above 1 × 10 to the power of 10 ohms under standard test conditions, and dielectric strength above 30 kilovolts per millimetre. Those figures are properties of the whole coating, but the pigment contributes to them: soluble ionic species raise conductivity, and the paste must not introduce them.
5. Legend ink is a different job
Legend ink, also called marking ink, prints the silkscreen identifiers, reference designators and logos. It is usually white, sometimes black or yellow, and its requirements differ from the mask. Hiding power is the dominant requirement, because the legend must read against the mask colour and against the copper underneath. A white legend ink based on rutile titanium dioxide at 50 percent pigment content, such as SEP-White, provides that hiding at a film thickness the printing process can achieve. Fineness requirements are also strict, because a legend stroke is narrow and a coarse particle breaks the line. The second requirement is contrast stability. A legend that yellowed during reflow would lose contrast against a light mask, and that change cannot be recovered.
6. SEP grade selection for solder mask and legend
The SEP series is an electronic-grade epoxy colorant system containing no organic solvents, with high fineness and narrow particle size distribution, specified for no interference with epoxy curing, migration resistance and stable insulation. Eleven grades are available, with pigment content from 15 to 65 percent, heat resistance from 200 to 300 °C, and imported pigments from Clariant, BASF, DIC Japan and LANXESS Germany.
Table 1 — SEP grades assessed for solder mask and legend ink
| Grade | Colour | C.I. | Pigment content | Heat resistance | Light fastness | Position in solder mask and legend |
|---|---|---|---|---|---|---|
| SEP-9673 | Epoxy Brilliant Green | PG7 | 25% | 260 °C | 8 | The conventional green mask shade; phthalocyanine absorption window is narrow, so photoinitiator matching is comparatively straightforward |
| SEP-9631 | Epoxy Brilliant Blue | PB15:3 | 30% | 260 °C | 8 | Blue mask; high tinting strength means a small addition reaches full shade |
| SEP-989 | Epoxy Jet Black | BK7 | 15% | 300 °C | 8 | Black mask; the lowest pigment content in the series is the useful property here, because it limits how much UV is absorbed before reaching the film base |
| SEP-White | Epoxy White | PW6 | 50% | 260 °C | 7 | White legend ink and light mask; rutile titanium dioxide at 50% gives the hiding that legend contrast requires |
| SEP-9326 | Epoxy Bright Red | PR254 | 30% | 260 °C | 8 | Red mask and identification; DPP pigment holds chroma through reflow at its 260 °C ceiling |
| SEP-9202 | Epoxy Bright Yellow | PY150 | 25% | 260 °C | 7 | Yellow identification; transparency helps where the mask must stay thin |
| SEP-9205 | Epoxy Golden Yellow | PY151 | 25% | 260 °C | 8 | Yellow identification with higher weather resistance |
| SEP-955 | Epoxy Violet | PV23 | 15% | 260 °C | 8 | Special identification colour; low pigment content suits thin films |
| SEP-9328 | Epoxy Brilliant Red | PR170 | 25% | 210 °C | 6 | Requires explicit verification before use in solder mask, because the 210 °C ceiling is below the 260 °C reflow peak |
| SEP-3130 | Epoxy Iron Oxide Yellow | PY42 | 65% | 200 °C | 8 | Not a solder mask grade: coarse inorganic body and high loading do not suit a thin UV-exposed film |
| SEP-3190 | Epoxy Iron Oxide Red | PR101 | 65% | 300 °C | 8 | Not a solder mask grade: excellent for thick castings but opaque to UV in a thin film |
Two selection principles follow from the table. For a black mask, SEP-989 at 15 percent pigment content is the configuration that best matches the physics of the problem. Hiding is achieved through the pigment's high tinting strength rather than through loading, so the film absorbs the minimum amount of UV consistent with a uniform black. For a white mask or legend, SEP-White at 50 percent rutile titanium dioxide provides the hiding power that legend contrast demands, while the same scattering that produces the hiding also limits UV penetration and therefore sets the exposure dose. The two iron oxide grades, SEP-3130 and SEP-3190 at 65 percent pigment content, are excluded from this application for a specific reason. Their value in thick-section casting, where inertness and thermal stability matter and where the light path is irrelevant, becomes a liability in a thin UV-exposed film.
7. Process practice
Volume competition with the filler. Every part of paste added displaces barium sulfate or an equivalent filler. In a thin film this affects levelling, gloss and the fill ratio the formulator had balanced for thermal expansion. The paste should be assessed as a filler substitution, not as an isolated addition. Exposure dose per colour. Each colour needs its own exposure dose, and the dose should be established on the actual film thickness with an adhesion test after development and a chemical resistance test after reflow. Copying the dose from one colour to another is the most common source of failure. Solvent system compatibility. The mask contains solvent for application. The paste carrier must be compatible with that solvent load so that the film does not shrink excessively on drying or develop surface defects. Fineness and distribution together. Fineness to GB/T 6753.1 and particle size distribution to ISO 13320 should both be specified, because the film thickness is tens of micrometres and the upper tail of the distribution is what breaks through it.
8. Verification checklist
Table 2 — Release criteria for solder mask and legend ink
| Item | Method | Purpose |
|---|---|---|
| Fineness | GB/T 6753.1 scraper method | Confirms no particle exceeds the film thickness |
| Particle size distribution | ISO 13320 laser diffraction | Confirms the upper tail, which sets pinhole risk |
| Viscosity | GB/T 9751 | Confirms the paste stays within the application window of screen, spray or curtain coating |
| Exposure window | Exposure step test at the specified film thickness with adhesion check after development | Confirms the paste reaches full cure at the bottom of the film |
| Reflow resistance | Multiple passes at 260 °C, or 288 °C short dwell for high reliability | Confirms no shade drift or surface degradation |
| Adhesion after reflow | Cross hatch or peel test on the cured mask | Confirms the pigment did not weaken the base of the film |
| Insulation resistance | Per the applicable industry method | Confirms no ionic contribution from the pigment |
| Legend contrast | Optical measurement against the mask shade | Confirms legibility after thermal exposure |
9. Frequently asked questions
Q1. Can the same colour paste be used for potting and for solder mask?
Not without re-evaluation. Potting asks for solvent-free formulation, low ionic content and no cure interference in a sealed volume. Solder mask asks for fineness below the film thickness, a workable UV exposure window and reflow resistance. The paste must be evaluated against the requirement of the specific application.
Q2. Why is black solder mask harder to cure than other colours?
Because carbon black absorbs across the UV range rather than scattering it, so little energy reaches the base of the film. The practical response is a low pigment content grade at high tinting strength, combined with a higher exposure dose.
Q3. Is white solder mask easier than black?
It is different rather than easier. Titanium dioxide scatters UV strongly instead of absorbing it, which limits penetration depth while still consuming dose through scattering. Both colours require their own exposure window.
Q4. Why exclude the iron oxide grades from solder mask?
Because their advantages elsewhere become liabilities here. A 65 percent loading and an inorganic coarse body do not suit a film tens of micrometres thick, and the same opacity that serves a thick casting blocks UV in a thin coat.
Q5. Is a 260 °C ceiling sufficient for lead-free reflow?
It matches the peak of a standard profile, and high-reliability processes test at 288 °C for short periods. Grades at 260 °C are normally acceptable for standard profiles, while the 210 °C grade requires explicit verification before use.
Key conclusions
- In photoimageable solder mask, the colour paste is not selected on shade but on the UV exposure dose it needs to reach full cure at the specified film thickness. Black absorbs UV, white scatters it, and coloured organic pigments transmit selectively, so each colour has its own exposure window.
- Fineness and particle size distribution are release criteria rather than production conveniences, because the cured film is only tens of micrometres thick and the upper tail of the distribution determines pinhole risk.
- SEP grade selection follows the physics rather than the palette. SEP-989 at 15 percent pigment content suits black mask because it limits UV absorption, SEP-White at 50 percent rutile titanium dioxide serves white mask and legend, and the 65 percent iron oxide grades are excluded here while remaining the reference grades for thick castings.