Purple PCBs: Treat Solder Mask Color as One Requirement in the Board Specification

A purple PCB usually means a board coated with purple solder mask. The underlying laminate and conductors still need their own specifications. Color can support appearance or product identification, but it does not establish layer count, electrical ratings, manufacturing quality or the material’s thermal properties.

Separate the visible surfaces

The solder mask covers selected areas, while connection pads remain exposed with their specified surface finish. Legend ink provides markings such as reference designators. These are separate materials and manufacturing functions even though they appear together in a board photograph.

OSH Park’s published service information provides an example of purple boards with an ENIG finish. That combination does not mean purple mask requires that finish everywhere or that the color itself produces the properties of the exposed metal.

Define the reason for the color

• State whether it is a cosmetic preference or a controlled product feature.
• Identify visible areas after assembly.
• Review contrast of markings and polarity indicators.
• Specify the acceptable sample or observation method if appearance is critical.
• Confirm the supplier’s supported mask formulation.
• Keep the laminate and finish requirements separate.

If purple is used to identify a variant, keep a formal part and revision label too. Color alone can become ambiguous when several revisions share the same coating or a later supplier produces a different shade.

Review process and inspection needs

Ask the fabricator to assess the actual mask openings and constrained features using its selected product. Do not assume that a process qualified for another formulation transfers unchanged simply because both materials are called solder mask.

Check the assembled board under the lighting used for inspection or the finished product. Reflections, flux residue and component placement can affect readability. A rendered CAD image cannot show every aspect of the final surface.

Approve appearance without losing engineering control

Retain an accepted sample when consistent appearance matters. Connect it to the mask product and process baseline rather than treating the sample as a substitute for all written requirements. Review a proposed formulation change if its optical or manufacturing properties affect the product.

Continue to specify electrical construction, finish, thickness and acceptance requirements in the usual way. A purple board can be suitable or unsuitable for the same engineering reasons as a board of another color.

Primary example: OSH Park’s published purple-board service description; confirm current order options directly. See the surface-finish guide and DFM checklist to keep appearance and manufacturing requirements clearly defined.