Solder mask — soldermask — is a thin polymer coating applied over copper traces to protect against oxidation, prevent solder bridging and improve long-term reliability. As pitches shrink, mask precision drives assembly yield and durability.
What solder mask does
- Protects copper from oxidation and corrosion
- Prevents solder bridges during reflow and wave soldering
- Provides electrical insulation between adjacent conductors
- Adds mechanical protection during handling and assembly
Types of solder mask
1. Liquid Photoimageable (LPI) — most common
Screen-coated or spray-applied as a liquid, exposed to UV through a phototool, then developed to open pads. High resolution, excellent adhesion, good chemical resistance. Best for HDI, fine-pitch BGAs, multilayer and high-volume production.
2. Dry Film Solder Mask
Laminated dry film with uniform thickness and good resolution. Less common in modern production, used in specialized applications.
3. Epoxy (Screen Printed)
Older, lower-cost approach. Lower resolution, less precise pad definition; rarely used in advanced fabrication.
Design rules
Solder mask expansion
Pads typically need mask openings slightly larger than the copper pad — commonly +3 to +4 mil, depending on fabricator capability.
Solder mask dams
The strip of mask between adjacent pads. Minimum dam width depends on fabricator capability, PCB class and pitch. Fine-pitch BGAs may require 3 mil or smaller dams with advanced imaging control.
Via tenting
Mask can fully cover vias to prevent solder wicking, reduce contamination and improve appearance. Not recommended for via-in-pad without proper fill.
Thickness
- Typical cured thickness: 0.8–1.2 mil (20–30 µm) over copper
- Slightly thinner over bare laminate
- Excessive thickness affects pad definition and fine-pitch tolerances
Colors
Available in green, black, blue, red, white, yellow and matte variants. Green remains most common because of inspection contrast and mature process control. Color generally has minimal electrical effect but can change AOI inspection quality.
Matte vs gloss
Gloss has the traditional appearance and good visibility. Matte reduces glare and improves AOI performance — popular for high-end consumer electronics.
High-speed and high-voltage considerations
- Mask slightly alters effective Dk on outer-layer microstrip — model in your field solver for precision RF.
- For high voltage, consider creepage/clearance, dielectric breakdown strength and surface insulation resistance. Specialized formulations may be required.
Manufacturing process (LPI)
- Surface preparation
- Coating (screen or spray)
- Pre-bake / tack dry
- UV exposure via phototool
- Development
- Final thermal cure
- Inspection
Standards
Solder mask is evaluated under IPC-SM-840, IPC-A-600 and IPC-6012. Boards may be built to IPC Class 2 or Class 3.
Design best practices
- Confirm minimum mask dam with your fabricator.
- Avoid extremely small isolated mask areas.
- Coordinate mask expansion with the assembly house.
- Consider matte finish for high-density boards.
- Engage DFM early to prevent assembly issues.
Common defects
- Misregistration
- Incomplete cure
- Pinholes
- Mask slivers
- Peeling or delamination
- Excessive mask on pads
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