Solder mask ink is a permanent protective coating printed on the surface of PCB boards. It is typically green but can come in various other colors. Its role goes far beyond mere “coloring”; it is a crucial part of PCB manufacturing.
★ Functions of Solder Mask Ink
✒︎ Insulation Protection: Prevents copper foil circuits from oxidizing and corroding in the air.
✒︎ Prevents Solder Short Circuits: During wave soldering or reflow soldering processes, it prevents solder from adhering to non-pad areas between circuits, causing bridging short circuits.
✒︎ Protects Circuits: Provides a layer of mechanical protection, reducing physical scratches and chemical erosion during transport, assembly, and use.
✒︎ Enhances Electrical Performance: Increases insulation resistance and voltage withstand capability, ensuring long-term reliability of the product.
✒︎ Aesthetic and Identification: Provides a neat appearance for the PCB and prints white (or other color) silkscreen layers for labeling component positions, version information, etc.
★ Components of Solder Mask Ink
✒︎ Resin
Film-forming substance that determines the basic physical and chemical properties of the ink (such as hardness, flexibility, heat resistance). Common types include epoxy resin, acrylic resin, polyurethane resin, etc.
✒︎ Photosensitizer
Makes the ink photosensitive. Under specific wavelengths of ultraviolet (UV) light, a chemical reaction occurs.
✒︎ Pigments
Provide color, such as phthalocyanine green for green, phthalocyanine blue for blue, etc.
✒︎ Fillers
Such as barium sulfate, silica, etc., used to adjust the flow, opacity, and hardness of the ink.
✒︎ Solvents
Adjust the viscosity of the ink to suit different coating processes.
✒︎ Additives
Such as defoamers, leveling agents, etc., used to improve the processing performance of the ink.
★ Classification of Solder Mask Ink
✒︎ By Photosensitivity Classification
𓀥 Photosensitive Development Ink
This is the mainstream process today. It forms patterns through exposure and development.
𓀥 Thermosetting Ink
Cured by heating, does not require exposure and development. It is lower in cost but not very precise, mainly used for low-end, single and double-sided boards with larger line widths and spacings.
𓀥 UV Curing Ink
Cured instantly by UV light, mainly used for character inks (silkscreen layers).
✒︎ By Curing Method Classification
𓀥 Thermal Curing: Requires high-temperature baking to fully polymerize and cure, providing the most stable performance.
𓀥 UV Curing: Fast curing speed.
𓀥 UV/Thermal Dual Curing: Initial UV curing for positioning, followed by thermal curing to achieve optimal performance, which is a common method for high-performance inks.
★ Solder Mask Process Flow
✒︎ Pre-treatment
Clean the copper surface and increase the roughness of the copper surface through mechanical or chemical etching methods to enhance the adhesion of the ink.
✒︎ Coating
Evenly coat the ink on the board surface. Common methods include:
𓀥 Screen Printing: Suitable for low-precision boards with line widths/spacings greater than 0.2mm.
𓀥 Spraying: Suitable for high-density interconnect (HDI) boards and irregular boards, providing an even coating without pressure.
✒︎ Pre-baking
Evaporate the solvent in the ink at low temperatures to form a non-tacky thin film for subsequent exposure.
✒︎ Exposure
Place a film with pad windows tightly against the ink and expose it to UV light. The ink under the transparent area of the film becomes photosensitive and undergoes a cross-linking reaction (curing), while the ink in the blocked area does not cure.
✒︎ Development
Wash the board surface with a dilute alkaline solution (such as 1% sodium carbonate solution). Unexposed ink is dissolved away, revealing the pads and electroplated areas that need soldering; the exposed ink is retained.
✒︎ Post-curing/Post-baking
Thoroughly cure the ink through high-temperature heating to achieve final hardness, adhesion, chemical resistance, and electrical performance.
★ Color Related to Solder Mask Ink
✒︎ Green
𓀥 The most historically established, mature process, stable supply, and lowest cost.
𓀥 Low reflectivity to light, making it the most eye-friendly during long-term SMT placement and manual inspection, reducing fatigue.
𓀥 Highest contrast with white silkscreen, making characters the clearest and easiest to read. ♡♡ Applications: Standard products in most consumer electronics, industrial control, automotive electronics, etc.
✒︎ Black
𓀥 High-end appearance, stable, and textured, commonly used in high-end consumer electronics (such as graphics cards, motherboards).
𓀥 Good opacity, providing better coverage of internal circuits.
𓀥 High heat absorption rate, which may have a slight adverse effect on heat dissipation in high-temperature environments.
𓀥 Manufacturing and inspection difficulties: Small scratches, copper spots, dust, and other defects are very noticeable against a black background, leading to a slightly lower pass rate than green. ♡♡ Applications: High-end consumer products, audio equipment, network devices, etc.
✒︎ White
𓀥 Mainly used for LED lamp boards, effectively reflecting light to enhance LED brightness and efficiency.
𓀥 Simple and modern appearance, suitable for light-colored product designs.
𓀥 Very easy to get dirty, leaving marks from hand sweat and stains.
𓀥 Prone to yellowing after reflow soldering, affecting aesthetics.
𓀥 Very sensitive to scratches and impurities during the manufacturing process. ♡♡ Applications: LED lighting boards, backlight boards requiring high reflectivity.
✒︎ Blue/Red/Yellow, etc.
𓀥 Eye-catching appearance, providing good product identification.
𓀥 Higher cost than green.
𓀥 Some colors (like blue) have contrast between green and black, with acceptable character clarity. ♡♡ Applications: Development boards, specific branded consumer electronics, boards with internal color coding.
★ Core Principles for Color Selection
1. Cost Priority Principle
If there are no special requirements for color, green is the first choice. This is the most economical and reliable option.
2. Application Scenario Driven Principle
𓀥 LED Products: Prefer white to maximize light reflection.
𓀥 High-end Consumer Products: Consider black or deep blue to enhance appearance quality.
𓀥 Development Boards/Education Kits: Choose blue, red, etc., to enhance brand recognition and visual appeal.
𓀥 High-density, high-precision boards: It is recommended to prioritize green to reduce production and inspection difficulties, ensuring yield.
3. Manufacturability and Maintainability Principle
𓀥 For boards with high repair/debug frequency (such as industrial control boards, experimental boards), avoid using black and choose green or blue with clear silkscreen.
𓀥 Consider the visual positioning system of SMT placement machines. Most systems recognize green boards best.
4. Supply Chain Stability Principle
The supply chain for green ink is the most complete. When choosing niche colors (such as purple, pink, matte colors, etc.), confirm whether stable supply can be ensured and whether the board factory has the corresponding process experience.
5. Aesthetics and Brand Image Principle
In fields where product appearance is crucial (such as gaming hardware, smart home), color is an important design element. At this point, compromises can be made on cost and process to choose the color that best represents the brand’s personality.
♡♡ Different solder mask colors also affect signal integrity for high-speed and high-frequency boards, so choose the appropriate color based on product requirements and application scenarios.
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