PCB silkscreen (also known as legend, nomenclature, or marking) refers to the text, symbols, or graphics on printed circuit boards, primarily used to provide critical information such as component locations, orientation, and part numbers for assembly, testing, maintenance, and traceability.
★ Silkscreen Fundamentals
1. Purpose
· Component Designators: Indicate component reference designators (e.g., R1, C2), part numbers, or parameters.
· Orientation Markings: Denote polarity direction for polarized components (e.g., diodes, electrolytic capacitors).
· Board Information: Include board name, revision number, manufacturing date, and manufacturer identification.
· Test Point/Interface Labels: Identify test points and connector functions (e.g., "USB1", "VCC").
2. Silkscreen Content
· Reference Designators: Correspond to schematic and BOM, such as U3 (IC), R5 (Resistor).
· Polarity Markings: +, -, △ (Pin 1 indicator), chamfered corner marks.
· Board Identifiers: Such as PCB-2025-V1.0.
· Safety Symbols: High voltage warnings, ESD-sensitive indicators, etc.
3. Process Types
· Screen Printing: Traditional process with low cost and limited resolution (line width typically ≥0.15mm).
· Inkjet Printing: High flexibility, suitable for small-batch rapid prototyping.
★ Design Guidelines
1. Placement and Layout
· Avoid Pads/Vias: Silkscreen must not cover pads, solder paste, or test points; maintain minimum 0.2mm clearance.
· Prevent Component Obstruction: Place silkscreen outside component body for visibility after assembly (e.g., beside ICs, left of resistors).
· Consistent Orientation: Recommend uniform character orientation (e.g., top-left to bottom-right) for readability.
2. Dimensions and Clarity
· Character Height and Stroke Width: Standard recommendation: height ≥1.0mm, stroke width ≥0.15mm; high-density boards can use 0.8mm/0.12mm subject to process capability verification.
· Aspect Ratio: Avoid excessively thin fonts to prevent print blurring or breaking.
3. Critical Markings
· Clear Polarity: Electrolytic capacitor (+) anode, diode cathode (vertical bar or notch), IC Pin 1 (dot or chamfer) must be clearly marked.
· Interface Labels: Connector pin definitions (e.g., TX, GND) should be placed near corresponding pins.
· Board Information: Revision numbers and serial numbers in fixed locations (e.g., board edge) for traceability.
4. Special Requirements
· Heat Dissipation Areas: Avoid printing near heat-generating components (e.g., power inductors) to prevent high-temperature discoloration or peeling.
· Impedance-Controlled Areas: Silkscreen ink may affect high-frequency signals; avoid RF traces or high-speed differential pairs.
★ Manufacturing and Inspection Requirements
1. Ink Selection
· Color Contrast: Typically white on green solder mask; other colors available for black, yellow, or dark-colored boards.
· Adhesion: Must pass solvent resistance and high-temperature tests (remain clear after soldering).
2. Process Limitations
· Resolution: Related to board surface flatness; rough surfaces (e.g., matte solder mask) may reduce clarity.
· Multilayer Boards: Silkscreen typically printed only on top layer (component side); bottom layer simple markings added when necessary.
3. Inspection Standards
· Readability: Characters clear without defects, overlapping, or incompleteness.
· Accuracy: Consistent with Gerber files; no misalignment or missing prints.
· Durability: Adhesion tested with 3M tape; remains identifiable after soldering.
★ Common Errors
1. Silkscreen Overlapping Pads: Causes soldering defects or cold solder joints.
2. Incorrect Orientation Markings: Reversed diode polarity leads to assembly short circuits.
3. Excessively Small Text: Laser engraving may appear blurry; silkscreen may smudge.
4. Missing Critical Information: Omitted revision numbers or polarity markings increase maintenance difficulty.
PCB Silkscreen does not affect electrical performance but is crucial for production efficiency and reliability. Design must balance readability with space constraints, following the principles of "clarity, accuracy, and clearance" while fully considering manufacturing feasibility. Proper silkscreen design significantly reduces assembly error rates and enhances product maintainability.
