In PCB manufacturing, the clarity of character printing directly affects production traceability, assembly efficiency and product reliability. The following systematically expounds on how to control the clarity of characters from aspects such as design specifications, process parameters, material selection and detection methods:
First, design specification control
Character size standard
Minimum line width and spacing: The character line width is recommended to be ≥0.15mm (6mil), and the character height should be ≥0.8mm to ensure recognition by both human eyes and machine vision.
Safe distance from the solder pad: The distance between the characters and the edge of the solder pad should be ≥0.2mm to prevent the carbonization of the character ink due to heat during welding and the contamination of the solder joint.
Font and layout optimization
Recommended fonts: Use equal-width sans-serif fonts (such as OCR-B, Helvetica), and avoid italic or decorative fonts that may cause character distortion.
Layout principles:
Key information (such as component position numbers) is close to the corresponding pads to reduce the visual search time;
Polarity markings (such as diodes, electrolytic capacitors) should be kept at a distance of ≥0.5mm from the component profile to prevent assembly misoperation.
Color and Contrast
Mainstream color combinations:
White characters (solder mask window) + green substrate (contrast >70%)
Black characters (covered with solder mask) + white substrate (contrast >85%)
Avoid low-contrast designs: For instance, light yellow characters printed on light green substrates may lead to misjudgment in AOI detection.
Second, process parameter control
Screen printing parameters
Web version parameters:
Mesh size: 325-400 mesh (applicable to character height 0.8-1.2mm)
Emulsion thickness: 10-15μm, ensuring uniform ink penetration.
Printing parameters:
Scraper Angle: 70°-75°, pressure 0.2-0.3MPa, speed 100-150mm/s, to prevent burrs on the edges of characters.
Ink selection and curing
Ink type:
Thermocuring type: Suitable for single-sided panels, curing temperature 150℃×30 minutes;
UV curing type: Suitable for double-sided or multi-layer boards, with a curing energy of 800-1200mJ/cm².
Performance requirements:
Adhesion: 100-grid test ≥4B (ASTM D3359);
Solvent resistance: No peeling after 50 wiping with isopropyl alcohol (IPC-SM-840E).
Direct imaging Technology (DI
High-precision inkjet: Utilizing piezoelectric nozzles, with a resolution of ≥1200dpi and a minimum character height of 0.3mm, it is suitable for high-density boards.
Multi-color printing: Supports 4-color (CMYK) or spot color printing to meet personalized needs such as brand logos.
Third, process quality control
First item inspection
Inspection items: Character integrity, position deviation (≤±0.1mm), color consistency (ΔE≤3).
Tools: Use a 3D microscope to measure the height of characters and a colorimeter to detect the Lab* value.
Online monitoring
AOI inspection
Character recognition rate ≥99.5%;
Defect classification: Missing pen, broken line, offset, ink accumulation.
Data traceability: Record the printing parameters and inspection results of each board, and generate SPC charts to monitor process stability.
Environmental control
Temperature and humidity: The temperature in the printing workshop is 23±2℃, and the humidity is 50±10% to prevent the characters from becoming blurred due to changes in ink viscosity.
Cleanliness: ISO level 7 (10,000 level) or above to avoid surface defects of characters caused by dust adhesion.
Fourth, industry trends and Best practices
Automation and Intelligence
Integrate the MES system to achieve real-time linkage between printing parameters and inspection data. For instance, automatically adjust the scraper pressure when character defects are detected.
Environmental protection requirements
Promote water-based inks or UV LED curing technology to reduce VOC emissions and comply with RoHS and REACH regulations.
Enhanced traceability
Superimpose QR codes or Data Matrix codes on the character layer to support full life cycle traceability, and the code size is ≥1.0mm×1.0mm.
Through strict design specifications, meticulous process control and intelligent detection methods, it can be ensured that the clarity of PCB character printing reaches the industry high standards (such as IPC-A-600H Class 2/3), meeting the demands of high-end fields such as automotive electronics and medical equipment.
