Bottom line: legend and 2D codes are not decoration — they are the interface between assembly error-proofing and lot traceability. Keep legend height at 0.8 mm or more, stroke at 0.15 mm or more, 0.15 mm clearance to mask openings, and screen-print registration within ±0.1 mm; keep the data module at 0.25 mm or more with a 4-module quiet zone and an ISO/IEC 15415 grade of C (2.0) or better, and read rates stay stable on the line.
1. Three hard limits for legend: legible, unambiguous, clear of pads
Legend over a pad traps ink that outgasses in reflow and leaves pinholes at the joint edge. Legend over a test pad stops the probe breaking through, so a good board is called open. The three limits are height, stroke width and clearance to openings. Break any one of them and the defect shows up not in the board shop but at the placement line and test fixture.
- Legend height 0.8 mm minimum, stroke width 0.15 mm minimum, character spacing 0.15 mm minimum.
- 0.15 mm or more from pad, opening and test pad edges; 0.5 mm or more from the board edge so routing does not clip it.
- Use a solid triangle or "+" for polarity, never a hairline outline; put diode orientation outside the component body outline, not under it.
2. Screen printing, inkjet and laser: where each one belongs
Screen printing is one pass and the cheapest, but registration error grows with panel size: on a 610×1100 mm panel the corner-to-centre difference reaches 0.1 mm, and corners push past ±0.15 mm. Inkjet prints per board from CAM data, holds ±0.05 mm, and can be added after lamination — good for frequent revisions and low volume. Laser marking is reserved for 2D codes and lot numbers: it images by contrast between mask and the copper beneath, cuts 8–15 μm deep, and must not penetrate the mask.
- Screen print: default for volume, white or black, cured at 150 °C for 30 min, cross-hatch adhesion 4B or better.
- Inkjet: prototypes and small lots, 0.15 mm stroke and up, height adapted to the space available, no screen and no exposure time.
- Laser: DataMatrix and lot codes, 8–15 μm deep; on green mask the contrast is too low, so open the mask to copper or add a white base layer first.
3. Whether a code reads depends on module size and quiet zone
A code that scans instantly on a bench reader often fails on a fixed-mount camera at the customer. The cause is rarely marking quality: the module is too small and the quiet zone has been taken by legend. A 0.25 mm module under a 5 MP camera with a 40 mm field gives about 3.2 pixels per module, just above the decoding threshold; at 0.2 mm you are at 2.5 pixels, where one specular highlight kills the read.
- Module 0.25 mm or larger; start at 16×16 or 20×20 and encode only lot, part number and serial.
- Quiet zone 4 modules or more, with no legend, no trace, no opening and no test pad inside it.
- Grade to ISO/IEC 15415 at C (2.0) or better, contrast 0.4 or better, first-read rate 99% or better on a fixed camera.
- On a multi-up panel, mark every piece individually — never copy one code across the panel; bind the code value to panel coordinates so you can trace back to the cavity.
4. Traceability granularity: from lot to cavity
A lot code traces to a batch, so a problem quarantines all of it. A cavity code traces to a position and shrinks the rework window from 500 pcs to under 50. The cost is one more read-verify step at marking and one coordinate column in the shop-floor system. For automotive and medical orders it is almost always worth it.
- Three levels: lot code (date and part number), panel code (working panel ID), cavity code (serial).
- Read-verify 100% after marking; stop the line on a no-read rather than keying data in by hand, which breaks the chain at its source.
- Bind to electrical test: write open and short results for each cavity into the same record so a complaint traces back to measured values.
5. Review checklist: five items on the drawing
The item most often missed in legend review is not typography but how the board will be assembled, tested and traced. Put these five on the checklist and most rework stops before material is cut, including a fiducial that turns out to be covered only after lamination.
- Legend: 0.8 mm height minimum, 0.15 mm stroke minimum, 0.15 mm clearance to openings.
- Process: screen print for volume, inkjet for frequent revisions, laser for codes with the 8–15 μm depth stated.
- Code: module 0.25 mm minimum, 4-module quiet zone, grade C (2.0) or better.
- Uniqueness: one code per piece, bound to panel coordinates, no panel-wide reuse.
- Keep-out: no legend and no code over fiducials, test pads, gold fingers or impedance coupons.
AIPCB.cc supports up to 44 layers, a maximum panel size of 610×1100 mm, an aspect ratio up to 20:1 (25:1 for backplanes), impedance tolerance of ±8% (standard ±10%), line width accuracy of ±8 μm and up to 13 OZ heavy copper, with 24-hour expedited prototypes. For legend layout and 2D code readability reviews, send the board outline, panelization and traceability requirements to info@ipcba.com.cn, or call +86 138 2310 9230 to work directly with engineering.
Send your Gerber files and stackup requirements; a 24-hour expedite channel is available.