HDI Gerber files hide problems that standard boards tolerate. Miss one item on the HDI Gerber checklist and you get a respin or worse — boards that pass incoming inspection but fail in assembly. These are the points that repeatedly surface in DFM reviews.
Verifying Stackup in HDI Gerber Output
Start with the stackup. HDI builds need every layer clearly identified — L1, L2, L3 with core and prepreg thicknesses. Many files arrive with generic layer names or missing dielectric info. The fab can't guess your intended impedance or copper weights.
Cross-check that the Gerber stackup matches your fab's capability. Thin dielectrics common in HDI leave little room for adjustment. A 0.05mm difference in prepreg can shift impedance out of spec before drilling even starts.
Drill Files and Via Considerations in HDI
HDI drill files need separate entries for laser vias, mechanical holes, and buried vias. Combining them or forgetting plated vs non-plated calls creates manufacturing headaches. Laser vias especially require exact depth control.
Check via aspect ratios. Anything over 0.8:1 on microvias pushes capability limits for many shops. Confirm your Gerber includes the correct drill sizes with tolerances. Missing backup drill data or wrong tool lists often delays the job.
Annular Ring and Registration for Microvias
HDI registration is tighter. Annular rings on laser vias should be at least 0.075mm. Smaller rings show up as breakout in the finished board. Many layouts cut this too close trying to maximize routing density.
Include all drill files in the HDI Gerber checklist review. One missing via layer and the whole build fails electrical test.
Impedance Control Requirements in Gerber Package
Impedance notes belong in the fabrication drawing, not just the Gerber. Specify single-ended and differential pairs clearly. HDI impedance depends heavily on copper thickness after plating and exact dielectric constants.
Many files lack reference layers or have inconsistent trace widths. The fab will ask questions, but better to catch it before upload. Tolerance of ±10% is common, but tighter values need callouts and possibly test coupons.
Solder Mask Design and Clearance Checks
Solder mask Gerber for HDI needs special attention around fine pitch pads. Expansion and registration tolerances are smaller. Overlapping mask between pads risks bridging; too much clearance exposes copper to oxidation.
Review mask openings against pad sizes. Typical HDI BGA mask defined pads should follow IPC recommendations but adjusted for your chosen fab process. Missing solder mask on via-in-pad areas is a frequent oversight.
Color and type (LPI, dry film) should be specified. HDI often uses thinner mask for better fine feature resolution.
Common Solder Mask Expansion Issues in HDI
Negative expansion values can pull mask off pads during processing. Positive expansion too large creates solder mask encroaching on pads. Both cause assembly problems.
Silkscreen and Legend Accuracy
Character (legend) files in HDI need clearance from pads. Fine pitch components leave little room. Silkscreen over pads or too close leads to ink on solderable surfaces.
Check line width and height. Minimum 0.15mm width is typical but many fabs prefer 0.2mm for readability after processing. Polarity marks and component outlines must not interfere with assembly.
Panelization and Array Setup in Gerber
Panelization Gerber must include proper routing, scoring or tabs. HDI panels are more prone to warpage so panel layout affects yield. Fiducials on the panel border and array are essential for assembly accuracy.
Leave adequate spacing between boards for depanelization. Tooling holes placement and size need confirmation. Missing panel Gerber or incomplete arrays cause the most last-minute questions from manufacturers.
For multi-image panels, verify all instances match and that coupon placement doesn't overlap active circuitry.
Final HDI Gerber Checklist Items Before Submission
Run a full design rule check on the exported files. Confirm all layers are present and named consistently. Include aperture list, README with special instructions, and fabrication drawing PDF.
- Netlist comparison between schematic and Gerber.
- Layer stackup PDF with tolerances.
- Drill report with all sizes and quantities.
- Impedance coupon locations if required.
- Board outline and panel outline clarity.
A thorough HDI Gerber checklist catches most issues that would otherwise delay production. Spend the extra hour reviewing these points. It saves days on the back end.
The manufacturer will still run their own checks, but clean files speed up the process and reduce the chance of engineering change orders after the order is placed.
