Outline routing is the last forming step of a board, yet it is often treated as a trivial "mill along the drawing" operation. On a large 610×1100 mm panel, mounting-hole position, edge parallelism and slot width together form one tolerance chain; once any link goes out of control, the failure returns to the design side during assembly as structural stress or misaligned screw holes. Managing the outline as a tolerance chain is the most direct way to protect assembly yield.
The tolerance chain starts with drawing annotations
Most outline problems are annotated in, not milled in. The design side should first make datums explicit:
- Hold mounting holes and datum holes to ±0.05 mm on size, and dimension their positions against a single datum. Do not chain "X mm from the edge" callouts that accumulate error link by link.
- Set the standard outline tolerance at ±0.1 mm; for mating edges, guide rails and slots, tighten to ±0.05 mm and mark them as fitting dimensions on the drawing.
- Keep non-plated holes at least 0.3 mm from the board edge and plated holes (including castellations) at least 0.5 mm; closer than that, routing stress is enough to tear the hole wall.
- Define rail width and fiducial positions for every panel, and put panel fiducials and board fiducials in one coordinate system so the datum does not drift after depaneling.
Routing quality: tool diameter, path and burrs
The three working variables of outline routing are tool compensation, feed conditions and burr control, and all three show up directly in hole position and edge quality:
- Enter tool compensation from measured values. The difference between a fresh and a worn tool reaches 0.02–0.03 mm, half of the outline tolerance; check tool diameter every 30–50 m of continuous routing and replace it once it drifts.
- Keep internal corners at R ≥ 1 mm or pre-drill relief holes; sharp internal corners concentrate stress and become the starting point of edge cracking during assembly.
- Control burr height within 0.1 mm; for gold-finger areas and card edges, remove burrs by edge brushing so insertion never scratches the connector.
- Schedule separate routing conditions for metal-core and heavy-copper panels: reduce feed by 20–30% on high-thermal-conductivity materials, or tool life and edge roughness degrade together.
Assembly stress: from drawing tolerance to real force
The last link of the chain is assembly. Screw preload converts outline and hole-position deviations into standing mechanical stress:
- Keep traces and components at least 2 mm away from mounting holes, and add annular slots around the holes to isolate stress where needed.
- On boards with edges of 300 mm or more, add one or two mid-span supports between corner mounts; unsupported spans beyond 150 mm raise fatigue risk markedly under vibration.
- Write the tightening sequence and torque into the assembly note. The difference between 0.4 N·m and 0.8 N·m at the same location is enough to change the direction of board deflection.
Inspection and delivery
- Fully inspect hole positions and outline dimensions on the first article with a vision measuring system, and archive the data with the stack-up table.
- Sample outline and hole size per AQL in volume production, keeping lot numbers on the records for assembly-side traceability.
- When a revision touches only outline or hole positions, re-check the entire datum chain so a local edit does not shift everything downstream.
Outline tolerance is not a single-point machine capability issue but a complete chain that runs from drawing annotation, through tool management, to assembly loading. When design writes datums and fitting dimensions clearly, the fabricator keeps tool diameter and feed under control, and assembly fixes torque and sequence, outline tolerances become genuinely manageable. AIPCB.cc supports up to 44 layers, a maximum panel size of 610×1100 mm, heavy copper up to 13 OZ, line-width accuracy of ±8 μm, impedance tolerance of ±8% (±10% standard), and aspect ratio up to 20:1 (25:1 for backplanes), with 24-hour rush prototypes. For engineering and quotations, contact info@ipcba.com.cn or +86 138 2310 9230.
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