PCB Routing Precision: Outline Tolerance and Edge Quality
Routing is the operation that gives a printed circuit board its final shape. A CNC cutter follows the outline, cuts slots and windows and separates the individual circuits from the production panel, and the accuracy of that operation decides whether the board fits the housing and whether a connector lands where the mechanical drawing says it should.
The industry separates standard routing from precision routing, and the difference is measurable rather than a matter of marketing. It appears in the outline tolerance, in the quality of the cut edge, in the capability to hold a slot position and in the process control that supports them.
What Routing Does
Routing covers the outer profile, internal slots and cutouts, connector windows and any non-standard shape that cannot be produced by scoring. The cutter removes material along a path defined by the board outline, and the resulting edge is a machined surface rather than a sheared or punched one.
Because the tool follows a path, the accuracy of the operation depends on the machine, the tool and the material. The same operation is used to cut a simple rectangular outline and to produce a board with a dozen internal features, but the tolerance that can be held is not the same in both cases.
Standard Versus Precision Routing
Standard routing holds an outline tolerance in the region of plus or minus 0.1 to 0.15 mm and is entirely adequate for a board that sits inside a housing with clearance. Precision routing holds a tolerance of plus or minus 0.05 mm or better, and it is used where the board edge is a mechanical datum.
The applications differ accordingly. A precision routed edge appears where a connector is positioned relative to the outline, where two boards plug together, or where the board is mounted against a machined surface. In each case the electrical function depends on the mechanical position, so the outline tolerance becomes part of the specification.
Tolerance Contributors
The machine contributes positioning accuracy and repeatability. The panel contributes its own dimensional change from lamination, which is a larger contributor than most designers expect, since the artwork is scaled to a nominal dimension and the pressed panel may not match it exactly.
The tool contributes a cutting width and a deflection. A small cutter deflects more under load, which pulls the edge away from the commanded path. The material contributes its behaviour: a brittle laminate chips, a soft one smears, and a metal backed board behaves differently from either.
Tool Wear and Feed Rate
A worn tool cuts a wider path and produces a rougher edge. The wear is predictable, and the fabricator compensates for it by measuring the cut width and adjusting the tool path, or by replacing the tool on a schedule based on the number of metres cut.
Feed rate is the other control. Cutting too fast loads the tool and produces burrs and delamination, while cutting too slowly heats the material and leaves a smeared edge. Both extremes are visible on the cut edge under magnification, which is why edge quality is inspected on a sample rather than assumed from the tolerance.
Slot and Cutout Geometry
Internal features add constraints. A slot narrower than the smallest available cutter cannot be produced, and a cutout with a sharp internal corner is impossible because the cutter is round. The design answer is a corner radius equal to at least half the cutter diameter, usually specified as a note on the drawing.
The position of a cutout is usually toleranced relative to the board edge or to a datum hole rather than to the artwork. Stating the datum explicitly avoids a disagreement at inspection, since the panel dimensions and the board dimensions are not the same frame of reference.
Depaneling and Stress
Routing is also the operation that separates the circuits from the panel. Where the board is held by tabs, those tabs are cut or broken in a separate step, and the choice of tab position decides how much stress reaches the components near the edge.
V-cut scoring is faster and cheaper but leaves a straight line and applies a bending load during breaking. Routing leaves a cleaner edge and allows a curved outline, at the cost of machine time. Many panels combine the two, scoring the long edges and routing the regions where geometry or components demand it.
Measurement and Inspection
Outline dimensions are measured with a vision system, a coordinate measuring machine or, in production, with a fixture that the board has to fit. The measurement method should be agreed in advance, because a coordinate measurement and a go no-go gauge answer slightly different questions.
Edge quality is checked visually and by cross section where delamination is a concern. Chip size, burr height and the presence of copper smear at the edge are the usual criteria, and they belong in the acceptance specification rather than in a verbal agreement.
Design Rules and Documentation
State the outline tolerance, the corner radii, the datum for every internal feature and the edge quality requirement on the fabrication drawing. Keep copper and components at least the tool radius away from the outline, and remember that a board edge with exposed copper laminate, such as a castellation or an edge plating, imposes additional process constraints on the router.
gopcb produces routed boards across the range from standard to precision outlines, with vision measurement of critical dimensions and edge inspection data for assemblies where the board outline is a mechanical datum.
CNC Milling Parameters in Practice
The parameters that a fabricator controls during CNC milling are the spindle speed, the feed rate, the number of passes and the depth of each pass. Running the cutter around the profile in several shallow passes produces a cleaner edge and less tool load than a single deep pass, and it is the usual approach on a laminate that is prone to chipping. The trade is machine time, which is why the choice of pass strategy is part of the quotation rather than an invisible detail.
Spindle speed and feed rate have to be matched to the tool diameter. A small cutter needs a high spindle speed to reach the correct surface speed, and if the machine cannot reach it the tool rubs rather than cuts, which burns the resin and produces a smeared edge. That is the mechanism behind many edge quality complaints that are attributed to the material, and it is worth confirming the tool diameter and the spindle capability before blaming the laminate.
Where PCB Routing Meets Assembly
The routed outline has a direct effect on the assembly line. A board that is held in a fixture by its edge needs that edge to be repeatable, and a board that is fed through a conveyor needs its outline and its tooling holes to be consistent from panel to panel. Where PCB routing holds a tolerance of plus or minus 0.05 mm, the fixture can be designed as a simple locating nest, while a looser outline forces a compliant fixture and a longer setup.
The edge also affects the components. A burr or a smear of copper at the outline can bridge to a chassis or create a short in a metal housing, and a delaminated edge can absorb moisture and fail a humidity test. Specifying the edge quality, and inspecting a sample from each production lot, keeps those problems on the fabrication side rather than in the field.
FAQ
When is precision routing worth the extra cost? When the outline is a datum for a connector, a mating board or a machined housing. If the board sits inside a housing with clearance, standard routing is normally sufficient.
Why can a slot not have a sharp internal corner? Because the cutter is round. The smallest achievable internal radius is half the cutter diameter, and that radius has to be drawn rather than assumed.
Does the laminate affect the achievable tolerance? Yes, indirectly. A material that chips or delaminates limits the feed rate and the tool life, which in turn limits how tightly the edge can be held over a production run.
Related reading: board outline and mounting design, PCB manufacturing tolerances, PCB manufacturing processes, and PCB capture pad design.
