Router Feed Rate and Edge Quality in PCB Depaneling

The router separates panels into individual boards by cutting along a path with a small carbide bit, and the feed rate is the speed at which that bit travels through the material. It is the parameter that balances cut quality against cycle time, and it interacts with spindle speed, bit geometry and stack height so closely that a change in any one of them moves the window for the others.

What the Router Does

A router removes material by milling a channel, typically 2 to 3 mm wide, along the outline defined in the programme. In production terms this is a depaneling operation, and it follows the same cutting rules as any other routing step, including the chip load and extraction limits that make the cut possible. The bit is small, the spindle turns at high speed and the cut is usually made in a single pass through a stack of panels held on a vacuum table.

Because the bit cuts the full thickness, the chip has to clear from a deep, narrow channel and the heat has to leave with it. That is the constraint that ties feed rate to bit geometry and to dust extraction, and it makes routing different from a shallow machining operation.

Feed Rate and Chip Load

Chip load is the thickness of material removed by each cutting edge in one revolution, and it is calculated from the feed rate, the number of flutes and the spindle speed. A load between about 0.02 and 0.05 mm per tooth gives a clean cut in FR-4, with the lower end used where the outline approaches a pad.

Feeding too slowly rubs the material rather than cutting it. The bit heats, the resin softens and the edge is left with a burnt, smeared appearance that hides small tears. Feeding too fast overloads the flutes, which produces a rough edge, a whistling cut and, on a small bit, a broken tool.

Spindle Speed

Spindle speed and feed rate are set together because only the ratio between them defines the chip load. A common combination for a 2 mm bit is 20,000 to 30,000 rpm with a feed of 1 to 3 metres per minute, and both figures scale with bit diameter.

Running the spindle at the top of its range shortens bearing life and raises the noise level, while running it low forces a slow feed to keep the load correct, which lengthens the cycle and increases the heat left in the panel rather than removing it with the chip.

Bit Geometry and Coating

Router bits are made in diameters from about 1.5 to 3 mm with two flutes, an upcut or a compression geometry and a diamond or titanium coating. The geometry decides where the chips go and how the top and bottom edges are finished, which is why a compression bit is used where both faces matter.

Coating life is measured in metres cut rather than in hours. A diamond coated bit may run several thousand metres in FR-4, while an uncoated one may be retired after a few hundred, and the record of metres per bit is what allows the tool change to be scheduled instead of discovered, and the bit wear measurement belongs with it.

Edge Quality and Burrs

The edge produced by routing is judged by burrs, fibre tear out, resin smear and discolouration. A light burr on the copper at the cut edge is normal, while torn laminate and a smear of resin across a track are defects that no deburring step will correct.

Both extremes of feed rate produce burrs, which is why the edge has to be inspected rather than assumed. The check is made on a coupon cut with the production programme rather than on a scrap panel, because a coupon can be measured and filed with the settings that produced it. Burrs from too slow a feed are thin and attached to the copper, while burrs from too fast a feed are larger, rougher and often accompanied by a broken edge in the laminate. A routing edge that is checked under low angle light at magnification tells the two apart.

Heat and Dust Extraction

Almost all of the heat generated at the cutting edge leaves with the chip, so extraction is a cutting parameter rather than a housekeeping measure. A blocked extraction path lets dust recirculate, which both reheats the bit and packs the channel ahead of it.

Extraction is normally specified as a vacuum at the foot of the bit of at least 15 metres per second of air velocity in the channel. Where a shop reduces extraction to lower the noise, the feed rate has to be reduced with it, and that trade is rarely made consciously.

Stack Height and Support

Panels are cut in stacks to share the setup, but the stack changes the job. A tall stack needs a longer flute to clear chips, presents more material to the bit and amplifies any movement of a panel that is not held flat against the table.

Support under the cut path matters as much as the vacuum. Where a panel is unsupported near the edge of the table, the material deflects under the cutting load and the bottom board of the stack is cut to a different width from the top, which appears as an outline tolerance problem rather than a routing one. The support plate is part of the answer, and it has to be flat enough that the stack does not shift when the bit enters the material.

Tool Wear and Breakage

Wear appears first as a change in the edge, then as an increase in cutting noise, and finally as a dimension that drifts out of tolerance. A worn bit also cuts a narrower channel for the same programme, and the outline moves with it.

Breakage is most often caused by an interruption rather than by steady cutting: entering the material at an unsupported point, crossing a slot, or a stack that shifts. The metres cut per bit should be recorded, and the limit should be set from the measurement of edge quality rather than from a supplier figure. A tool changed on a metrage limit rather than on a failure is the simplest way to keep the edge inside specification.

Setting and Verifying the Window

The window is described by spindle speed, feed rate, plunge rate, bit diameter and geometry, stack height and extraction. It is established for each material and stack, and it should be re-established when the laminate supplier changes. The plunge rate deserves the same attention as the feed rate, because the entry point is where most small bits break.

Verification is a first article cut from a test panel and checked for outline dimension, burr height and laminate tear out, with the results filed against the parameters. Deburring and edge quality are the two figures that a customer’s inspection will look at, and both are set by the parameters above.

FAQ

What chip load should a router bit run at? About 0.02 to 0.05 mm per tooth in FR-4, calculated from the feed rate, the spindle speed and the number of flutes.

Does a faster feed rate reduce edge quality? Beyond the correct chip load it does, producing rough edges and broken laminate, while feeding too slowly rubs the material and burns the resin.

How long does a router bit last? It is measured in metres cut rather than in hours, and it ranges from a few hundred metres for an uncoated bit to several thousand for a diamond coated one.