PCB Routing Edge Burr Control: Cut Quality, Dust and Delamination
Depaneling by pcb routing removes material with a rotating cutter rather than a shearing blade, which gives a clean profile but also produces burrs, dust and local heating. The quality of that cut depends on the bit, the feed rate and the support under the board, not on the machine brand.
Why Routing Leaves a Burr on the Board Edge
A burr forms because the copper foil and the epoxy resin behave differently under the cutter. Copper is ductile and stretches ahead of the cutting edge before it fractures, while the resin is brittle and fractures cleanly. The result is a thin lip of copper that remains attached to the edge, sometimes folded back over the top surface where it can short to an adjacent feature. Glass bundles that are not fully encapsulated push the same lip outward, so the burr height on a woven glass laminate is usually greatest where a warp or weft bundle meets the cut line.
Burr severity rises with tool wear, with low feed rate, and with a cutter that has a negative or neutral rake angle. It also rises sharply when the bit has been used past its rated life, because a dull edge rubs rather than cuts and the friction heats the resin until it softens. In that regime the burr is accompanied by smeared resin and a widened cut, and reworking the edge with a deburring tool simply pushes the copper further into the laminate.
Router Bit Geometry, Coating and Wear
Depaneling bits are almost always two flute carbide end mills with a diamond like carbon coating. The diameter matters more than most operators expect: a 2.0 mm bit cutting at 30,000 rpm gives a peripheral speed that shears cleanly, while a 3.0 mm bit at the same speed removes more material per pass and loads the spindle harder. The flute count controls chip clearance, and on a standard two flute bit the chip load per tooth should be between 0.01 and 0.02 mm.
Coating wear is the practical life limit. Diamond like carbon coating survives roughly 30 to 60 metres of cut on FR-4 before the edge is exposed, and after that the coefficient of friction rises and the edge temperature climbs. Log the cut length for every router bit and replace on distance rather than on appearance. A bit that looks sharp at 80 metres of cut will already be producing burrs 20 percent higher than one at 20 metres.
Feed Rate, Spindle Speed and Chip Load
Feed rate and spindle speed have to be set together, because the chip load is simply feed rate divided by spindle speed and flute count. Cutting a 1.6 mm FR-4 panel usually calls for 20,000 to 40,000 rpm and a feed of 10 to 25 mm per second, which puts chip load in the recommended band. Feed too slow and the edge rubs, generating heat and burrs; feed too fast and the bit deflects, leaving a tapered wall and sometimes cracking the glass bundles.
Verify the setting from the cut rather than from the dial. A correct cut leaves a matte, uniform wall with visible but shallow tool marks and fine dust rather than chips. A cut that produces brown discolouration, a shiny glazed wall or resin smoke is running hot, and the answer is more feed per tooth, not more spindle speed. Record the parameters with the panel type, because thin panels and high glass content laminates need lower feed rates than standard 1.6 mm FR-4.
Edge Burr Measurement and Acceptance Limits
Burr height is measured at the edge profile, ideally on a polished cross section or with a stylus profilometer, and the acceptance limit should be written on the fabrication drawing. For most products a burr of 25 microns or less is acceptable, and anything above 50 microns must be removed before the board is assembled. Where the cut runs within 0.2 mm of a conductor, the limit tightens, because a folded burr that reaches a pad creates a short that will not be visible after coating.
Measurement is only meaningful if it is repeatable. Fix the magnification, fix the lighting direction, and measure the same three locations on every panel. A deburring pass with a ceramic fibre brush or a fine diamond file removes the lip without rounding the profile, but it must be followed by a cleaning step, because the brushing embeds copper particles in the resin that later show up as ionic contamination.
Delamination, Whitening and Resin Smear
Routing damage is not limited to the visible edge. Heat generated by a dull bit can drive moisture in the laminate above its boiling point, and the resulting pressure separates the glass weave from the resin, producing whitening that extends several tenths of a millimetre into the board. The same heat softens the resin so that it smears across the glass bundles instead of fracturing, and a smeared edge will not accept a clean coating. Both effects are common on boards that were baked for moisture removal immediately before routing, because the bake leaves the laminate dry but hot.
Check for delamination by looking at the edge under 20x magnification with grazing light, then confirm with a microsection on a sample from each panel batch. The pass criterion is no separation between the resin and the glass with any visible gap, and no crack that extends beyond 0.1 mm from the cut face. Any board that shows whitening should be checked for moisture uptake before it is committed to a reflow profile, because the same moisture will expand again at 260 C.
Dust Extraction and Static Control in the Router Enclosure
Routing generates a fine, abrasive dust that is roughly 40 to 60 percent glass by weight, and it must be extracted at the cut point rather than allowed to settle in the enclosure. A dust extraction nozzle placed within 10 mm of the cut, with an air velocity above 20 m per second at the intake, captures most of the debris; a cabinet filter alone does not, because the dust has already travelled across the panel before it reaches the filter.
The dust is also a static hazard. Dry glass filled dust moving through a plastic hose generates a charge that can reach several kilovolts, and if the panel is not grounded through the fixture, that charge will discharge into the nearest conductor. Ground the panel through the tooling pins, use conductive hose, and verify the path with a resistance measurement below 1 x 10^9 ohms. Combine dust extraction with the same floor zoning discipline used elsewhere in the shop so that routing dust does not migrate into the printing area.
Panel Support, Tooling Pins and Vibration Control
Support under the cut line decides whether the panel deflects during routing. If the board is unsupported within 10 mm of the cut, the panel acts as a cantilever, the cut edge chips on exit and the risk of cracking a nearby via rises sharply. Tooling pins should be positioned within 25 mm of the cutting path at both ends of every segment, and the panel should be held down without bowing it more than 0.1 mm across the routed span.
Vibration control matters equally for thin panels. A 0.8 mm board routed on a machine set up for 1.6 mm stock will resonate, and the resonance shows up as a wavy profile and a higher burr. Reduce the spindle speed, reduce the depth of cut per pass, and support the panel from below with a sacrificial backing sheet. Where the design allows, routing in two passes of 0.8 mm each produces a better edge than a single 1.6 mm pass at the same feed.
Program Design: Tabs, Breakaway and Cutting Order
The routing program determines how much the panel moves while it is being cut. Tabs should be placed so that the last material removed is symmetric about the centre of the part, and they should be long enough that the part does not rotate once the first tab is severed. A tab of 2.5 to 3.0 mm width with a 0.5 mm deep score on each side is a common compromise between stiffness during routing and clean separation afterwards.
Cutting order also matters. Routing the outer profile first and the internal cut outs last keeps the panel stiff for as long as possible, whereas removing the perimeter early leaves a floppy part that deflects under the cutter. Where a board carries a breakaway rail with a V-score, route the rail with a shallower depth of cut and a higher feed so that the V-score edge is not rolled over. Review the program against the tooling hole panel design before the first production run rather than after the first batch of rejects.
Records, Bit Life and Change Control
Keep a router bit log that records installation date, cumulative cut length, spindle speed, feed rate and the burr measurement from the first article. Replace the bit at the interval defined by the log rather than when the operator notices a poor edge, and record the replacement in the same file. When a panel type changes, re verify the first article rather than assuming the previous settings transfer, because glass content, thickness and copper weight all shift the optimum feed rate.
Change control closes the loop. Any change to bit supplier, coating, spindle, backing material or program should be assessed against a defined first article that includes a burr measurement and a microsection of the edge. The cost of that check is a few minutes of machine time; the cost of discovering a delamination problem after coating is a batch of boards that cannot be reworked.
FAQ
What burr height is acceptable on a routed PCB edge? For most assemblies, 25 microns or less is acceptable without rework and anything above 50 microns must be removed. Where the cut is within 0.2 mm of a conductor, apply the tighter limit, because a folded copper burr can bridge to a pad and will not be visible once the board is coated.
Why does the routed edge turn brown or shiny? That is heat damage. It means the chip load is too low, so the cutting edge rubs instead of shearing. Increase the feed per tooth, check that the bit is not past its rated cut length, and confirm that dust extraction is pulling air across the cut point.
Does baking before routing cause edge damage? It can. Baking removes moisture but leaves the laminate warm, and a warm laminate smears rather than fractures cleanly. Let panels cool to room temperature before routing, and route panels that will be reflowed within the same shift so that they do not reabsorb moisture.
