The one thing to get right
Attribute before you change. A defect that disappears after you swapped the bit, the parameters and the board in the same shift has not been diagnosed — it has been masked. Change one thing, re-run, and only then move to the next.
The twelve defect questions
These twelve classes cover most of what reaches inspection. Each answer names the layer or the setting that owns the cause.
Why do drill bits break?
Most breakage traces to the stack rather than the bit: a scratched or creased entry board, a bowed backup board, debris trapped between panels, or poor chip removal. Use flat boards with a surface hardness suited to the bit, and check that the flutes are clearing chips.
What causes hole damage?
Hole damage usually means the entry aluminum or the backup board is missing, or the backup board has been placed the wrong way round. Both layers must be present, and orientation must be consistent across the stack.
Why are holes out of position?
The entry board is the usual suspect: one that is too soft or too hard lets the bit wander at first contact, and scratches or creases in the aluminum steer it off. A high-density 0.50 mm mineral-filled entry board, or a composite entry board with thin aluminum faces over a fibre core, holds position better. Machine, program and stack also move position — see hole position accuracy.
Why does hole size drift out of tolerance?
Hole-size distortion is mainly a worn-bit and parameter problem rather than a board problem. Limit the regrind count — in multilayer work, regrind every 500 holes and replace the bit after two to three regrinds.
Why are holes missed?
Missed holes usually come from the program or from a broken bit rather than the material. After a stop, resume by stepping back one to two holes so the interrupted hole is re-drilled.
What causes burr on the exit side?
Exit burr points at the backup board: density too low, debris between panels, or a curved panel leaving a gap between the stack and the board. Density-test the backup board, press to close the gaps, and make sure the entry aluminum is present so the bit starts clean. The board-side detail is in exit burr control.
Why is the drill not breaking through?
An incomplete-drilling defect usually means the backup board thickness is uneven, or the machine table is not flat. Measure thickness consistency across the board and replace the board if it varies.
What causes curled debris?
Curled debris appears when there is no entry board, or when the parameters are wrong for the stack. Add a suitable entry board and adjust the feed and speed — lower the feed and raise the speed.
Why do holes become blocked?
Three common causes: drilling too deep into the backup board (hold it to about 0.5 mm), reusing a backup board past its condition, and moisture in the laminate. Bake the panels at 145 °C ± 5 for 4 hours and replace the board.
Why is the hole wall rough?
Rough hole walls trace to the wrong entry board material, or to insufficient vacuum. Change the entry board to a grade suited to the laminate and check the vacuum system.
What is the white ring at the hole edge?
A white ring around the hole is a thermal and mechanical stress mark, and it can also signal a marginal base material. Change the bit and lower the feed rate, and review the laminate if it persists.
What causes a nail head at the hole mouth?
A nail head at the exit mouth points at the backup board; a nail head at the entry mouth points at an entry board that is too thin. Use a smooth, hard backup board, change or flip it each stack, and thicken the entry board where the entry side is the problem.
Parameters and setup questions
What is the correct presser-foot pressure?
Around 7.5 kg of pressure with the foot about 0.80 mm above the board face is the setting we work to. Confirm it on your own machine, because stack height and panel size shift the right figure.
How deep should the drill enter the backup board?
About 0.5 mm. Shallower leaves the exit unsupported and produces burr; deeper packs the flutes and blocks holes.
How tall should a drilling stack be?
About six times the drill diameter for double-sided panels and about two to three times the diameter for multilayer panels.
Why bake panels before drilling?
To remove absorbed moisture. In our process the bake is 145 °C ± 5 for 4 hours; undried laminate releases moisture at the hole wall and causes resin smear and blocked holes. The parameters guide sets out the full setup order.
How many holes should a drill bit last?
In our field records, about 3000–3500 holes in double-sided work and around 500 in high-density multilayer work, with FR-5 reducing life by roughly 30 percent.
Which board to change, and when
Should I change the backup board first when a defect appears?
Usually not. The backup board is where an exit-side defect shows most clearly, which is why it is blamed first and often at fault last. Rule out the bit, the parameters and the equipment first, then change one stack variable at a time.
Which layer owns which part of the hole?
The entry board owns the bit's entry and the heat path at the top of the stack; the backup board owns the exit side. Neither owns hole position — that belongs to the machine, the program and the stack geometry. The full layer-by-layer split is in our PCB drilling defects guide.
How do I pick a backup board without testing every grade?
Start from material class and hole size rather than from a catalogue. HDF is the cost-first default for 2–12 layer work, melamine where a harder fused face is needed, and phenolic where positional accuracy at small diameters is the constraint. The backup board material comparison ranks the families, and the selector tool shortlists a grade from your inputs.