The one thing to get right
Separate the two halves before you spend money. Position error that repeats across every panel is the machine, the program or the stack. Position error that appears with a creased or wrong-grade entry board, and disappears when it is replaced, is the board. Measure which one you have.
Stack height is a positioning trade, not just a capacity number: a taller stack-up adds alignment error layer by layer, and hole location accuracy depends on how much stack the panel program actually needs. Sizing the stack to the job is the cheapest accuracy gain there is.
What owns hole position
Position accuracy is the sum of a chain, and the board sits at only one link. Sort the causes before changing anything:
| Source of error | What it looks like | Who owns it |
|---|---|---|
| Spindle, table and machine geometry | Position drifts with machine, not with material | The machine |
| Program coordinates and registration | Error repeats at the same coordinates on every panel | The program |
| Stack rigidity and clamping | Error grows with stack height and at the panel edges | The stack and the presser foot |
| Bit-tip first contact | Error appears with a soft, hard or creased entry surface | The entry board |
The first three rows are worth ruling out first, because they are the ones a board cannot fix. Everything below is about the fourth row — the one link where the entry board is a genuine variable.
Why the entry board moves the hole
At the instant the bit tip touches the top of the stack, the flutes are not yet engaged and the bit is at its least stable. The surface it meets either guides it straight or lets it deflect:
- Too soft — the tip sinks before it bites, and the bit wanders before the cutting edges take hold.
- Too hard or too thin — the tip skids on the surface instead of entering cleanly, again shifting the start of the hole.
- Scratched or creased — a crease in the aluminum is no longer a flat surface; it is a physical ramp that steers the bit off-centre. This is the most common entry-board cause of position error, and it is a handling problem rather than a grade problem.
The same first-contact behaviour is why an entry board with the wrong rigidity shows up on small holes first: a small bit is less stiff and deflects more readily than a large one. The alloy and temper of the aluminum set that rigidity, which is what the entry alloy calculator is for.
Specifying the entry board for position
Where position at small diameters is the constraint, two entry-board constructions are used:
- High-density mineral-filled board, ~0.50 mm. A hard, flat surface that gives the tip a stable first contact without the softness of a plain sheet.
- Composite board. Two thin aluminum faces bonded over a fibre core — in the construction we see most, roughly 0.06 mm of aluminum on each face over a core, for a total of about 0.35 mm. The thin, flat face gives a clean first contact; the core keeps the sheet rigid, so the tip meets a surface that neither sinks nor skids. It is the form usually chosen for the smallest holes.
For standard entry work the common aluminum runs at a nominal 0.20 mm, which in delivered stock is often nearer 0.18 mm; the base material itself is covered on the aluminum entry board page. Whichever construction you choose, crease-free handling is not optional — a creased board of the right grade will out-position-error a flat board of the wrong one.
The machine and program half
Before attributing position error to the board, confirm the half that the board cannot influence:
- Table flatness and spindle condition — check that the position error is not drifting with machine geometry.
- Program and registration — an error that repeats at the same coordinates across panels is a program or registration problem, not a consumable one.
- Stack rigidity and pressure — position error that grows with stack height points at clamping and presser-foot settings rather than at the board; the parameters guide covers the foot pressure and height.
These overlap with the general defect check: position offset is one of the twelve defect classes, and the order to work through them is set out in the defect check order.
How to verify a position fix
- Measure the baseline. Record hole position against a fixed reference with the current stack, before changing anything.
- Change one variable. Swap only the entry board — for a crease-free, correctly specified grade — and hold the machine, program and stack fixed.
- Re-measure. Compare against the baseline at the same coordinates and the same hole size.
- Attribute. If position moves, the entry board was a contributor; if it does not, look at the machine, program and stack.
This is the same one-variable discipline that applies to every other drilling defect: a fix you cannot attribute is not a fix. To size the effect before running the trial, the positional tolerance calculator converts mean offset and spread into the share of holes that miss the target window.
Questions engineers ask
What actually controls PCB hole position?
Most of it belongs to the machine, the program and the stack: spindle and table geometry, the program's coordinates, panel registration and stack rigidity. The entry board adds one specific influence — the first contact between the bit tip and the top of the stack.
Does the entry board affect hole position accuracy?
Yes, at first contact. An entry board that is too soft lets the bit wander before it bites; one that is too hard or too thin can let the tip skid. Scratches and creases become a physical guide that steers the bit off-centre — which is why crease-free handling matters as much as the grade.
What entry board holds position best for small holes?
Two routes are used where small-hole position is the constraint: a high-density 0.50 mm mineral-filled entry board, and a composite entry board that sandwiches a fibre core between thin aluminum faces. The composite form is the one usually chosen for the smallest holes.
How thick should the entry aluminum be?
The common standard entry aluminum runs at a nominal 0.20 mm, which in delivered stock is more often around 0.18 mm. A composite entry board takes a different route: two thin aluminum faces of about 0.06 mm bonded to a fibre core, for a total of about 0.35 mm.
How do I confirm the entry board is the cause of a position problem?
Change one variable and re-measure. Hold the machine, program and stack fixed, swap only the entry board for a crease-free, correctly specified grade, and measure hole position against the same reference. If position does not move, the entry board was not the cause.