PCB Entry & Backup Board Selection Guide

Entry boards and backup boards are the top and bottom of the same drilling stack, but they are selected on different criteria. The entry board controls where the drill enters; the backup board controls what happens where it exits. Selecting them together — rather than as two unrelated purchases — is what removes exit burr, hole-position drift and inconsistent hole depth from a process.

This page is the map. It covers the three variables, a selection matrix that crosses hole size with layer count, the aperture threshold that changes the answer, and a checklist you can run against your own stack. The material-level detail lives on the individual material pages linked throughout.

The Three Variables That Decide the Material

Every selection question reduces to three inputs. Get these wrong and no amount of supplier documentation will fix the process; get them right and the grade usually falls out of the matrix on its own.

Variable	What it controls	Why it dominates
Smallest hole diameter	Required surface hardness and density	As the drill gets smaller, the board acts less like support and more like a forming surface. Below 0.20 mm, small differences in hardness show up as measurable differences in hole-position accuracy and exit burr.
Layer count / stack height	Thickness, flatness and compression resistance	A taller stack amplifies any non-flatness or compression in the board underneath it: the panel lifts, the vacuum seal breaks, and Z-axis depth drifts. Thickness tolerance and flatness matter more as layer count rises.
Stack environment	Moisture and surface-seal requirements	Wood-fibre cores absorb moisture, which warps the board and breaks the vacuum seal. In humid plants a sealed or coated surface is a process requirement, not a premium option.

If you can only measure one thing before ordering, measure your smallest hole diameter. It is the variable with the steepest effect on results. See exit burr control and moisture control for the two failure modes these variables actually cause.

Selection Matrix: Hole Size × Layer Count × Material

The matrix below crosses the two dominant variables. Read the hole-size row first — it sets the minimum hardness — then confirm the layer-count column for thickness and flatness. Material families are named generically so the logic holds whatever grades your supplier offers.

Smallest hole	Minimum hardness	Simple / low layer count (1–6)	Multilayer (6–24)	High-density / micro-via
≥0.25 mm	70 HD	HDF, standard density	HDF, standard density	HDF is usually sufficient — verify flatness
0.20–0.25 mm	70–75 HD	HDF, standard to high density	HDF high density, or UV-coated	UV-coated melamine
0.10–0.20 mm	75–80 HD	UV-coated melamine	UV-coated melamine, mid to high density	UV-coated melamine high density
<0.10 mm	80+ HD	Phenolic or high-density melamine	Phenolic, or high-density melamine	Phenolic or high-density melamine; confirm with a drill trial

Hardness and density bands shown here are the ranges published across this site's material and selection pages. They are starting points for a drill trial, not guarantees — the binding figure is always the one measured on your own stack, at your spindle speed and hit count.

Two notes on reading the matrix. First, the entry board and the backup board are selected independently: an aluminum entry board can sit above a stack whose backup board is HDF, and that is a normal combination, not a mismatch. Second, the layer-count columns affect thickness and flatness, not hardness — a 20-layer board with generous hole sizes does not need a harder backup board, it needs a flatter one.

Why the Aperture Threshold Changes the Answer

Selection guides usually stop at "use a harder board for smaller holes." The more useful question is where that stops being true, because above that point you are paying for a property your process cannot detect.

An independent industry drilling study covering seven backup-board material types at three hole diameters (0.15 mm, 0.20 mm and 0.35 mm) reported a consistent pattern:

• At 0.15–0.20 mm, material choice has a large effect. The harder, denser boards produced measurably better hole-position accuracy, less deviation and smaller exit burr.
• At 0.35 mm, the differences between board materials essentially disappear. Positional capability converged across all seven types.

The relative performance ranking reported at the small-diameter end, from best to weakest, was: phenolic (paper-based) → hot-pressed melamine → fast-cycle melamine → cold-pressed melamine → phenolic wood → UV-coated → standard high-density fibreboard. Two caveats reported alongside it: the phenolic paper board gave the best hole-position accuracy but wore drill bits faster, and the highest-density fibreboard performed worst on exit burr and hole-wall roughness even though it is the cheapest option.

That gives a directly usable rule. Below roughly 0.20 mm, select for hardness and density; at and above roughly 0.35 mm, select for flatness, thickness tolerance and cost. If your hole range sits entirely above 0.35 mm, moving to a cheaper board is not a compromise — on the study's data it is the correct selection. If your range straddles the threshold, split the stack or standardise on the grade your smallest hole requires.

External figures on this page describe generic material types. Actual performance should be confirmed by sample testing on your own stack.

Selecting the Entry Board (Top of the Stack)

The entry board's job is to guide the drill at the moment of contact, before the bit reaches copper. Its selection is driven almost entirely by the smallest hole and the burr target, not by layer count.

Condition	Entry board direction	Primary reason
Standard holes, cost-sensitive	Plain aluminum entry board (1100-series, full-hard)	Provides a consistent entry interface and heat path at the lowest material cost
Sub-0.20 mm holes, burr-sensitive	Coated or lubricated aluminum entry sheet	The coating reduces friction and heat at contact, which is where small-diameter burr and drill wear originate
Back-drilling / stub control	Back-drill aluminum entry sheet	Selected for controlled depth removal rather than for the entry interface alone
Very small vias, high aspect ratio	Thinner gauge, tighter thickness tolerance	Gauge consistency translates directly into entry-point consistency across the panel

Compare what an entry board is and coated aluminum before choosing between plain and coated; the coating decision is driven by your burr measurement, not by hole size alone. The entry board selector returns a starting point from hole size and board type.

Selecting the Backup Board (Bottom of the Stack)

The backup board is where most process problems surface, because the drill exits through it at full speed. Hardness sets exit burr; density and flatness set depth consistency; surface treatment sets moisture behaviour.

Material family	Pick it when	Watch out for
HDF	Holes are ≥0.25 mm and cost per panel dominates	Lower surface hardness — confirm exit burr on your own stack before committing
UV-coated melamine	Sub-0.20 mm holes, or a humid plant needing a sealed surface	Higher cost than HDF; verify the coating survives your hit count
Melamine laminate	You need the hardest surface with a resilient fibre core, FPC and multilayer work	Priced above UV-coated; check flatness on wide panels
Phenolic	Sub-0.10 mm holes or dimensional stability is the binding constraint	Reported to wear drill bits faster — factor bit cost into the comparison
Kraft paper	You need the base-material view — kraft is what phenolic boards are made from, and it doubles as lamination press packing	A base sheet, not a finished drill-exit board — exit specs come from the cured phenolic grade
2.7 mm coated wood	The stack needs more exit body than the 2.0/2.5 mm grades give, with a coated face for consistent breakout	Gauge code alone is not a spec — quote density and hardness together with the thickness

Whatever family you pick, three physical properties decide whether it behaves: flatness, thickness tolerance and internal bond strength. They are covered in how to choose a backup board, together with the density bands used on this site.

Verifying a Selection on Your Own Stack

A selection matrix narrows the field; it does not close the question. The only binding specification is the one measured under your own process. Run these six checks before committing a grade to production.

• Record your smallest hole diameter and the drill bit geometry used for it
• Confirm layer count and total stack height, so thickness and flatness requirements are explicit
• Measure baseline exit burr on your current board — without a baseline the trial has no reference
• Check hole-position capability (CPK or equivalent) across a full panel, not a coupon
• Log drill bit wear and hit count at change-out, so bit cost enters the comparison
• Record plant humidity and storage conditions, and note any warping at the panel edges

Run the trial with the same drill programme and parameters for both boards. Changing the board and the parameters at the same time produces a result you cannot attribute — see multilayer drilling for parameter interactions that commonly confound a board comparison.

Entry-Side Notes for Micro-Hole Work

At micro-hole diameters the entry layer stops behaving like a protective cover and starts behaving like part of the tooling. The entry sheet does three jobs at once — it guides the bit at touchdown, takes the first part of the heat cycle, and clears the chip away from the cut — and as the diameter falls those three stop scaling together: a thin bit has less stiffness to be guided with and less mass to carry heat away, so the margin between working and not working narrows in all three jobs at once.

Two consequences matter for selection. First, a symptom appears suddenly rather than gradually — the process crosses a threshold where one of the three jobs stops being satisfied. Second, the fix is usually a trade rather than an improvement: a harder sheet helps guidance and can hurt chip clearance, and a more lubricated surface helps the friction side and puts guidance back under pressure. The choice is between a rigid, hard layer that stabilises the entry (plain hard aluminium) and a layer that manages friction at the point of contact (lubricated or coated), read against which of the three jobs is failing.

Where the entry layer stops helping

Three cases account for most of the time lost on entry-material trials that do not converge:

• The bit is past its usable life. A worn edge rubs rather than cuts, and no entry material makes a dull bit sharp — entry heat and wall condition respond to bit condition before they respond to the sheet.
• The heat is arriving from the stack, not the entry. A deep stack accumulates heat through the cut; the entry sheet changes the first part of the cycle, not what the laminate does with the rest.
• The symptom is at the exit. Burr and breakout are governed by the backup layer and the exit condition — work the symptom through the backup side rather than re-specifying the entry layer.

A fourth case is a reporting error: breakage attributed to the entry material because it was the last thing changed. Thin bits break for reasons that live in the program, the stack and tool management. When verifying an entry change, hold the bit, the parameters, the panel lot and the stack position, record where in the bit’s life the measurement was taken, and compare rates rather than counts from runs of different size.

Run the Numbers with the Selectors

If you would rather start from a returned grade than read a matrix, three calculators on this site take the same inputs used above and return a starting point:

• PCB backup board selector — returns a starting grade from smallest hole, layer count and burr target
• PCB entry board selector — returns an entry board direction from hole size and board type
• PCB melamine backup grade selector — narrows within the melamine family once you have decided to use it

Supporting calculations that feed the same decision: stack height, hole aspect ratio, exit burr and drill tool life.

Selection FAQ

References

• An independent industry drilling study of seven generic backup-board material types at three hole diameters (no public URL)
• How to choose the right PCB backup board
• Exit burr control