The one thing to get right
Decide which entry-side symptom you are buying against. If it is nail head, white ring and a bit that wanders in the first revolutions, you are buying guidance and heat path. If it is position drift at very small diameters, you are buying rigidity and dimensional stability. Those two lead to different classes, and a supplier who quotes you the same sheet for both has not read your stack.
What the entry side has to do
The bit touches the entry board before it touches anything else, and what happens in that first fraction of the cut sets up the rest of it. Three things are being asked of the sheet:
- Guide the bit. The sheet holds the bit on axis for the first revolutions, before the panel's own material can support it. Weak guidance shows up as nail head and a white ring at the hole mouth.
- Carry heat away. The entry point is where spindle heat and cutting heat first meet the stack. A metal sheet conducts that heat away far more effectively than a polymer or paper sheet, which is why aluminium is the mainstream default.
- Stay flat and stay put. The sheet has to lie flat across the panel and hold position through the run. Any dimension change in the sheet shows up as stack-height variation, which is a position problem rather than an entry problem.
The backup board does the mirror-image job at the other end of the hole. If you are still sorting out which end of the stack a symptom belongs to, the defect diagnosis order separates them, and the coated aluminium entry board explainer covers what a coating adds.
The classes at a glance
Each of these is a material class, not a single product. Within the aluminium class the alloy and the gauge are separate decisions, and within the coated class the chemistry of the coating is a third one.
| Class | What it contributes at the entry | Reach for it when | What you accept |
|---|---|---|---|
| Plain aluminium sheet (AA1100 H18) | Bit guidance in the first revolutions plus a high-conductivity path for entry heat; full-hard temper, dimensionally stable | The mainstream case: single and multilayer work across the normal hole range | No interface layer — the sheet and the bit interact directly, so the surface finish and cleanliness of the sheet matter |
| Lubricated / coated aluminium sheet | Adds a coating layer at the bit–sheet interface, changing friction and heat behaviour at the entry point | Dense drilling patterns where heat per unit area at the entry is the limiting factor | A coating chemistry to qualify against your cleaning line, and an extra removal step to plan for |
| Phenolic paper entry board | Rigid, flat, paper-based sheet that contributes to positional stability at small diameters | Very small holes where the entry side is part of the accuracy budget rather than only a heat path | Lower thermal conductivity than a metal sheet, so the heat has to be managed elsewhere in the stack |
| Melamine-coated phenolic entry board | Rigid phenolic body with a hard fused surface presented to the bit | Where a harder entry surface is wanted on top of the phenolic body | Surface hardness and bit wear sit on the same side of the ledger — watch bit life |
| Back-drill aluminium sheet | An entry-side aluminium sheet specified for stacks that are also back-drilled | The entry layer and the back-drilling step are planned together | It is a different specification from a standard entry sheet; do not substitute one for the other without checking |
Class-level guidance only. Within the aluminium class the alloy, the temper and the gauge each move the outcome, and the coated class adds coating chemistry on top of all three.
Water-soluble against non-water-soluble
Once a coating is in the picture, the first question is not how much it helps but whether you can get it off. That is the water-soluble split, and it is decided by your process before it is decided by the drill:
- Water-soluble coating. Designed to come away in a water-based cleaning step, so it can be removed in a sequence you already run. It fits plants that want the entry-side benefit without adding a dedicated removal stage.
- Non-water-soluble coating. Remains as a film after drilling and needs its own removal route. It suits plants that already run a compatible cleaning step and would rather not have the coating leave early.
Check the chemistry against three things before the drill: your cleaning sequence, your resist system, and your disposal route. A coating that survives into a later process step is worse than no coating, and that failure is invisible on the drilling bench. The coating thickness calculator gives you a first read on how much coating the job implies, and the lubricant and coated aluminium range covers the entry-side products built on this principle.
Aluminium against phenolic
This is the comparison that most often gets decided on the wrong grounds. Aluminium wins on heat path and on the mainstream hole range; phenolic wins where the entry side has to hold geometry.
| Factor | Aluminium entry sheet | Phenolic entry board |
|---|---|---|
| Primary contribution | Guidance plus heat conduction away from the entry point | Rigidity and positional stability at the entry |
| Thermal behaviour | High conductivity relative to FR-4, so entry heat spreads and leaves | Lower conductivity; heat has to be handled elsewhere in the stack |
| Where it leads | Mainstream single, double-sided and multilayer drilling | Very small holes where position accuracy is the binding constraint |
| Material identity | Full-hard commercial-purity aluminium — an AA1100 H18 sheet is ≥99.0% aluminium with controlled residuals | Laminated phenolic resin, paper-based, with a high decomposition temperature |
| What to watch | Sheet flatness and surface cleanliness, because there is no interface layer to absorb a problem | Bit wear against the harder surface, and the thermal load moving to the backup side |
The two are not competitive substitutes so much as answers to different symptoms. If the entry-side problem is heat and nail head, move within the aluminium class — plain to coated, or a different gauge — rather than switching families. If the problem is position drift at the smallest hole, the phenolic PHE board and its phenolic entry-side counterpart are where the accuracy budget lives, and the HDI and micro-via selection guide works that case through. For the alloy and temper side of the aluminium decision, the aluminium entry alloy calculator is the quickest entry point.
Gauge, hole size and density
Gauge sets two things at once: how much guidance the sheet gives the bit before it reaches the panel, and how much heat it can carry off. Both depend on the smallest hole and the hole density read together, not on the drill diameter alone. A dense pattern of mid-size holes can put more heat per unit area into the entry sheet than a sparse pattern of smaller ones, and that is a gauge and class question rather than a diameter question.
Two practical rules hold across classes. First, treat the gauge as a specification with a tolerance, not a nominal number — our published aluminium entry gauge is 0.18 ±0.015 mm, and the tolerance is part of what you are buying. Second, re-qualify when the smallest hole changes; the gauge that held at one diameter is not automatically the gauge for the next regime down. The entry board thickness calculator and the panel hole density calculator cover the two inputs separately.
Choosing without a full trial
You can shorten the list before you spend machine time by answering four questions:
- Which symptom are you buying against? Entry heat and nail head point at the aluminium class; position drift at the smallest hole points at the phenolic class.
- What does your cleaning line already do? This decides the coated class and the coating chemistry before drilling enters the conversation.
- How dense is the pattern? Density, not diameter, is what drives entry-side heat per unit area.
- Is the stack also back-drilled? If so, the entry layer is specified with the back-drilling step, not independently of it.
Send those four answers and the class narrows to one or two candidates, at which point a short controlled trial settles it. The back-drilling material selection guide covers the case where the last question is a yes, and the engineering desk will take the four answers directly.
A side-by-side trial that produces usable data
When two candidate sheets are close, a split-panel trial settles it in one run. Keep the same backup board, the same drill programme and the same bits on both halves of the panel, and change only the entry sheet. Measure three things on each half:
- Entry and exit burr height — 30 holes per side, per product.
- Hole location accuracy — X/Y deviation sampled in each panel quadrant.
- Bit wear — hits accumulated before regrind or replacement.
Compare the data, not the brand labels. If the two sheets are equivalent on those three measurements, the decision becomes commercial; if they differ, the measurement that moved tells you which class property you were actually buying.
Questions buyers ask
What is the difference between plain and lubricated aluminium entry boards?
The aluminium sheet does the same job in both cases — it guides the bit through the first revolutions and carries heat away from the entry point. The lubricated variant adds a coating layer that changes how the bit and the sheet interact at the interface, which matters most where hole density is high and the heat per unit area is therefore high. Treat the coating as a process-dependent addition: qualify it on your own parameters rather than assuming a fixed improvement.
Water-soluble or non-water-soluble lubricant coating — which one?
The distinction is what the coating does after the cut. A water-soluble coating is designed to be removed with water in the normal cleaning sequence, while a non-water-soluble one stays as a film and needs its own removal step. That is a chemical and process-compatibility question first and a drilling question second: check it against your cleaning line, your resist system and your disposal route before you check it on the drill.
Aluminium or phenolic entry board for small holes?
Aluminium is the default where the job is to guide the bit and conduct heat away, and it is well suited to the mainstream hole range. Phenolic entry boards are paper-based and rigid, and are chosen where the entry side has to contribute to positional accuracy at very small diameters. The deciding factor is whether your limiting symptom appears at the entry side as heat and nail head or as position drift.
Does entry board gauge have to match the drill diameter?
It has to be matched to the job rather than to the individual drill. Gauge sets how much guidance the sheet gives the bit before it reaches the panel and how much heat it can carry off, and both of those depend on the smallest hole and the hole density together. Confirm the gauge and its tolerance against the current data sheet rather than scaling it from a rule of thumb.