Phenolic Entry Board

The one thing to get right

Buy it for rigidity, not for heat. A laminate entry sheet trades thermal conductivity away in exchange for positional stability, and that trade is only worth making when position is what is failing. If your entry-side symptom is nail head, white ring or bit wander from heat, the answer is inside the aluminium class, not in a different material family.

Construction

The sheet is a laminate of paper impregnated with thermosetting phenolic resin, consolidated under heat and pressure into a rigid board. Like the backup-side PHE grade, the resin runs through the thickness rather than sitting on a face, so the sheet behaves uniformly and holds its dimensions well. It is a dielectric material rather than a metal, which is what sets up the trade described below.

In the stack it is the first thing the bit meets. Its job in a standard stack would be guidance; in a precision stack it is guidance plus a contribution to where the hole ends up, because a rigid sheet that does not move or flex also does not let the bit wander in the first revolutions.

What it contributes at the entry

Three properties, in order of how often they decide the selection:

• Rigidity. A stiff sheet resists the small deflections that let a fine bit start off-axis. This is the property the material is bought for, and it matters most as the hole diameter falls.
• Dimensional stability. The sheet holds its shape across the run, so the entry condition at the last panel resembles the entry condition at the first. That is a repeatability contribution rather than a peak-performance one.
• A hard, uniform surface. The bit starts against a consistent surface across the whole sheet, which removes one source of variation from the entry side.

What it does not contribute is heat removal. That limitation is real and should be planned for rather than discovered: if the entry-side thermal load is high, the heat that a metal sheet would have carried away has to be managed somewhere else — by the parameters, by the bit, or by the backup side of the stack.

Against an aluminium entry sheet

This is the comparison that decides most entry-side specifications, and it comes down to which symptom is limiting.

Factor	Phenolic entry board	Aluminium entry sheet
Primary contribution	Positional stability at the entry through rigidity	Bit guidance plus a high-conductivity path for entry heat
Thermal behaviour	Lower conductivity — the heat has to be managed elsewhere in the stack	Conducts heat away from the entry point effectively
Dimensional behaviour	Rigid laminate, stable across the run	Full-hard metal sheet, dimensionally stable and flat
Gauge basis	Set per job against the smallest hole and the hole density	Published entry gauge of 0.18 ±0.015 mm in the AA1100 H18 sheet
Leading use	Very small holes where the entry side is part of the accuracy budget	Mainstream single, double-sided and multilayer drilling

Read the table as a symptom map rather than a ranking. Entry heat, nail head and a white ring at the hole mouth point at the metal side; position drift at the smallest diameter points at the laminate. The process reasoning behind the second case is on the drilling defect diagnosis order, and the entry-side heat case is covered under coated aluminium entry materials; the micro-hole side of that choice is worked through in the micro-hole entry board guide.

Choosing the gauge

Gauge sets how much of a rigid body the bit has to pass through before it reaches the panel, so it is chosen against the job rather than against the drill diameter alone. Two rules hold:

• Match the gauge to the smallest hole and the density together. A dense pattern of mid-size holes puts a different demand on the sheet than a sparse pattern of small ones, and the two together decide the gauge.
• Treat the gauge as a specification with a tolerance. The tolerance is part of what the stack height depends on, so it belongs on the drawing and on the acknowledgement, not just in the description.

The entry board thickness calculator works the gauge from the stack inputs, and the panel hole density calculator gives the density side a number. Confirm the supply and tolerance of the specific gauge you settle on against the current data sheet before it goes into a program.

Pairing across the stack

A rigid entry sheet is only half of a precision stack, and pairing it with a rigid backup material is the right move only when position is the governing constraint at both ends of the hole.

• Both ends positional. Phenolic entry with a rigid backup side, in IC substrate and precision work where the smallest hole leads the decision.
• Entry positional, exit thermal. A rigid entry sheet with a lubricated backup board or a sealed face, where the exit side is limited by heat or moisture rather than by position.
• Entry thermal. Nothing in this family — go back to the aluminium class and work the gauge and the coating instead.

Where the stack is also back-drilled, the entry layer is specified with the second pass rather than independently of it, and the back-drilling selection guide covers that case. Send the layer count, the smallest hole, the stack and the format, and the pairing can be confirmed as a stack rather than as two separate parts.

Questions buyers ask