Entry Material: 5 Rules for Hole Quality

Entry material is the sheet that sits on top of the panel when it is drilled. It looks like a consumable detail, and it decides a surprising amount about the quality of the hole: it cushions the drill as it enters, it holds the copper down at the surface and it takes some of the heat out of the tool.

The choice is usually made once and then left alone for years, which is a mistake. As tools get smaller and hole counts rise, the material that suited a two layer board with a generous annular ring may be the reason a fine line job is producing rough hole walls.

What Entry Material Does in the Stack

When the drill touches the stack, the first material it meets is the entry sheet rather than the copper. A rigid sheet supports the tool tip and reduces the whip that a long, thin drill experiences, while the top copper is pressed against the laminate instead of being lifted.

The sheet also absorbs some of the heat generated at the point of cut and carries debris away from the hole as the flutes lift it. Both functions depend on the material and on how well it is held against the panel, which is why a loose sheet performs worse than a clamped one.

Thickness, Hardness and the Surface

A thick sheet supports the tool better and is preferred for small drills, while a thin sheet is used where the stack height matters or where the cost per panel dominates. The surface finish of the sheet also matters, because a rough sheet leaves debris that can be dragged into the hole.

Aluminium, phenolic and coated paper products are all used, and each has a different effect on the hole wall. The choice should follow the tool diameter and the panel rather than a general preference, because the same material that behaves well on one job can mark the surface on another.

Hole Wall Quality and the Cut

The quality of the hole wall is judged after plating, when the copper has covered it. Roughness, resin smear and fibre protrusion all start as a drilling result, and the entry material influences how clean the cut is at the point where the drill enters the copper.

Where the walls are rough across every hole, the tool condition and the stack are the first suspects, and where the roughness changes with position on the panel the machine or the fixture is more likely. Our backup material notes explain how the bottom of the stack contributes to the same result.

Drill Wear and the Heat Behind It

Entry material that is too hard for the tool increases edge wear, and one that is too soft allows the drill to wander at the surface. Either way the wear is not uniform, so the tool is replaced earlier than it needs to be and the hole count per tool falls.

The temperature at the cutting edge is the other factor. Excessive heat dulls the edge quickly and can leave a smear that has to be removed by desmear, so the entry material is part of the same thermal story as the spindle speed and the feed.

Burr Formation at the Copper Surface

A burr is the ridge of copper pushed up around the hole as the drill breaks through the surface. It is unsightly, it can lift the mask later, and it interferes with a pad that has to take a probe or a solder joint.

The entry sheet presses the copper down while the cut is made, which is the main reason it reduces the burr. Where a burr still appears, the sheet has been used too many times, the stack is not clamped evenly, or the tool is dull. Our drill accuracy notes cover the machine side of that check.

Backup Material and Its Own Rules

The backup sheet at the bottom of the stack has a different job. It supports the laminate, prevents exit burrs and reduces the chance of the drill pushing debris into the hole from below. Its selection follows the same logic as the entry sheet but with the opposite surface requirement.

The two should be specified together rather than separately, since a stack with a well chosen top and a poor bottom will still produce a burr on the exit side and a rough wall at the bottom of the hole. The drill chart is the natural place to record both.

Entry Material for Microvias and Small Tools

Small tools are more flexible and much more sensitive to the surface they enter. For laser drilled microvias the entry material also has to be compatible with the laser, because a sheet that absorbs the beam differently from the copper will change the energy needed for the same hole.

Where a shop drills both through holes and small vias on the same machine, the entry material should be selected for the smallest tool in the program. Selecting for the average tool is how a program with a handful of small holes ends up with a rough wall on every panel.

Reuse, Contamination and Storage

Entry sheets are sometimes reused for a second job, and the practice is defensible only where the sheet was removed clean and flat. A used sheet is already perforated, so it no longer supports the tool, and it carries debris from the previous panel into the next one.

Storage matters as much. A sheet that sits in a humid room absorbs moisture and behaves differently, and a dusty sheet transfers that dust into the hole. Our drill chart notes describe how the stack is specified and recorded for each job.

Cost, Consumption and the Trade Off

The material is a consumable, and the cost per panel looks trivial until it is compared with the cost of a rough wall, a rejected batch or an extra desmear step. The comparison should be made on total cost rather than on the purchase price of the sheet.

Consumption also gives a useful check on the process. A shop that suddenly uses twice as much entry material for the same job has probably started scrapping sheets or running a different stack, and both are worth understanding.

Records and First Panel Checks

The record should name the entry and backup materials with their supplier and thickness, together with the tool list they were used with. That combination is what makes a hole quality problem traceable to a change rather than to a memory.

The first panel check should include a burr inspection at the surface and a hole wall check after plating. Our annular ring notes cover the measurements that sit alongside them, and the IPC documentation standards describe the records expected for the stack.

FAQ

Does entry material matter on a thick panel? It matters more, because a thick stack needs more support to keep the tool straight. The sheet is the first thing the drill sees, and on a deep hole any wandering at the surface is amplified by the time the tip reaches the bottom.

Can the sheet be omitted to save time? It can be, and the result is a burr on every hole and a rougher wall, both of which are paid for later. The saving is a fraction of a minute per panel against a cost that is counted in rework.

Is one material enough for the whole shop? A single product may cover most jobs, and it is worth reviewing when the tool list changes. A shop that moves towards smaller holes or heavier copper should re-check the choice rather than continue with what has always been used.