Drill Entry Material And Its Effect On Holes

Drilling a hole through a stack of panels requires a sheet on top of the stack and a sheet beneath it. The top sheet is the entry material and the bottom one is the backup or exit material, and both change the hole that results far more than their cost suggests.

This article covers what each material does, how the choice affects burrs, smear and bit life, and how it is specified.

What The Entry Material Does

The entry material sits directly on the top copper of the first panel. It performs three functions. It lubricates and cools the cutting edge at the point where the bit is hottest and most loaded, which reduces the temperature reached by the resin. It supports the copper foil so that the bit does not lift or tear it as it enters. And it absorbs the initial shock of contact, which would otherwise be transmitted into the laminate and the foil.

The result is a cleaner entry hole with less burr and less resin smear at the top of the hole. Without an entry material the top of the hole is where most of the damage occurs, and that damage is in the same region where the plating will be thinnest and where a barrel crack is most likely to start. The mechanisms of the resulting defects are described under copper plating defects prevention.

Types Of Entry Material

The two common types are an aluminium foil and a resin coated or phenolic sheet. Foil is thin, inexpensive and effective at lubricating and cooling, and it is used for a single panel or a short stack. A coated sheet is thicker and provides more mechanical support and better lubrication, and it is used for taller stacks and for thicker panels where the drilling heat is greater.

The choice interacts with the bit and the laminate. A hard, brittle laminate produces more abrasive debris, which wears the bit faster, and a lubricating entry material reduces that wear and extends the life before the hole quality degrades. Because the entry material changes the wear rate, it also changes the correct hit count, so a change of entry material is a process change rather than a consumable swap. The way the count is set is described under PCB design and fabrication.

What The Backup Material Does

The backup material sits beneath the stack and supports the last panel at the point where the bit exits. Without it, the bit pushes the laminate away from the hole wall as it emerges, which produces a burr on the exit side and can delaminate the copper from the resin. It also reduces the deflection of the panel under the drilling force, which improves the position of the hole at the bottom of the stack.

The backup material has to be rigid enough to support the panel and soft enough not to dull the bit, and it is normally a phenolic board or a high density particle board. It wears as holes accumulate, so it is replaced at intervals; a backup board that is perforated from a previous run no longer supports the panel and the exit burr returns. The board also has to be flat, because a warped backup board tilts the stack.

Burrs And Their Consequences

A burr at the edge of a hole is raised copper that was not cut cleanly. On the entry side it appears around the rim; on the exit side it appears as a ring of copper pushed out. A burr matters because it is a raised feature that interferes with the plating, with the solder mask and with the solder joint, and because it indicates that the material around the hole was deformed rather than cut.

Small burrs are removed by the deburring step that follows drilling, which uses a mechanical brush or a chemical treatment. A large burr cannot be removed without removing copper, and it becomes a defect. The measurement is a visual check of a sample of holes after drilling and before plating, or a section that shows the profile of the rim. Reducing the burr is a matter of the entry and backup materials, the bit condition and the drilling parameters, and it is one of the reasons those three are controlled together.

Effect On Bit Wear

The entry material is the first thing the bit touches, so its hardness and its abrasiveness affect the cutting edge directly. A material that lubricates reduces the friction and the temperature, which slows the wear of the tungsten carbide. A material with hard inclusions, such as an unfilled phenolic with filler particles, can abrade the edge on every hit and shorten the life considerably.

The effect is measurable as a change in the number of holes drilled before the hole quality deteriorates, and it is normally established by the same experiment that sets the hit count. Because the effect is significant, substituting an entry material without repeating the test is a common cause of a sudden change in drilling quality that no other parameter explains.

Specifying And Controlling

The specification should state the entry and backup materials by type and thickness, along with the drill parameters and the hit count, because the three are one system. It should also state the conditions under which the materials are changed, including the interval for replacing the backup board, which is normally based on the number of holes rather than on appearance.

Control in production is by recording the materials used for each run and by checking the hole quality on a sample. A change of supplier for a consumable that is considered equivalent is not always equivalent in a drilling stack, and the check on the sample is what detects the difference before a panel is scrapped. The qualification of a new process combination is part of the programme described under multilayer prototype requirements.

FAQ

Can the entry material be omitted? It can be omitted on a single thin panel where the drilling load is light, and doing so usually produces more burr and more smear at the top of the hole. On a stack or a thick panel it is not advisable.

How often should the backup board be replaced? Based on the number of holes, because a board that is perforated no longer supports the exit side. The interval comes from the hole quality observed at the exit of the last panel.

Does the entry material affect the drilled hole position? It affects it indirectly, by reducing the deflection of the stack under the drilling force. A well supported stack holds position better, particularly at the bottom of the stack.