Backup Material Selection and Drilling Hole Quality Guide

Backup material sits underneath the panel in the drilling stack and does several jobs at once: it supports the copper as the drill exits, it dissipates heat, and it helps to clear the chips from the hole. It is a consumable that costs very little compared with a drill bit or a scrapped panel, and choosing the wrong one is one of the most common causes of exit burrs and rough hole walls.

What Backup Material Does

As the drill breaks through the bottom of the panel, the copper is unsupported and tends to tear outward. A backup board provides a support layer that holds the copper in place until the drill has passed completely through it.

The board also absorbs heat from the drill and helps to carry the chips away from the cutting edge. Both effects reduce the temperature at the exit, which is where the drill is most likely to burn the resin and leave a rough wall.

Types of Backup Board

The traditional material is a high density fibreboard or a phenolic laminate, both of which are dimensionally stable and inexpensive. Phenolic boards are harder and give better support, while fibreboard is softer and easier on the drill but wears faster.

Composite backup materials with an aluminium facing or a resin coated surface are used where the hole quality requirement is high. The choice depends on the hole size, the panel thickness and the acceptable burr height. Our hole types guide describes the features the drilling has to produce.

Entry Material and Its Role

Entry material sits on top of the panel and does a related job. It stops the drill from wandering as it starts, it lubricates the cutting edge, and it prevents the top surface copper from being lifted as the drill enters.

The classic entry material is an aluminium foil laminated to a paper or a phenolic carrier. The aluminium lubricates and conducts heat away, while the carrier holds the foil flat against the panel and can be removed after drilling without leaving residue.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/pcb-assembly-tests.webp" alt="Microsection of a drilled hole wall after deburring” />

Burr Formation and Control

A burr is a raised lip of copper around the exit of the hole. It forms when the drill pushes the copper outward instead of cutting it, and it is worse with a worn bit, a high feed rate or a soft backup material.

The burr has to be removed before plating, because a folded burr traps chemistry and can create a void in the barrel. The mechanical deburring step removes the burr, but it also removes some copper from the surface, which is why prevention is better than removal. Our drill bit guide covers the wear mechanisms that make burrs more likely.

Heat Dissipation in the Stack

Drilling generates a large amount of heat in a very small volume. If the heat is not carried away, the resin softens and smears over the copper of the inner layers, which produces a smear that later causes an intermittent connection.

The stack materials are part of the thermal path. A dense backup board conducts heat away from the exit side, and an aluminium entry foil helps on the entry side. Both reduce the peak temperature at the cutting edge and extend the life of the bit.

Drill Wander and Hole Position

A drill that is not held straight at the start will wander, and the hole will be out of position at the exit even if it started correctly. Entry material with a hard, smooth surface helps to keep the drill on axis.

Backup material affects the same problem from the other end, because a soft or uneven board lets the drill deflect as it exits. Registration is therefore a function of the whole stack rather than of the drilling machine alone. Our hole copper guide explains how the resulting wall quality affects plating.

Selecting the Right Combination

The selection starts from the hole size and the panel thickness. A small hole in a thick panel needs a hard entry surface and a backup board that supports the exit without deflecting, while a large hole in a thin panel is more forgiving.

The number of holes per stack also matters, because the backup board degrades as it is drilled. A board that is reused for too many stacks stops providing support and starts producing burrs, even though it looks intact.

Symptoms of the Wrong Backup

The symptoms are exit burrs, a rough or smeared hole wall, a nail head or a lifted pad on the exit side of an inner layer, and a hole that is out of position at the bottom of the panel. Each of them is visible in a microsection.

A single stack drilled with two different backup materials, and then sectioned, is a convincing way to establish the difference. Our quality documentation describes how these conditions are classified at gopcb.

Process Control Points

The controls are the backup and entry material specification, the number of holes and stacks per sheet, the drill parameters, the deburring process and the inspection of the finished hole wall.

Each of those is recorded per job, and the material usage should be tracked rather than left to the operator. A stack that is allowed to run past its useful life produces defects that are difficult to trace back to the consumable.

Process Control and Verification

On a design of this kind, entry material is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Process Control and Verification

On a design of this kind, entry material is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

FAQ

Why does a burr form at the hole exit? Because the copper is unsupported as the drill breaks through. A harder backup material supports the copper and reduces the burr, and a sharp drill also helps.

Can backup boards be reused? They can be used for a limited number of stacks, and the limit should be defined. Beyond it the board stops supporting the exit and burrs return even though it looks intact.

What does entry material do that backup cannot? It controls the start of the hole rather than the exit. It stops the drill wandering, lubricates the cutting edge and protects the top surface copper.