Drilling Entry Material and Burr

Drilling a printed board is a machining operation, and like any machining operation the result depends on the tool, the material being cut and what is on either side of the workpiece. Entry and backup material sits on the top and bottom of the stack, and it decides how much burr and how much damage the hole wall carries away from the drill.

Why Entry Material Is Used

The entry material holds the copper down as the drill enters, which reduces the burr that would otherwise be pushed up around the hole. It also lubricates and cools the cutting edge and helps to locate the drill as it touches the surface.

A hard entry foil behaves differently from a soft one. A rigid aluminium foil resists the cutting edge and helps to hold the surface, while a softer material conforms and cushions. Our article on laser versus mechanical drilling covers the alternative process.

Backup Material and Exit Burr

The backup board supports the laminate as the drill exits and stops the exit burr from forming. It also cleans the drill as it passes, which reduces the smear that would otherwise be dragged up the barrel.

A backup board with the wrong density leaves a burr on the last layer, and the burr is worse when the drill is worn. The material and the drill life are therefore linked, and both should be specified together.

Hole Wall Quality

The hole wall has to be clean enough for the plating to adhere. Smear, resin residues and torn glass fibres all reduce the adhesion, and the defect appears later as a barrel crack or as a void in the plating.

Drill parameters, entry material and the number of hits on a drill all affect the wall. The usual test is a microsection after desmear and plating to confirm that the barrel is sound. Our article on hole copper explains what the plating has to achieve.

Burr and Its Consequences

A burr on the entry side can short a pad to a neighbouring pad or lift a surface mount pad out of plane. A burr on the exit side interferes with the plated surface and can break away later as a loose particle inside the assembly.

The burr is often an indicator rather than a problem in itself, since the same conditions that produce a burr also produce a rough wall. Our article on fabrication notes checklist lists the requirements that should accompany the drill data.

Drill Life and Register

A drill wears at the cutting edge, and the wear increases the hole size, raises the burr and produces a rougher wall. The hit count limit should come from measurement rather than from a supplier recommendation, because the laminate and the hole size change it.

Register between the drill and the pads is a separate variable, but the two interact: a worn drill wanders more as it enters, so a marginal register becomes a defect. Our article on tooling holes and registration covers the datum side.

Aspect Ratio and Small Holes

As the hole diameter falls, the drill becomes more flexible and the chip load per flute becomes harder to control. The entry material then has to be thin and flat, and the backup has to give the drill a clean path out.

The limits of the process are set by the aspect ratio as much as by the diameter. Our article on aspect ratio explains how the plating side of the same limit is calculated.

Verification

A coupon drilled with the production stack and microsectioned is the standard verification, and it should include the smallest hole on the design. The images should be compared with the previous lot so that a slow drift is visible.

The records should tie the coupon to the drill program and the material lot, which is what allows a later problem to be traced to a specific build. Our article on microsection preparation describes the preparation method.

Checks Before Release

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

Verification and Records

The sequence of operations is part of the specification, because a different order produces a different result from the same steps. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

Points to Confirm at First Article

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

FAQ

Is entry material always necessary? It is standard for through holes in standard laminate. For very thick panels or unusual materials the trades change, and the decision should come from a microsection result.

How does a worn drill show up? As a larger hole, a heavier burr, a rougher wall and more smear. The hit count limit should be set from those measurements rather than from a general recommendation.

Can a burr be removed after drilling? It can be brushed or etched back, but the removal process has its own effect on the copper, so preventing the burr at the drill is the better approach.