PCB Drill and Hole Quality

A drilled hole is the foundation of every plated connection, and its quality determines whether the plating adheres and whether the barrel survives thermal cycling. The defects are small and the consequences are large.

The process variables are the drill condition, the entry and exit material, the stack up and the feed and speed. Each produces a characteristic defect that can be seen in a cross section.

Drill Wear and Hole Size

A worn drill cuts a smaller hole and produces a rougher wall, and it also generates more heat. The hole size drifts over the life of the bit, which is why the drill is replaced on a hit count rather than on inspection.

The finished hole size after plating depends on the drill size and on the plating thickness, so both are specified. A hole that is at the low limit after plating may not accept the component lead.

Burrs and Entry Damage

A burr is a ridge of copper left at the entry or the exit of the hole, and it interferes with the pad and with the component. The entry material and the drill condition determine how much is produced.

The burr removal step should be defined, since a burr that is left in place is a defect that appears later as a poor joint or as an assembly problem.

Smear and Resin Transfer

During drilling, the resin can heat and smear across the copper of an inner layer, which prevents the plating from bonding to it. The result is an intermittent connection that appears as an open after thermal cycling.

Smear is controlled by the drill parameters, the stack up and the desmear process. A cross section shows the smear as a dark line between the copper and the plating.

Wall Roughness and Nail Heading

A rough wall makes the plating uneven and can trap chemistry, while nail heading describes the copper of an inner layer being pushed into the hole by the drill. Both reduce the reliability of the barrel.

Nail heading is a sign of a drill that is too dull or a feed that is too high, and it is visible in a section as a distorted inner layer.

Desmear and Chemistry

The desmear step removes the resin smear and prepares the wall for the plating, and it has to be matched to the resin system. An under treated wall leaves smear, while an over treated one roughens the resin excessively.

The control is a coupon that is sectioned and examined, since the result is not visible from outside.

Back Drilling and Depth

Where a stub is removed as described for back drilling, the depth control is the critical parameter and the hole quality requirements are the same as for a plated hole.

The operation has to leave the remaining barrel clean and intact, which is a requirement on the drilling rather than on the design.

Aspect Ratio

The ratio of board thickness to hole diameter determines how difficult the plating and the chemistry are. A high ratio requires better control across the whole process, and it limits which fabricators can build the board.

The ratio should be kept within the demonstrated capability of the supplier, as described for supplier qualification.

Verification

The verification is a cross section on a coupon, measured for plating thickness, wall roughness, smear and the inner layer connection. The coupon should travel with the panel.

Where a defect is found, the section shows the cause, which is why the coupon is worth more than a dimensional check.

Records

The records should include the drill hit count, the parameters and the section results. A drift in hole quality is usually visible in the hit count before it is visible in the product.

They belong with the fabrication evidence described for manufacturing tolerances.

Process Control and Verification

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. 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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

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 tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

FAQ

How often should a drill be replaced? On a hit count set by the manufacturer and adjusted by the stack up and the material.

What is the most serious hole defect? Smear on an inner layer, because it produces an intermittent connection that is hard to detect.

Can hole quality be inspected without sectioning? Not directly, which is why a coupon is built and sectioned with the panel.

Does the stack up affect the drilling? It does, because different resin systems behave differently at the drill temperature.