Counterbore vs. Countersink: Which Non-Plated Mounting Hole Is Better for Circuit Boards

If you’ve worked on DIY woodworking projects, you might have had to compare counterbore vs. countersink when handling screws. 

You can find the same terms in PCBs. And they refer to mounting holes, just like in woodworking. Which of these non-plated mounting holes is better? Let’s find out.

What Is a Counterbore?

A counterbore is a cylindrical hole with a flat bottom. The walls of the hole drop down straight with no angle. So the fasteners that fit in counterbores have zero-angled sides with a flat bottom.

A circuit board with a counterbore mounting hole

A circuit board with a counterbore mounting hole

The device used to make these holes is also known as a counterbore. So the term is interchangeable to refer to both.

What Is a Countersink?

Countersinks are cone-shaped holes that match conical screw heads. So like counterbores, the slot is enough to fit flat-head screws perfectly inside while flush or slightly sunk.

But the primary defining feature of these holes is their angled, conical shape. The standard angles are 60°, 82°, 90°, and 100°.

A printed circuit board with a countersink mounting hole

A printed circuit board with a countersink mounting hole

Others like 110° and 120° are not as popular, but you can get countersinks and their accompanying screws in these angles. The most popular ones are 82°, 90°, and 100°.

If you mix up the angles, you can compromise the strength of the PCB. Why? Because the head will sit on a tiny portion of the wall instead of the entire conical-shaped hole.

Counterbore vs. Countersink: Shape and Angle

Both counterbores and countersinks serve a similar purpose (secure mounting). But they have some differences. The most obvious one is the shape and angle.

The cylindrical form of counterbores allows screws to sit flat at the bottom. This coaxial hole enlargement design can fit washers, which sit under the screw as you fasten.

The width is the same at the top and bottom. So the washer can slide inside. Washers enable the screw to push the PCB evenly from all sides for better fastening.

A set of steel washers for fasteners

A set of steel washers for fasteners

But remember, fitting a washer can make the screw heat sit above the PCB surface. So only use it if the head is thin or the hole is deep enough to accommodate both.

However, counterbores are generally deeper than countersinks. So they are usually enough to keep the head flush or slightly under with both inside.

On the other hand, the conical form of countersinks means they have a wider top and narrow bottom. So you cannot fit washers to spread the pushing force from the fastener.

Countersink mounting hole drilling (note the wide top and narrow bottom)

Countersink mounting hole drilling (note the wide top and narrow bottom)

Also, countersinks are generally not as deep as counterbores. But their depth varies depending on the angle.

Counterbore vs. Countersink: Symbol

When drawing or designing the PCB, you can indicate the presence and position of either drill hole using a symbol.

Counterbores have square shoulders due to non-angled sides and a flat bottom when looking at them from the sides. So the counterbore symbol resembles a boxy letter U. Some refer to it as an unfinished square (without the top part).

But conical countersink holes resemble the letter V. Therefore, their countersink symbol is V-shaped.

Counterbore vs. Countersink: Strength

For the same screw size, counterbores have a higher holding strength than countersinks. This difference arises from the shape of the hole.

The force applied by a flat-shaped screw on the flat-bottomed hole is parallel to the hole axis. And if the fastener head is narrower than the flat surface, you can use a washer to cover the whole area.

This layer will spread the force evenly across the base to secure the board firmly.

But countersinks have conical walls, meaning the mounting force has an angular spread. So instead of pushing the print circuit board down, countersink screws push the surface down and out.

This angular force cannot match the holding strength provided by counterbore screws.

Counterbore vs. Countersink: Drill Bits

You can use Forstner bits when counterboring because they leave flat bottoms and clean finishes. The second best option is a spade bit because it does not create finishes as neat as Forstner bits.

And the process should start with drilling the counterbore, then the pilot hole.

A counterboring tool

A counterboring tool

But countersinks have a wide diameter at the top that gradually narrows downward. So you can use cross-hole, flip-style, fluted, or rocker-pro drill bits.

However, drilling countersinks is trickier than making counterbores. First, you must drill the pilot hole to guide the bit. But more importantly, ensure you match the countersink drill bit size to the screw.

If the countersink is broader than the screw head, it will swallow the fastener and reduce the mounting force.

And if too small, the head won’t fit inside the countersink’s wall surface. Forcing it in will only worsen the situation because you can break the material around, making the hole larger.

Different drill bits for making counterbore and countersink mounting holes

Different drill bits for making counterbore and countersink mounting holes

Therefore, measurements are critical when drilling countersinks. Vital things to check are the cone angle, drill bit depth, countersink size, and center stop location.

We recommend trying these settings on a piece of scrap PCB, wood, or plastic before drilling your PCB board. 

This testing should inform any required countersink tool adjustments to avoid damaging the circuit board.

Counterbore vs. Countersink: Applications

There are two factors to consider when determining the application of counterbores and countersinks.

  • Holding/connection strength
  • Neatness (smooth, perfect fitting)

Counterbores are not as neat as countersinks, especially on printed circuit boards. Also, they consume more space on the surface. 

The looks might not matter when dealing with wood or metal, but they might count in electronics.

An LED circuit board with two mounting holes

An LED circuit board with two mounting holes

But these circular holes create stronger connections, leading to firm holding when mounting the board.

So electronics that don’t need tight fitting, such as mobile phones and wearable devices, can have conical-shaped holes.

 But electronic machines that vibrate a lot need tight-fitted PCBs.

For instance, washing machines vibrate when washing and spinning. 

Using countersinks in these devices can weaken the joints gradually, cutting parts of the material. So counterbores are better for such machines.

A washing machine circuit board under repair

A washing machine circuit board under repair

Another factor to consider is space. As stated earlier, counterbores take up more real estate on PCBs. So they are not the best for compact devices. Use countersinking screws instead.

Which Screw Hole Is Better?

Although countersinks are better for portable devices, manufacturers prefer counterbores to avoid unnecessary PCB damage.

 Remember, the conical-shaped holes spread the mounting force down and out, which can damage the walls.

On top of that, the drilling process requires more work and resources, which can increase the manufacturing cost and time.

 The complex drilling process can also damage the board.

Two mounting holes in a PCB

Two mounting holes in a PCB

And with counterboring, the screws can double up as current return paths to drain excess current. Add to these benefits the secure mounting strength counterbores provide, and we have a winner.

Wrap Up

Although the counterbore method has emerged victorious, it does not mean you should ignore countersinks. 

You should consider using countersunk screws in tiny, portable devices if the mounting factors favor them. But generally, the counterboring method is better. 

That’s it for this article. Comment below if you found the article insightful. We want to hear your feedback. Cheers!