For the last 90 years or so, it was a common belief that bacteria giving off carbon dioxide was responsible for the holes in Swiss cheese. This theory has now come into dispute by a new one, which states that the formation of the holes (or eyes) in Swiss cheese is due to tiny particulates that weaken the curd allowing gas to form. Whichever theory is correct, the result is numerous holes that take up a lot of space in the cheese.
Circuit boards are also full of holes and are often referred to as being “swiss cheesed.” While some of the holes are necessary for mounting devices onto the board, most will be used to interconnect the electrical nets between the multiple layers of the PCB. Every one of these holes will have to be drilled for fabrication on the board. A more in-depth look at this PCB drilling process will provide a comprehensive understanding of the process of building a circuit board.
The Different Holes Drilled in a Printed Circuit Board
A raw circuit board can have many fabricated properties, including cut-outs, slots, and the finished board’s overall shape. The largest number of such properties will be the holes that are drilled into the board. The purpose of these holes can be broken down into three categories:
Small holes plated with metal are used to conduct electrical signals, power, and ground through the board layers. These holes are known as vias, and they come in different types depending on what is required:
- Thru-hole: This is the standard via, and it goes through the entire board layer stackup from top to bottom. These vias can connect to traces or planes on as many internal layers as needed.
- Buried: These vias start and stop on internal board layers without extending to an external layer. This structure takes much less room than a standard thru-hole via, making them very useful in high-density interconnect (HDI) circuit boards. However, they are also much more expensive to fabricate.
- Blind: A blind via starts on an external layer but only goes partway through the board. As with buried vias, they are more expensive to fabricate, but they free up room for routing, and their shorter barrel can also help improve the signal integrity of high-speed transmission lines.
- Micro: This via has a smaller hole size than the others because it’s drilled with a laser. Microvias are usually only two layers deep due to the difficulties of plating smaller holes. These vias are necessary for HDI boards or high pin-count fine-pitch devices like BGAs that require their escape vias to be put into their solder pads.
While surface mount parts are used for most active and discrete components on circuit boards today, many parts are still preferable to use with a thru-hole package. These components are typically connectors, switches, and other mechanical parts that need the more robust mounting that a thru-hole package provides. Additionally, thru-hole packages are usually preferred for power components such as large resistors, capacitors, op-amps, and voltage regulators due to the current and heat conduct.
A circuit board will usually have mechanical features or objects attached to it, such as brackets, connectors, or fans, which require holes for mounting. Although these holes are generally not plated with metal, they can be if the mounted feature requires an electrical connection to the board, such as chassis ground. In some cases, mounting holes are used to disperse the hot components’ heat to the inner plane layers. Another purpose of mechanical holes is to aid in the manufacturing of the board. These holes are often referred to as “tooling holes.” They are used to align the board on automated manufacturing equipment.
Those are the different holes in the board that require drilling. The next step is to detail the drilling process itself.
The PCB Drilling Processes Used for Hole Fabrication
While there are many different types and styles of numerically controlled (NC) drilling machines used for printed circuit boards, the larger production versions can drill up to 30,000 holes per hour. These machines use automated optical guidance systems for precision and are equipped with multiple air-bearing spindles that can drill at speeds reaching 110,000 RPM. The drill bits themselves are handled with automated changing systems and laser-inspected to ensure the maximum quality in the drilled holes.
The standard method of drilling a thru-hole in a circuit board is to run the board with all of its layers laminated through the NC drilling machine. Later on, the holes will be plated inside and out as required to complete fabrication. This same process is used for all thru-holes, whether they are vias, components, or mounting holes. However, the process is different for vias that don’t drill through the board.
In the sequential buildup process of PCB fabrication, blind and buried vias are drilled before the full board layer stackup is laminated together. In the case of a blind or buried via that spans only two layers, the appropriate layer pair will have those vias drilled and plated before the final lamination. For those vias that traverse more layers, the board’s fabrication has to be carefully sequenced with the appropriate steps of bonding, drilling, and plating to create the vias correctly. Blind vias can also be fabricated with controlled-depth drilling where the drill only goes through the fully laminated board partway. This method of fabricating blind vias is less expensive than the sequential buildup method, but there are limitations to the hole size and how the circuitry is routed under the hole to be drilled.
Microvias are drilled with a laser, and as such, they are smaller than the holes drilled mechanically. And while their smaller size is beneficial to dense designs, it also prevents them from spanning more than two layers at a time. To connect microvias together vertically, they must be stacked together in layer pairs. Like a standard buried via, microvias are typically fabricated sequentially, and those that are buried filled with electroplated copper for connectivity between stacked vias.
How the Proper Drill Selections Can Help with the Manufacturing of Your Circuit Board
Beyond the basics of the drilling process in a circuit board, there are some ways that layout engineers can help that process with board design:
- Same drill diameter: If your board has a high number of drills on it that are all the same diameter, it may force drill bit changes in the middle of drilling. This changeover can introduce tolerance errors between holes of the size. The better practice is to change some of the drill sizes to decrease the number of drill holes for one size.
- Minimum drill diameter: Mechanical drill sizes should be six mils or larger for boards that are 62 mils thick, with the preference being for eight mils. Smaller drill sizes are more challenging to drill mechanically, plus the smaller drill aspect ratio can make them harder to plate. This issue could force the board into the more expensive laser drill process. For boards that are thicker than 62 mils, larger drill sizes will be required.
- Avoid using blind and buried vias: If blind and buried vias aren’t a requirement for high-speed circuitry or high-density design, don’t use them. The sequential buildup process for blind and buried vias makes the raw board fabs much more expensive.
There are many aspects of drilling a printed circuit board that will directly affect its cost and how well it can be manufactured. The size of the drilled holes alone has a significant impact on the price. If the holes are too small, the cost of fabricating the board can take a serious jump. On the other hand, if the holes are too large, it can complicate the assembly process, increasing assembly expenses.
At VSE, we can mitigate these kinds of problems during your circuit board’s design. Our engineers and technicians have spent the last 30 years collaborating with designers like you to understand the PCB drilling process and precisely what is needed to get their board manufacturable and cost-effective.