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Everything You Need to Know about PCB Drilling

People believed that bacteria expelling carbon dioxide were responsible for the holes in Swiss cheese for almost a century. This theory was replaced by another one that claimed the Swiss cheese holes are caused by small particles that weaken the curd, allowing the formation of gases. Whichever concept is correct, the truth is that there are numerous holes that make up almost three-quarters of Swiss cheese. 

Printed circuit boards (PCBs) also contain multiple holes and have been nicknamed “Swiss-cheesed.” PCB holes are essential in mounting components onto bare boards and interconnecting the electrical nets between different PCB layers. It would be best if you professionally drilled these holes to serve their purposes well. This article will take a dip dive into the PCB drilling process to offer a comprehensive understanding of the PCB manufacturing process.

What is PCB Drilling?

PCB drilling is the process of creating holes in bare circuit boards. This process is carried out to facilitate components placement and the interconnection of electrical nets between various PCB layers. 

Drilling is the most expensive and time-consuming step in PCB manufacturing. Furthermore, you must carefully do it because even a slight mistake can cause a severe impact on the PCB quality. Therefore, designers must always consider the PCB manufacturer’s capacities before making an order. 

The PCB drilling process acts as the foundation for vias and the interconnectivity of different layers. The miniaturization of electronic devices, like TVs and smartphones, has initiated the shift from stationary to portable PCB drilling. High-quality micromachining is needed to minimize the sizes of electronics. The drilling process plays an important role in achieving device miniaturization. 

PCB Drilling Methods

Drilling is one of the basic and most frequently performed processes in PCB manufacturing. There are many methods of creating micro-holes, such as conventional punches and dies, electrical discharge machining (EDM), vibration drilling, laser machining, and others.    

We use electrical discharge machining in graphite-epoxy laminates since the graphite fibers are electrically conductive. Since the process generates high temperatures and currents that can lead to melting of the composite surface, thermal expansion of the graphite fibers, and debonding between fibers and the matrix, the production rate of the electrical discharge machining method is a bit slow. 

Vibration drilling is a branch of vibration cutting that fundamentally differs from conventional drilling. The latter is a normal cutting process, while the first is a pulsed intermittent cutting process facilitated by piezoelectric crystal oscillators. Vibration thrust and torque are smaller than standard values by 20-30% under the same drilling parameters. While vibration drills prevent burr and enhance the cutting stability, there are still additional issues it causes.   

Laser machining is a drilling method that eliminates delamination. The technique was developed as a solution to small hole sizes, but it also has its shortcomings. A printed circuit board consists of three basic materials – copper, glass fiber, and resin – that are also made up of different materials and optical properties. This makes it challenging for a laser beam to neatly and efficiently cut through a circuit board. Laser beam holes in standard boards are sometimes thin. This is why laser machining is mainly used to create small blind and buried vias. 

Mechanical drilling is suitable for creating through-holes in motor protection circuit breakers (PCBs) due to the need for improved accuracy, low-temperature, and deformation. 

Automated drilling machines make holes in PCBs by controlling the whole process using a computer. When you need to drill several holes of different sizes and dimensions, CNC machines are the most effective automated drilling machines you can use to save time and production expenses. 

For registration holes that require further accurate drilling on the centers of their inner layers, you can use an x-ray drill. This method is also useful when via holes are connected to copper layers; hence, you should drill the holes on the leaded parts.

Different Holes Drilled in a PCB

A bare circuit board contains several fabricated properties, such as cut-outs, slots, and the end product’s overall shape. The biggest number of these properties are the holes that are drilled into the board. The role of these holes can be divided into three classes:

1. Via Holes 

Small holes plated with metal can be applied to transmit electrical signals, power, and ground through the PCB layers. These holes are called vias, and there are various types based on the requirements. 

Through-hole. A Through-hole is a standard via that extends from the upper to the lower part of the board. These vias link traces or planes to various layers. 

Buried. A buried via starts and ends on the inner PCB layers without extending to the surface layers. They consume less space than through-hole vias, making them suitable for high-density interconnect (HDI) boards. Nevertheless, buried vias are also costly to create.  

Blind. These vias start from surface layers but only percolate through halfway the board. Earlier on, we mentioned that blind vias are expensive to create but create adequate space for routing. Their shorter barrel can enhance the signal quality of high-speed communication lines. 

Micro. Micro vias contain smaller holes than others because they are made using laser machines. They are typically two layers deep because of the challenges of plating smaller holes. Micro vias are suitable for HDI boards or high pin-count fine-pitch gadgets, like BGAs that require insertable escape vias. 

2. Component Holes 

Though surface mount parts are applied mostly inactive and discrete PCB components, most of them still integrate well with through-hole packages. These components include connectors, switches, and other mechanical components that require strong mounting guaranteed by a through-hole package. Besides, these packages are perfect for power components, like big resistors, capacitors, op-amps, and voltage regulators because of their current and heat transmission capacities.

3. Mechanical Holes

A PCB normally contains mechanical objects, like brackets, connectors, and fans, attached to it. These objects need holes for mounting purposes. While these holes are rarely plated with metal, they can be if the mounted item requires an electrical connection to the PCB, like a chassis ground.  

In some instances, mounting holes are used to conduct heat away from hot inner layers. They can also be used to streamline the PCB manufacturing process. Mechanical holes are sometimes called “tooling holes,” primarily when used to align the PCB to an automated manufacturing machine.

Aspects to Consider in PCB Drilling

There are two primary aspects you should consider during the PCB drilling process.

Aspect Ratio

Aspect ratio (AR) is the feature that defines the PCB reliability.

In a through-hole PCB, AR is the ratio between the board thickness and the drilled holes’ diameter. For micro vias, it is the ratio between the hole depth and the diameter of the drilled hole. 

AR defines the ability to deposit copper inside the vias quickly. The copper plating of the inner parts of the holes becomes challenging when the diameter is minimized, and the depth increased. This calls for a copper plating bath with a greater throwing capacity to jet liquid into the small holes and deposit copper. 

To get the AR of a through-hole, you divide the board thickness by the diameter of the via.

Because micro vias do not penetrate through the whole PCB, you calculate their AR by dividing the drill depth by the diameter of the via. 

Drill-to-Copper

The Drill-to-copper aspect is the land clearance between the via edge and the adjacent copper object. The object can be a copper trace or any other active copper region. The drill-to-copper aspect is a significant factor because even a minor deviation can cause circuit disruptions.

How Precise PCB Drilling Help Minimize Costs

Drilling costs will be less if the operation is done at optimum speed. During drilling, each phase should be carried out hand in hand. By making the vias faster, the rate must also be regulated to avoid issues of tool breakage. By controlling the time you spend on your PCB layout, you will have minimized the manufacturing expenses.

Common Drilling Issues and their Solutions

It is essential to attach a Drill table, also called a Hole Schedule, to your PCB quote. This table lists all the different types of holes on the PCB, their drill diameters, and whether they are Electroplated through holes (PTH) or Non-electroplated through holes (NPTH). 

In the case of Multilayer boards, especially where Blind and buried vias are used, holes in different layers may overlap on top of each other. It is advisable to remove the overlaps where possible. 

As per our manufacturing process, we suggest using the following guidelines for holes:

  1. Hole diameter tolerance: PTH = ±076mm, NPTH = ±0.05mm.
  2. Through-hole copper thickness: minimum 18um, average 20um.
  3. Solder mask thickness: 10 ~ 50 um, minimum 5um on trace corner.
  4. Bow and Twist≤75%.

Drilling Validation Checks

This is what you should check to validate your PCB drilling process:

  • The AR should be kept low to prevent drill wear.
  • The higher the number of separate drill sizes in a PCB design, the higher the drill bits you have to implement. Instead, you can minimize drill sizes to reduce drilling time. 
  • Verify whether the non-plated drills have connections. 
  • Check the drill count/size between the drill file and fabrication print.
  • Confirm whether the drill type is a PTH or NPTH. 
  • Check whether your board has holes of less than 0.007. If you find some, you can space them apart or, if possible, delete some of them. 
  • Check whether mouse bits are available – perforated breakaway tabs. They form a line of small holes in a printed circuit board, similar to the holes surrounding a coupon. Mouse bits provide a better grip. 
  • Confirm whether the drills and other related copper layer features are placed outside the PCB profile.
  • Allow a minimum distance of 0.01 between the hole edge and the PCB profile.
  • Confirm whether the via size should be reduced to meet the minimum AR requirement after considering the drill tolerance.
  • Plated holes should have a tolerance of less than +/- 0.002 and +/- 0.001 tolerance for non-plated holes. 
  • Confirm whether the fab drawing for arcs display NPTH drill or cut-out points but lacking the drill file. 
  • Provide the via sizes that should be filled.
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