PCB Hole Selection to Optimize PCB Performance and Cost

Drilling holes in a PCB is not simply about making physical spaces for component leads; it’s a crucial part of ensuring the electrical functionality of the entire board. In addition to accommodating components, the process also involves creating vias—small holes that allow electrical signals to pass between different layers in a multi-layer PCB. These holes are essential for establishing reliable connections across the layers, and the precision of the drilling process directly impacts the board’s electrical performance and mechanical integrity.

With the increasing complexity of modern PCBs, which often feature multiple layers and high-density interconnections (HDI), drilling has evolved into a highly specialized and technical process. Different types of holes are required for various functions, and advanced drilling techniques are needed to meet the demands of these intricate designs. Let’s dive into the various types of PCB holes and the sophisticated drilling methods used in today’s manufacturing processes.

Types of PCB Holes

1. Through-Hole (Plated and Non-Plated Through-Holes)

A Through-Hole is the most basic and widely used type of hole in a PCB. It refers to a hole that goes completely through the board, from the top layer to the bottom layer. These holes are critical for connecting components with leads (such as resistors, capacitors, and ICs) or for making electrical connections between different layers of the PCB.

• Plated Through-Holes (PTH): These holes are coated with conductive metal along their interior walls, allowing them to serve as electrical connections between different layers of the PCB. Plated through-holes are often used to insert leaded components, such as connectors and large through-hole devices. The plating ensures that current flows from one side of the PCB to the other or between inner layers in multi-layer boards.

• Non-Plated Through-Holes (NPTH): Non-plated through-holes do not have conductive plating on the walls and are used strictly for mechanical purposes, such as mounting screws or components that do not require electrical connections between layers. They are common in applications where the hole is used only for mechanical support, like PCB standoff points, or for tooling alignment.

2. Vias

Vias are a special type of hole used specifically to make electrical connections between the layers of a multi-layer PCB. They are significantly smaller than through-holes for component leads and are not used for mounting components but purely for interlayer electrical routing.

Vias can be further classified into three main types based on their functionality and location within the PCB:

• Blind Vias: Blind vias connect one of the external layers of a PCB to one or more internal layers, but they do not extend through the entire board. This type of via is particularly useful in multilayer PCBs where designers need to connect external traces to internal layers without wasting space by extending the hole through the entire PCB. They are often used in high-density interconnect (HDI) boards where surface real estate is at a premium.

• Buried Vias: Buried vias connect internal layers without reaching the external surfaces of the PCB. This allows for complex internal layer connections while leaving the outer layers free for component placement or routing. Buried vias are only visible during the fabrication process and become “buried” within the final board once the layers are laminated together.

• Through Vias: Through vias are the most common type of via and extend from one outer layer of the PCB through all the internal layers to the opposite outer layer. While similar to plated through-holes, through vias are typically smaller and used for electrical interconnects rather than mounting components.

3. Microvias

Microvias are an advanced form of via used in HDI PCBs, with diameters generally smaller than 150 microns (0.15 mm). They are created using laser drilling and are essential for miniaturized electronics such as smartphones, wearables, and other devices that require fine-pitch components and high-density interconnections.

Microvias come in several forms:

• Blind Microvias: These connect an outer layer to the nearest inner layer, similar to blind vias, but on a much smaller scale. They are commonly used to route signals between layers in HDI PCBs and are critical for reducing space requirements in dense designs.

• Stacked Microvias: Stacked microvias involve multiple microvias placed directly on top of one another, typically in consecutive layers. They enable vertical interconnections between multiple PCB layers and are used when dense routing is required.

• Staggered Microvias: Staggered microvias are similar to stacked microvias but are slightly offset from each other in adjacent layers. This technique is used to reduce stress on the PCB material caused by aligning multiple vias in a stack, which can sometimes lead to mechanical weaknesses in high-density designs.

• Filled Microvias: To increase the reliability of microvias, especially in high-frequency or high-current applications, they are sometimes filled with conductive material (e.g., copper or epoxy). This process creates a more robust electrical connection and improves thermal conductivity, which can be beneficial for power integrity and signal performance.

4. Backdrilled Vias (or Stub Vias)

Backdrilling is a process used to remove the unused section of a via, particularly in high-speed signal boards where signal integrity is critical. In standard through-vias, the portion of the via that extends beyond the last layer that needs electrical connection can act as a “stub,” which causes signal reflections and impairs high-frequency performance.

• Backdrilling removes these unnecessary sections of the via, reducing signal loss and improving overall signal integrity. This technique is particularly important in PCBs used for high-speed digital applications, such as network equipment and telecommunications.

5. Teardrop Via (or Teardrop Hole)

Teardrop vias or teardrop holes involve creating a teardrop-shaped pad where the trace meets the via hole. This method increases the mechanical strength and reliability of the connection between the trace and the hole, reducing the risk of damage due to stress or misalignment during the drilling process.

• Teardrop vias are especially beneficial for high-vibration environments or during the PCB assembly process, where mechanical stresses could otherwise weaken standard vias or traces. By gradually transitioning from the trace to the hole, teardrop vias minimize the potential for trace breakage or drill misalignment.

6. Countersink and Counterbore Holes

While not as common as other types of PCB holes, countersink and counterbore holes are used when specific mechanical requirements exist, such as for mounting screws or other hardware.

• Countersink Holes: These are used to create a conical recess in the PCB, allowing flat-head screws to sit flush with or below the surface of the board. This type of hole is used in applications where components or hardware need to be mounted without protruding above the surface.

• Counterbore Holes: These are used to create a flat-bottomed recess, which allows a socket head or cap screw to sit below the surface of the PCB. Counterbore holes are primarily used in situations where the head of the screw or bolt needs to be recessed for mechanical assembly purposes.

7. Via-in-Pad (VIP)

Via-in-Pad is a technique where the via is placed directly beneath a component pad, rather than in an adjacent area. This method is increasingly used in HDI and compact designs because it saves surface area and allows for more efficient routing in tight spaces.

• In VIP designs, the vias are often filled with conductive or non-conductive materials and then capped with copper to provide a flat surface for soldering components. This technique is particularly useful in designs with fine-pitch BGAs (ball grid arrays) or other surface-mount components where available board space is limited.

• The primary benefit of VIP is the reduction in trace lengths and parasitic inductance, which can improve signal integrity and reduce transmission line effects in high-speed designs.

8. Tenting Vias

Tenting refers to the practice of covering vias with solder mask to prevent solder from flowing into the holes during the assembly process. This is commonly done with through-vias that are not meant to be soldered but need protection from contamination or to improve the aesthetic of the PCB surface.

• Tenting can either fully or partially cover the via. Full tenting involves covering the entire via opening, whereas partial tenting leaves a small opening for gas venting or inspection purposes.

9. Test Holes

Test Holes are small, non-functional holes created for inspection or quality control purposes during the PCB manufacturing process. These holes allow manufacturers to visually inspect the inner layers or confirm the integrity of the plated through-holes. Test holes are often placed in non-critical areas of the PCB and do not serve any electrical or mechanical function in the final product.

PCB Drilling Techniques

Drilling holes in PCBs is a critical stage in the manufacturing process, ensuring both mechanical integrity and electrical connectivity. Depending on the type of hole, the number of layers, and the complexity of the design, different drilling techniques are applied. Below is an expanded overview of modern PCB drilling techniques, including the use of advanced coating technologies and hybrid systems to meet increasingly demanding production requirements.

1. Mechanical Drilling

Mechanical drilling is still one of the most widely used methods for creating larger holes in PCBs, such as through-holes, non-plated holes, and some vias. It involves the use of a high-speed drill bit, usually made of tungsten carbide, rotating at high speeds to penetrate the PCB’s layers.

However, as PCB designs become more intricate and hole sizes shrink, maintaining the durability and precision of drill bits has become a challenge. To address this, manufacturers have introduced specialized coating technologies for drill bits, particularly for very small sizes like 0.1 mm and below.

Coating Technologies for Drill Bits

To extend the lifespan of drill bits and improve performance, especially for small diameters like 0.1 mm, several types of coatings have been developed:

• Diamond-Like Carbon (DLC) Coating: DLC coating is applied to the surface of drill bits to increase hardness and reduce wear. This coating mimics some properties of diamond, such as extreme hardness and low friction, which helps extend the life of the drill bit and maintain sharper cutting edges. DLC-coated bits are particularly useful for drilling hard PCB substrates like FR4 or ceramic-filled materials.

• Titanium Nitride (TiN) Coating: TiN coating is one of the most common coatings for drill bits in PCB manufacturing. This coating provides increased hardness, reduced friction, and improved heat resistance. These benefits result in longer tool life, especially in high-speed, high-volume drilling operations where heat buildup can otherwise degrade performance.

• Titanium Aluminum Nitride (TiAlN) Coating: TiAlN-coated drill bits are even more heat-resistant than TiN-coated bits, making them ideal for high-temperature drilling applications. TiAlN coatings are also highly resistant to oxidation, allowing the drill bits to last longer, especially in applications that involve drilling through tough materials or thick stacks of PCB layers.

• Multi-Layer Coatings: Modern drill bits often come with multi-layer coatings that combine different materials to optimize hardness, heat resistance, and friction reduction. These coatings provide a balance between toughness and longevity, particularly in scenarios where the PCB material is abrasive or the drill bit must penetrate multiple layers of copper and substrate.

Using these coating technologies, manufacturers can produce drill bits that maintain their cutting precision for longer periods, reducing the need for frequent replacement and minimizing production downtime.

Mechanical Drilling Challenges and Solutions

• Small Hole Drilling: As PCB designs shrink and demand tighter tolerances, mechanical drilling faces limitations with holes smaller than 0.1 mm. High-speed drills with specialized coatings allow for smaller hole sizes, but for extremely small microvias, mechanical drills may struggle, and laser drilling is often preferred.
• Drill Wear: Drill bit wear is a constant challenge in mechanical drilling, particularly for high-volume production. Regular maintenance and replacing worn bits with coated alternatives reduce the risks of defects like burrs, rough hole walls, and misalignment.
• Stack Drilling: Mechanical drilling can be optimized by drilling multiple layers simultaneously, known as stack drilling. However, this requires extreme precision to ensure hole alignment, especially when different layers have varying dielectric or copper thicknesses.

2. Laser Drilling

Laser drilling is a highly precise technique used primarily for drilling small holes like microvias, which are essential in HDI (high-density interconnect) PCBs. Laser drilling uses focused laser beams to vaporize material and create clean, accurate holes, often smaller than what mechanical drills can achieve.

Types of Lasers Used in PCB Drilling:

• CO₂ Lasers: These are used for removing non-metallic materials such as dielectric layers in PCBs. They work by vaporizing the non-conductive materials between copper layers. CO₂ lasers are fast and efficient for dielectric removal but are not effective at cutting copper.

• UV Lasers: UV lasers (ultraviolet lasers) are much more precise and can be used to drill through both copper and non-metallic layers. They are capable of creating extremely small holes, even less than 20 microns in diameter, making them ideal for the microvias required in HDI PCBs.

Advantages of Laser Drilling:

• Extreme Precision: Laser drilling offers unparalleled precision, especially for creating microvias. Lasers can create consistent holes with very tight tolerances, reducing the likelihood of defects like misalignment or irregular hole sizes.
• Non-Contact Process: Since laser drilling is a non-contact method, there is no tool wear, unlike mechanical drills. This allows for consistent hole quality over long production runs.
• High Aspect Ratios: Laser drilling is ideal for holes with high aspect ratios (depth-to-diameter ratio), which is essential for creating deeper vias in multilayer PCBs.

Laser Drilling Challenges:

• Speed for Larger Holes: While laser drilling excels at small holes, it can be slower than mechanical drilling for larger through-holes. Hybrid systems that combine both mechanical and laser drilling are often used to balance precision and efficiency.
• Cost: Laser equipment is expensive compared to mechanical drills, making it a more costly option, especially for simpler PCBs where precision is not as critical.

3. Controlled Depth Drilling

Controlled depth drilling is a technique used to create blind or buried vias, where the hole does not extend all the way through the PCB. This method is critical for multi-layer PCBs where precise interconnections between specific layers are required.

Key Considerations for Controlled Depth Drilling:

• Precision Depth Control: Controlled depth drilling involves stopping the drill at a specific layer, requiring highly accurate machinery. Any miscalculation in depth could result in a via that doesn’t connect properly or penetrates unintended layers.
• Combination with Laser Drilling: In high-density boards, controlled depth drilling may be combined with laser drilling to ensure smaller vias are created with high precision, while larger vias or through-holes are drilled mechanically.
• Tool Wear: Even with controlled depth drilling, maintaining the sharpness and precision of drill bits is essential, especially for complex, multi-layer boards.

4. Sequential Lamination and Drilling

In sequential lamination, the layers of a multi-layer PCB are fabricated and laminated together in stages, with drilling occurring at different points in the process to create buried and stacked vias. This method is particularly important for HDI designs.

• Buried Vias: Vias connecting only internal layers are drilled after a specific subset of layers is laminated, then the next layers are added on top, burying the vias.
• Stacked Microvias: Stacked microvias involve drilling vias layer by layer and stacking them to create vertical connections between adjacent layers. This technique is especially common in HDI PCBs, where high-density routing is required.

Sequential lamination allows for more complex routing, higher component density, and greater electrical performance in a small space. However, this process is more time-consuming and costly due to the multiple lamination and drilling steps required.

5. Plasma Etching

Plasma etching is an advanced technique used to remove material from the PCB surface or within drilled holes, typically for cleaning up and smoothing the walls of vias and microvias. Plasma, an ionized gas, chemically reacts with the surface materials and removes unwanted substances like excess resin or dielectric materials.

• Desmearing: Plasma etching is often used after mechanical drilling to clean the inner walls of vias, removing resin smears left from the drilling process. This step is critical to ensure good electrical connections when the vias are plated.
• Dielectric Removal: Plasma etching can also be used to remove dielectric materials between copper layers during the creation of microvias, leaving behind a clean path for electrical connectivity.

While plasma etching offers excellent precision, it is a slower and more costly process compared to traditional desmearing methods.

6. Back Drilling

Back drilling is used to remove the unused portion of a plated-through-hole (PTH), especially in high-speed PCBs where signal integrity is a concern. The unneeded stub left by a through-hole can cause signal reflections and degrade high-frequency signals, so back drilling eliminates this issue by removing the excess copper that extends beyond the necessary layer.

• Precision Control: Back drilling must be performed with high precision to remove only the unused portion of the via without damaging the required connections. This technique is critical for reducing signal loss in high-frequency PCBs used in telecommunications and data networking.

7. Hybrid Drilling Systems

Hybrid drilling systems combine the benefits of mechanical and laser drilling, allowing for more flexibility in production. These systems use mechanical drills for larger through-holes and plated holes, while lasers are used to create smaller microvias and high-precision vias. This approach maximizes efficiency and cost-effectiveness while maintaining the precision required for complex designs.

The Impact of Hole Selection in PCB Design

In PCB design, selecting the right type of holes significantly impacts the board’s performance, manufacturing cost, and production complexity. Designers must weigh various factors when deciding which type of hole to use, including performance requirements, the feasibility of manufacturing processes, production cost, and design complexity. Beyond the choice between blind vias and back drilling, other considerations include mechanical versus laser drilling, back drilling versus buried vias, and planning for HDI (high-density interconnect) stack configurations. Below, we discuss strategies for choosing the most common hole types to help designers optimize both quality and manufacturing costs.

1. Back Drilling vs. Buried Vias

The choice between back drilling and buried vias is key to optimizing cost, signal integrity, and manufacturing complexity in multi-layer PCBs. Back drilling is particularly effective for high-speed designs where reducing signal reflection is critical. By removing the unused portion of the via, back drilling eliminates the “stub” that could negatively impact signal integrity. It is typically less complex than buried vias, which require multiple lamination and plating steps, making back drilling more cost-effective in high-performance designs where reducing manufacturing steps is essential.

On the other hand, buried vias are essential in designs requiring internal layer connections without surface interruptions. However, their use introduces additional complexity and cost due to the need for precise multi-step lamination processes. While buried vias provide excellent connectivity for internal layers, substituting them with back drilling where possible simplifies the manufacturing process, reduces costs, and speeds up production. Back drilling is often the preferred choice in designs prioritizing signal performance, whereas buried vias are better suited for highly compact designs that demand internal layer-to-layer connections without surface layer interference.

Recommendation: For high-speed designs where signal integrity is critical, back drilling is the optimal choice, offering reduced manufacturing complexity and better cost control. Buried vias are better reserved for dense, multi-layer designs where internal layer connections are needed, despite the increased manufacturing complexity.

2. Blind and Buried Vias: Mechanical Drilling or Laser Drilling?

Blind and buried vias are commonly used in PCB design to connect internal layers with external layers. When deciding how to implement these types of vias, designers must choose between mechanical drilling and laser drilling. Mechanical drilling is ideal for larger vias, offering cost-effectiveness at larger diameters and fewer layers, while laser drilling is suited for smaller vias and high-density designs, providing precision in multi-layer boards. Laser drilling, though more expensive, enhances board performance and space utilization, particularly for microvias in HDI designs.

Recommendation: Use mechanical drilling for large vias (diameter >0.2 mm) in fewer layers, and laser drilling for smaller, high-density designs with microvias, especially in multi-layer HDI boards where space efficiency is crucial.

3. HDI Stack-Up Planning: Fewer Layers with More Stacks vs. More Layers with Fewer Stacks

In HDI design, deciding between fewer layers with more via stacks and more layers with fewer stacks affects PCB performance and cost. Reducing layers with more stacks leads to thinner, more compact boards but increases laser drilling and manufacturing complexity. Conversely, using more layers with fewer via stacks simplifies drilling but increases material cost and board thickness.

Recommendation: For designs requiring dense signal interconnections, fewer layers with more stacks provide compactness but add manufacturing complexity. For large-scale production or cost-sensitive projects, more layers with fewer stacks reduce complexity and manufacturing risk, making it ideal for high-volume production.

Conclusion

Selecting the right hole types in PCB design, such as back drilling versus buried vias, mechanical versus laser drilling, or optimizing HDI stack configurations, is crucial for balancing performance, cost, and manufacturing complexity. By carefully considering your design’s specific needs, you can make informed decisions that enhance signal integrity, reduce production steps, and lower costs. Whether it’s choosing back drilling for high-speed designs or opting for laser drilling in HDI boards, each choice plays a key role in delivering a high-performance, cost-effective product. Collaborating with expert manufacturers like Highleap Electronic ensures that your PCB design meets both performance goals and budget constraints, offering top-tier manufacturing capabilities without compromising on quality.