As electronic products continue to become smaller, thinner, and more highly integrated, circuit designers need increasingly compact interconnection structures. Microvia technology has therefore become an important part of modern high-density PCB manufacturing, particularly for high-density interconnect (HDI) boards.

A microvia can be produced using several different manufacturing technologies. The two major approaches discussed in this article are Laser Drilling and Mechanical Drilling. The appropriate method depends on factors such as hole diameter, dielectric material, board structure, production volume, equipment capability, reliability requirements, and overall manufacturing cost.

Understanding the differences between these technologies helps PCB engineers and process engineers select an appropriate solution for a specific board design.

What Is a Microvia?

A Microvia is a small-diameter via, generally associated with HDI structures and typically formed by laser drilling through a thin dielectric layer. In industry practice, microvias are commonly associated with hole diameters of approximately 150 μm or less, although the actual definition can vary depending on manufacturing standards and application requirements.

Microvias are primarily used to connect adjacent conductive layers and are especially valuable when conventional through-hole vias would consume excessive routing space.

PCB vias can generally be categorized into three major types:

• Blind vias
• Buried vias
• Through vias

A blind via connects an outer layer to one or more inner layers without passing completely through the PCB. Its depth is therefore limited by the required layer-to-layer connection.

A buried via is located entirely within the internal layers of a multilayer PCB and does not directly reach an external surface. It is normally formed in an internal substructure before the final multilayer lamination process.

A through via extends through the entire PCB thickness and can provide electrical connections between multiple layers. Through vias may also be used for through-hole components, depending on the PCB design.

Microvias are most commonly associated with blind-via structures and are particularly important in HDI PCB manufacturing.

Why Microvias Are Important in HDI PCB Manufacturing

As PCB routing density increases, conventional through-hole vias can consume significant amounts of routing space. Their larger drilled holes and larger antipads can restrict the available routing area on inner layers.

A Microvia provides a much smaller interconnection structure. This allows designers to:

• Increase routing density
• Reduce via-related space requirements
• Support finer-pitch components
• Improve layer-to-layer interconnection
• Enable high-density BGA fanout
• Reduce unnecessary through-hole structures
• Support compact PCB designs

For this reason, microvias are widely used in smartphones, wearable electronics, networking equipment, advanced computing systems, automotive electronics, and other compact electronic products.

Choosing a Microvia Formation Process

The selection of a Microvia Drilling process should not be based only on the purchase price of the drilling equipment.

The total manufacturing cost can be affected by:

• Cost per hole
• Equipment utilization
• Drilling speed
• Tool life
• Tool replacement
• Setup time
• Material compatibility
• Registration requirements
• Hole quality
• Scrap rate
• Inspection requirements
• Production volume

For relatively large holes, conventional mechanical drilling may offer a cost-effective solution. As hole diameter decreases and the required aspect ratio becomes more challenging, Laser Drilling becomes increasingly attractive.

Therefore, process selection should consider the complete manufacturing process rather than comparing individual equipment prices.

Mechanical Drilling for PCB Vias

Mechanical drilling remains one of the most widely used processes in PCB Manufacturing.

Modern CNC drilling systems have become significantly more accurate and productive. Improvements in machine rigidity, spindle technology, automatic tool changing, vision alignment, motion control, and process monitoring have improved drilling performance for multilayer PCBs.

Mechanical drilling is particularly suitable for:

• Through holes
• Larger blind vias
• Plated through-holes
• Component holes
• Mounting holes
• Tooling holes
• Other mechanical features

High-speed spindles can support very high rotational speeds, while automated drilling systems can process large numbers of holes with repeatable positioning.

However, mechanical drilling also has limitations when very small holes are required. As the drill diameter decreases, factors such as drill breakage, tool wear, runout, board material, hole quality, and production efficiency become increasingly important.

Blind-Via Depth Control in Mechanical Drilling

When mechanical drilling is used to produce blind vias, accurate depth control becomes critical.

Unlike through-hole drilling, a blind via must terminate at a specific layer without damaging the target layer or creating an excessively shallow connection.

Modern drilling systems can use advanced sensing, machine vision, spindle monitoring, depth-control mechanisms, and automated process compensation to improve blind-via accuracy.

Accurate detection of the drilling position and surface reference helps reduce variations caused by:

• PCB surface height differences
• Panel thickness variation
• Material deformation
• Debris
• Tool wear
• Machine vibration
• Registration errors

Process monitoring can also help identify abnormal drilling conditions and reduce the risk of defective holes.

Tool Wear and Drill Life

Drill-bit performance directly affects the cost and quality of PCB Drilling.

Small-diameter carbide drills are particularly sensitive to mechanical stress. Tool wear can gradually affect hole diameter, hole-wall quality, positional accuracy, and drilling reliability.

PCB manufacturers therefore monitor factors such as:

• Number of holes drilled
• Drill diameter
• Material stackup
• Copper thickness
• Board thickness
• Spindle speed
• Feed rate
• Tool condition

Optimizing drill geometry and cutting parameters can extend tool life and reduce the manufacturing cost per hole.

However, there is no single drill design or parameter set that is suitable for every PCB. The optimal process depends on the laminate system, copper structure, hole size, board thickness, and production requirements.

When Laser Drilling Becomes More Suitable

When the required hole diameter becomes very small, Laser Drilling offers significant advantages over conventional mechanical drilling.

Laser drilling is widely used to form microvias in HDI PCBs because it can create small holes without requiring a mechanical drill bit to physically pass through the material.

Typical laser-drilled microvias may have diameters around 50–150 μm, although the achievable size depends on the laser source, material system, process parameters, and equipment capability.

For many HDI applications, laser drilling is preferred when:

• Microvia diameters are very small
• Thin dielectric layers are used
• High-density BGA fanout is required
• Fine-pitch interconnections are needed
• Sequential buildup structures are used
• High positional accuracy is required

Main Laser Drilling Methods

Several laser processes can be used in Microvia Drilling. Two important approaches are direct laser drilling and conformal-mask laser drilling.

Direct Laser Drilling

In direct dielectric drilling, the laser beam is focused onto the dielectric surface. Laser energy removes or vaporizes the resin material to form the required opening.

After the dielectric has been removed, subsequent desmear, metallization, and copper plating processes can establish the electrical connection.

CO₂ lasers are widely used in PCB manufacturing because they interact efficiently with many organic dielectric materials.

However, the actual minimum hole size is not determined by the laser wavelength alone. It also depends on beam optics, pulse conditions, dielectric composition, copper thickness, process control, and the required hole quality.

Conformal Mask Laser Drilling

In conformal-mask processing, a patterned copper or mask structure defines the target area for laser processing.

The laser selectively removes the exposed dielectric material, while the mask helps control the area being processed.

This method can be useful when the desired hole diameter is smaller than the effective laser beam diameter or when specific process control requirements must be met.

The choice between direct drilling and conformal-mask processing depends on the PCB structure, material system, production volume, equipment, and required feature dimensions.

Laser Drilling vs. Mechanical Drilling

The differences between Laser Drilling and Mechanical Drilling can be summarized as follows:

The best choice depends on the complete PCB design and production strategy.

Microvia Reliability Considerations

Producing a small hole is not enough to guarantee a reliable Microvia.

Reliability depends on the complete structure, including dielectric thickness, copper plating quality, target-pad design, via geometry, thermal expansion, resin characteristics, and manufacturing control.

Important reliability factors include:

• Microvia geometry
• Copper plating thickness
• Interface quality
• Resin system
• Coefficient of thermal expansion
• Target pad structure
• Stacked-via configuration
• Staggered-via configuration
• Thermal cycling
• Registration accuracy

Stacked microvias can provide excellent routing density, but they also introduce additional manufacturing and reliability considerations. Copper-filled structures may be required for certain stacked microvia configurations.

For demanding applications, microvia reliability should be evaluated through appropriate thermal cycling, cross-sectional analysis, and other qualification methods.

The Role of Registration Accuracy

As PCB features become smaller, registration accuracy becomes increasingly important.

A microvia must be accurately aligned with its target pad and the surrounding circuit pattern. Excessive registration error can reduce the effective capture area and potentially result in an open connection or reduced reliability.

For HDI PCB production, manufacturers therefore need precise control over:

• Imaging alignment
• Layer registration
• Laser positioning
• Lamination movement
• Material expansion and contraction
• Drill-to-pad alignment

Good stackup design and manufacturing process control are equally important because registration performance is influenced by both design and production conditions.

Cost Considerations for Microvia Manufacturing

The cost of PCB Manufacturing is influenced by much more than the price of the laser or drilling machine.

A realistic cost evaluation should consider the entire process chain.

For mechanical drilling, cost factors may include:

• Drill-bit consumption
• Tool changing
• Spindle maintenance
• Drilling speed
• Tool breakage
• Hole inspection

For laser drilling, cost factors may include:

• Laser equipment investment
• Energy consumption
• Equipment maintenance
• Beam calibration
• Process setup
• Throughput
• Material compatibility
• Cleaning and subsequent metallization

The manufacturing volume also has a significant effect on the economics of each process.

A process that is more expensive per machine hour may still be economically attractive if it provides substantially higher throughput or enables a PCB design that cannot be efficiently manufactured using conventional drilling.

How to Select the Right Microvia Process

When selecting a Microvia Drilling technology, PCB engineers should evaluate the following questions:

1. What is the required hole diameter?
2. How thick is the dielectric layer?
3. Is the via blind, buried, or through?
4. What material system is being used?
5. What is the required positional accuracy?
6. What is the required production volume?
7. What is the required via reliability?
8. Can the existing equipment meet the required process capability?
9. What are the expected tooling and consumable costs?
10. Does the final design meet the manufacturer’s DFM requirements?

This approach helps prevent the selection of a process based solely on nominal hole size.

Kingda’s Microvia and HDI PCB Manufacturing

At Kingda, microvia production should be evaluated as part of the complete PCB Manufacturing process rather than as an isolated drilling operation.

For HDI designs, factors such as material selection, stackup configuration, dielectric thickness, laser drilling, mechanical drilling, copper plating, registration accuracy, and reliability testing must work together to achieve stable production results.

The appropriate process is selected according to the actual PCB design requirements, including feature size, layer structure, material characteristics, production volume, and reliability expectations.

Kingda can support customers in evaluating manufacturability early in the design stage, helping optimize the relationship between PCB density, manufacturing capability, reliability, and cost.

Conclusion

The development of Microvia Drilling technology has played an important role in the evolution of high-density PCB technology.

Mechanical Drilling remains an efficient and economical solution for many conventional PCB holes, particularly through holes and larger-diameter features. However, as hole sizes become smaller and HDI structures become more complex, Laser Drilling provides important advantages for forming microvias in thin dielectric layers.

The optimal process is not determined by hole diameter alone. Material properties, board structure, registration accuracy, reliability requirements, production volume, equipment capability, and total manufacturing cost must all be considered.

For modern HDI PCB production, selecting the appropriate microvia formation process at the design stage can significantly improve manufacturability, reliability, production efficiency, and overall product cost.