Blind and buried vias are common features in high-density multilayer PCB design, and both can be formed with either mechanical drilling or laser drilling. In HDI designs, stacked microvias are a common strategy to route signals between multiple layers in a compact footprint. However, mechanically drilled blind or buried vias cannot be stacked in the same way due to the manner in which sub-lamination and sequential lamination processes build up a PCB stackup.

This article examines the differences in manufacturing process sequences, drilling precision, plating requirements, and design constraints that make stacked microvias possible, but stacked mechanically-drilled blind/buried vias impossible given the practical approaches to PCB stackup fabrication. The different stackup processing approaches will impact how a PCB layout can be routed and, ultimately, the number of layers needed to solve a PCB layout.

Definitions and Context

Before looking at why stacking works in one case but not the other, it’s important to define the two cases clearly. The decision to use sub-lamination or sequential lamination to fabricate blind/buried vias will impact the available hole sizes and routing density available in the design.

• Blind via: A via that starts on an outer layer and ends on an inner layer, without passing through the entire board.
• Buried via: A via that connects two or more inner layers without reaching the outer layers.
• Microvia: A blind or buried via with a diameter ≤ 0.15 mm (about 6 mil), typically formed with a laser drill and extending only through a single dielectric layer in an HDI build-up process.

While blind and buried vias can be formed mechanically, HDI microvias are laser-drilled. The choice of process dictates what structures are feasible in manufacturing and whether stacking is possible without unacceptable yield or reliability losses.

Manufacturing Process Comparison

The ability to stack vias depends heavily on when the holes are drilled and plated during the PCB fabrication sequence. Although both mechanical and laser-drilled vias can be plated and connected to multiple layers, the process in which they are created fundamentally changes what can be done with them. Mechanical drilling tends to be used in bulk steps that occur either before or after full lamination, while laser drilling is tightly integrated into a sequential build-up process that adds layers one at a time. This difference means mechanical blind/buried vias are essentially "fixed" once they are made, whereas laser-drilled microvias can be incrementally added and stacked during each lamination cycle.

Mechanically Drilled Blind/Buried Vias

1. Sub-lamination prep: The stackup design is divided into sub-laminations (core sections) with their own blind/buried vias.
2. Buried via drilling and plating: Each sub-lamination has its layers drilled and plated to the required via wall thickness and final copper weight before they are laminated into the full stack.
3. Final lamination: All sub-laminates are bonded together with prepreg layers.
4. Through-hole drilling: All through-holes are drilled and plated in the last drilling step.
5. Final plating: The through-holes and surface layers are plated to the required via wall thickness and final copper weight.

In this process, all mechanically drilled blind vias that start on the same surface are drilled and plated at the same time. They cannot be stacked vertically because drilling through an already plated via would damage it.

Laser-Drilled Microvias (HDI Sequential Build-Up)

1. Laminate core: Start with a core stack and plate any through or buried vias.
2. First build-up dielectric layer: Apply thin dielectric and copper foil.
3. Laser drill microvias: Ablate through one dielectric layer into the pad below.
4. Plate and fill: Plate the hole walls, then copper-fill to create a flat pad.
5. Repeat build-up: Apply another dielectric/copper layer and repeat drilling and plating.

Because each build-up layer is laminated, drilled, and plated before the next layer is added, microvias can be stacked vertically with reliable copper continuity. This all relies on the fact that the laser drill does not destroy the copper pad when it drills through the dielectric layer.

Why Mechanical Stacking Doesn’t Work

Even if the via diameter in an HDI design were increased to a size that could be handled by a mechanical drill, mechanically drilled blind vias still could not be reliably stacked. One of the biggest limitations is depth control. Lasers can ablate through a dielectric layer and stop cleanly on the copper pad beneath, whereas mechanical drills have difficulty stopping within such tight tolerances. This problem is compounded by drill bit runout, vibration, and flex, all of which make it easy to over-drill and damage the target pad.

Other issues include:

• Registration limitations: Stacked vias require precise alignment over the previous via. Mechanical drilling tolerances are typically ±2–3 mil, while laser drilling can achieve ±1 mil or better. This higher precision makes lasers much more suitable for stacking.
• Resin smear formation: Mechanical drilling produces molten resin that coats the copper landing pad, which must be removed through a desmear process. In thin HDI dielectric layers, removing smear without damaging the pad is difficult.
• Plating challenges: Mechanically drilled blind vias often have higher aspect ratios and rougher walls, making it difficult to plate and fill the via uniformly. This can lead to weak spots at the via knee, which is the most common failure location in stacked structures.

Taken together, these factors make stacked mechanically drilled blind/buried vias highly unreliable in production. Even if the geometry seems feasible in CAD, the limitations of mechanical drilling, cleaning, and plating introduce risks that outweigh the potential benefits.

 

 

Practical Implications for PCB Designers

The choice between mechanical blind/buried vias and stacked microvias has significant implications for manufacturability and reliability. Mechanical blind and buried vias are suitable when connecting deeper layer pairs that are not adjacent, especially in designs with lower routing density or when HDI is not justified from a cost perspective. They are also appropriate in cases where stacking is not required and via counts remain moderate.

Stacked microvias are better suited for high-density packages such as fine-pitch BGAs, where multiple signal layers must fan out from the component in a compact footprint. In sequential build-up HDI designs, stacked microvias enable shorter interconnects, reduce routing congestion, and make it possible to achieve denser layer utilization without excessive via-in-pad usage on deeper layers.

 

Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your design is fully manufacturable at scale. Contact NWES for a consultation.

 

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