Vias are the vertical interconnects that let signals travel between layers in a multilayer printed circuit board. Without them, the copper layers would be electrically isolated by the insulating material between them. The three most common via types—through holes, blind vias, and buried vias—differ in how they are drilled, how much board real estate they consume, and how much they cost to manufacture. This article explains each type and the process considerations that matter when you specify them.

Through holes: the standard interconnect

A through hole is drilled through the entire board, connecting the top and bottom surfaces and every layer in between. It is the most straightforward and widely used via because it can be drilled after lamination in a single operation. The hole is then plated to create electrical continuity between the copper layers it passes through.

Through holes are also used for component mounting, but a via specifically is not intended for inserting component leads or reinforcement materials—it is a plated hole for layer-to-layer conduction. In high-density designs, through holes can consume routing space on every layer, which is why blind and buried vias become attractive.

Blind vias: connecting outer to inner layers

A blind via connects an outer layer to one or more adjacent inner layers without passing through the entire board. Because it does not emerge on the opposite side, it is “blind” to the other surface. Blind vias free up space on the layers they do not penetrate, allowing denser routing.

The key manufacturing challenge is depth control. The hole must reach the target inner layer precisely, and the depth is typically limited to a ratio of the drill diameter. If the depth is too shallow or too deep, plating becomes difficult and reliability suffers. Blind vias can be formed by drilling after lamination with controlled depth, or by drilling individual subassemblies before final lamination—the latter requires precise registration and alignment.

Buried vias: inner-layer connections only

A buried via connects inner layers and does not reach either outer surface. It is completely enclosed within the board after lamination. Buried vias offer the highest routing density because they do not interfere with surface layers, but they are also the most complex and expensive to produce.

Buried vias cannot be drilled after the final lamination. They must be drilled on individual subassemblies, plated, and then laminated together with the other layers. This adds process steps—drilling, plating, and multiple lamination cycles—which is why buried vias are typically reserved for high-density boards where the extra space is essential.

Via plugging and surface finish considerations

Via plugging is often required to prevent solder or contamination from entering the hole during assembly. The requirements vary by via type and end use:

  • Through vias used only for conduction: Copper plating inside the hole is sufficient; solder mask may or may not fill the hole.
  • Vias that must be solderable: The hole should be filled with solder (tin-lead or lead-free) to a specified thickness—often around 4 µm—and must be free of solder mask ink to avoid trapped solder balls.
  • Vias that must be sealed: Solder mask plugging is required to make the via opaque, with no solder rings, balls, or surface irregularities.

These plugging options interact with the surface finish. For example, ENIG, lead-free HASL, immersion tin, immersion silver, and OSP each have different abilities to cover or fill via holes. Discuss your via plugging and surface finish requirements with your fabricator during design for manufacturability (DFM) review.

Why drilling quality matters

Drilling is a critical step in PCB fabrication. The drill must create clean holes with proper wall quality to support uniform plating thickness in the barrel. Poor drilling can lead to rough walls, smear, or inaccurate registration, which in turn cause plating voids or weak connections. In extreme cases, defective vias can prevent components from being securely mounted or cause open circuits, leading to board scrap.

Inspection methods such as cross-section analysis, X-ray inspection, and electrical test (flying probe or test fixture) are used to verify via integrity. First article inspection (FAI) and automated optical inspection (AOI) further ensure that drilling and plating meet specifications before volume production.

What this means for your build

  • Choose the simplest via type that meets your routing needs. Through holes are the most cost-effective and readily available; blind and buried vias add process steps and cost.
  • Specify via plugging clearly. Indicate whether vias must be open, filled with solder, or sealed with solder mask, and note any thickness or opacity requirements.
  • Account for depth and aspect ratio in blind vias. Work with your fabricator to ensure the drill depth is controllable and the aspect ratio is within their process capabilities.
  • Plan for additional lamination cycles with buried vias. These affect lead time and yield, so incorporate them early in your design reviews.
  • Include via inspection in your quality plan. Cross-section and X-ray checks catch plating defects that electrical test alone may miss.

Source: compiled from public web sources.