How Are Multi-layer PCBs Structured? A Look Inside Their Laminates and Interconnects

October 18, 2025

When PCB engineers first encounter multi-layer circuit boards, it's easy to get overwhelmed. With as many as ten or eight layers, the traces resemble a spider's web. Below are several internal structure diagrams of multi-layer PCBs, using three-dimensional graphics to display the internal architecture of PCBs with various stack-up structures.

01 The Core of High-Density Interconnect (HDI) Boards Lies in the Vias

The trace processing of multi-layer PCBs is no different from that of single or double-layer boards; the biggest difference lies in the via process.

Traces are etched out, while vias are drilled and then copper-plated. Those involved in hardware development are familiar with this, so we won’t go into further detail.

Multi-layer circuit boards generally come in several types: through-hole boards, 1-step HDI boards, 2-step HDI boards, and 2-step stacked vias boards. Higher-order boards, such as 3-step HDI boards and any-layer interconnect boards, are rarely used in everyday applications and are extremely expensive, so we won’t discuss them in detail for now.

Generally, 8-bit microcontroller products use 2-layer through-hole boards; 32-bit microcontroller-level smart hardware uses 4 to 6-layer through-hole boards; Linux and Android-level smart hardware uses 6-layer through-hole to 8-layer 1-step HDI boards; compact products like smartphones typically use 8-layer 1-step to 10-layer 2-step circuit boards.

8-layer 2-step stacked vias, Qualcomm Snapdragon 624

02 The Most Common Through-Hole

There is only one type of via, drilled from the first layer to the last layer. Whether for outer or inner traces, the holes are drilled through, hence the name "through-hole board."

Through-hole boards are not defined by the number of layers. The 2-layer boards commonly used are through-hole boards, while many switch and military circuit boards with up to 20 layers are still through-hole. The board is drilled through with a drill bit, and then the holes are copper-plated to form conductive pathways.

It’s important to note that the inner diameter of through-holes is typically 0.2mm, 0.25mm, or 0.3mm. However, 0.2mm holes are generally significantly more expensive than 0.3mm holes. This is because thinner drill bits break more easily and drill more slowly. The additional time and cost of the drill bits are reflected in the higher price of the circuit board.

03 Laser Vias in High-Density Interconnect (HDI) Boards

This picture shows the stack-up structure of a 6-layer 1-step HDI board. The outer two layers use laser vias with an inner diameter of 0.1mm. The inner layers are mechanical vias, equivalent to a 4-layer through-hole board with two additional layers on the outside.

Laser drilling can only penetrate glass fiber substrates, not metal copper. Therefore, drilling on the outer surface does not affect the inner traces. After laser drilling, the holes are copper-plated to form laser vias.

04 2-Step HDI Board: Two Layers of Laser Vias

This diagram shows a 6-layer 2-step staggered HDI board. While 6-layer 2-step boards are less common (most start from 8-layer 2-step), the principle for higher layers is the same as for 6 layers.

The term "2-step" means there are two layers of laser vias. "Staggered vias" means the two layers of laser vias are offset from each other.

Why stagger them? Because the copper plating doesn’t fully fill the vias, leaving them hollow. Thus, you cannot drill directly on top of them; instead, you must offset the next layer of vias by a certain distance.

A 6-layer 2-step board is equivalent to a 4-layer 1-step board with two additional outer layers.
An 8-layer 2-step board is equivalent to a 6-layer 1-step board with two additional outer layers.

05 Stacked Vias Board: More Complex Process, Higher Cost

In stacked vias boards, the two layers of laser vias are aligned on top of each other. This allows for a more compact layout. The inner laser vias must be electroplated and filled flat before the outer laser vias are added. This process makes stacked vias boards more expensive than staggered vias boards.

06 Extremely Expensive Any-Layer Interconnect Board: Multi-Layer Laser Stacked Vias

In this type of board, every layer uses laser vias, and every layer can be interconnected. Layout engineers can route traces and place vias as they wish. Just thinking about it feels exhilarating—no more worries about being unable to complete the layout! However, procurement specialists might want to cry, as these boards cost more than ten times the price of ordinary through-hole boards.

Thus, only products like the iPhone can justify using them. No other smartphone brands are known to use any-layer interconnect boards.

Summary

Finally, here’s a diagram for a detailed comparison. Please note the size of the vias and whether their pads are enclosed or open.