Single-layer and double-layer PCBs are the two fundamental types of printed circuit boards, consisting of only one or two layers of conductive material on the substrate. When more routing space or improved signal integrity is required, multi-layer PCBs are the optimal solution. Unlike their single-layer and double-layer counterparts, multi-layer PCBs combine multiple layers of conductive and insulating materials, forming a complex network of connections. The typical stack-up includes copper layers, cores, substrates, and prepreg. Prepreg and core represent two types of insulating or dielectric materials used in PCBs.
Figure 1: Multilayer PCB stack-up
What is prepreg?
Prepreg, short for pre-impregnated, is a fiberglass cloth impregnated with uncured epoxy resin, without any attached copper foil. In a PCB stack-up, prepreg functions as both an insulating layer and an adhesive. During PCB construction, prepreg is positioned between adjacent copper layers or between a copper layer and a core material. Through high heat and pressure during manufacturing, the epoxy resin flows, bonding the layers together and resulting in a solid, rigid board.
Figure 2: Prepreg
What is the core of a PCB?
The core of a PCB is the central layer that provides mechanical support and rigidity. It is typically made of a laminate material consisting of thin copper foil layers bonded to a rigid substrate. The copper foil on the core serves various purposes, such as creating a ground or signal plane. In multi-layer PCBs, several core layers may exist, with prepreg layers in between the cores and outer copper layers.
Figure 3: PCB Core
HDI Stack-up
In contrast to traditional PCB stack-ups that use through-hole vias to connect layers, HDI stack-ups incorporate micro-vias, blind vias, and buried vias to enhance routing density and electrical performance. The X-N-X structure is commonly used to describe the HDI stack-up configuration, where X represents the number of HDI layers with micro-vias, and N denotes the number of core layers between these HDI layers. Typical configurations include 1-N-1, 2-N-2, and 3-N-3.
To better understand the X-N-X structure in practice, consider a 1-4-1 stack-up configuration. This configuration features one HDI layer with micro-vias at the top of the PCB, followed by four core layers without micro-vias in between, and another HDI layer with micro-vias at the bottom.
Figure 4: HDI Stack-up Example
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