Printed Circuit Boards today face an ever-increasing demand in capabilities including high speed, low weight, higher component density, miniaturization, and more. Multi-layer PCBs can help meet those increased demands. Although they are not simply laid one on top of the other, a multi-layer PCB is created from two or more PCBs stacked together. Instead, additional dielectric material will connect the PCBs according to predetermined standards.
A well-designed “stackup” (the arrangement of layers that make up a PCB) can reduce some of the factors that can impact signal quality including: external noise, impedance mismatch, radiation, and signal cross talk.
Today’s article will look at some requirements to consider when designing a successful multi-layer PCB.
How Does It Stack Up?
Multi-layer PCBs are typically built in even-numbered layers. Stackups can consist of several different types of layers including signal, ground, and power layers. Here are some examples of standard PCB stackups used at Bay Area Circuits.
The most common method of stackup fabrication is to start with a solid core, then add prepreg between cores for inner layers or between a core and foil for outer layers, such as a 4-layer PCB stackup, as shown in the illustration.
This is followed by a lamination process detailed in an article titled Manufacturing of Double-Sided and Multi-Layer Printed Circuit Boards.
Other less common and more expensive fabrication options include:
Buried and Blind Vias (internal layers are built using either foil or cap lamination): The process is essentially the same as a standard stackup and lamination. However, the lamination process may need to be repeated several times to capture the vias internally and still complete the needed electrical connections.
Cap Lamination: Instead of starting with a core and building out using prepreg and foil, this process puts the core on the outer layers and then prepreg in the middle. The process to laminate is less stable and requires the correct press and tools to accomplish.
Not only does material composition of the boards need to be considered, but so does the spacing between layers. There are two types of spacing materials that can be used: core (copper-plated, glass-reinforced epoxy laminated sheets with a typical thickness between 0.004” and 0.040”) or prepreg (a reinforced sheet impregnated with resin, typical thickness between .002” and .007”).
A well-designed stackup and effective fabrication method can help you ensure the right build for your multi-layer PCBs. While most PCB manufacturers utilize a set of standard stackups, it’s also very common to utilize a stackup defined as a part of the project requirements. Whenever possible, it’s a good idea to leverage your PCB manufacturer’s knowledge & experience to validate a proposed stackup.