PCB Layers Explained: Multilayer Boards and Stackup Rules

Imagine trying to build a house from the roof down instead of from the foundation up. Sound crazy? Sure—it is. Building a house requires a very organized, layered approach. Once a construction team receives the plan and lays the foundation, they can build the stud walls and establish the cut-outs for doors and windows. Then, the trusses can fit on top of the stud walls. Sheeting overlays the trusses and adds cover while establishing structural stability. A layer of sheeting also goes onto the outside stud walls and—as with the roof trusses—adds more stability. After a crew completes the rough work, tar paper and shingles provide another layer of protection on the roof. 

The organization of the house and the layered approach also sets up routing for electrical, HVAC, and plumbing services. Drywall, paint, finish lumber create layers of aesthetic appeal that transform the shell of the house into a home. Much like a house, a printed circuit board too needs a strong organizing principle. 

Every PCB Needs a Good Foundation: Assembly Instructions

Foundational aspects of a PCB include the dielectric material, copper, and trace dimensions along with the mechanical or dimension layer. The materials used as dielectrics provide two basic functions for the PCB. As we build complex PCBs that handle high-speed signals, the dielectric materials isolate the signals found on adjacent layers of the PCB. PCB stability depends on the consistent impedance of the dielectric over the entire plane and the consistent impedance over a wide frequency range.

While the copper as a conductor seems obvious, other functions exist. Different weights and thicknesses of copper impact the ability of a circuit to achieve the correct amount of current flow and define the amount of loss. In terms of the ground and power planes, the quality of the copper layer impacts the impedance of the ground plane and the thermal conductivity of the power plane. Matching the thickness and length for differential signal pairs solidifies the stability and integrity of the circuit—especially for high frequency signals.

Determining your circuit board’s size and shape is pretty important to get at the beginning of the design

Physical dimension lines, dimension markings, datasheets, cut-out information, vias information, tooling information, and assembly instructions not only describe the mechanical or dimension layer but also serve as the measurements for the PCB foundation. The assembly information controls the mounting and location of the electronic components. Because the process of “printed circuit assembly” connects the functional components to the traces on the PCB, the assembly process requires that design teams focus on signal management, thermal management, pad placement, the relation between electrical and mechanical assembly rules, and if the physical mounting of a component matches mechanical requirements.

Every PCB design requires assembly documentation like that found in IPC-2581. Other documents include the Bill of Materials, Gerber data, CAD data, the schematic, fabrication drawings, notes, assembly drawings, any test specifications, any quality specifications, and all regulatory requirements. The accuracy and detail included in those documents lessens the opportunity for any errors in the design process.

Must Follow Rules: Keep Out and Routing Layers

An electrician installing electrical wiring in a house must follow rules to ensure that the wire will not have sharp bends or become vulnerable to the nails or screws used to install drywall. Running the electrical wire through stud walls requires a consistent approach to the depth and height of the wiring path. 

The keep out and routing layers establish the same constraints for a PCB design. A keep out layer defines physical constraints—such as component placement or mechanical clearance—for the design software or electrical constraints like route keepouts. Routing layers establish the interconnections between components. Depending on the application and type of PCB, a routing layer can be placed on the top and bottom layers of a PCB or within the internal layers.

Finding Space for the Ground Plane and Power Plane

Every house has a main electrical service panel or load center that receives incoming electricity from an electrical utility and distributes power to circuits that supply lights, outlets, appliances, and devices. The ground planes and power planes of a PCB provide the same functionality by connecting the circuit to ground and by distributing different onboard voltages to components. As with a service panel, the power and ground planes can contain multiple copper sections that allow circuits and sub-circuits to connect to different potentials.

Protect the Board, Protect the Traces

Professional house painters carefully document the colors and finishes used for ceilings, walls, and trim. On a PCB, the silkscreen layers use text to designate the locations of components on the top and bottom layers. Having the information available through a silkscreen saves a design team from referring back to assembly documents.

The primers, paint, stains, and varnishes applied by house painters add appealing colors and textures. In addition, those finishes protect surfaces from deterioration. In the same way, the thin solder mask layers on a PCB protect traces from shorting when some type of debris falls across the traces.

Positives for PCB Layer Stackup Organization

As PCB technologies have improved and consumer demands for faster, more powerful products have increased, PCBs have changed from the basic two-layer boards to boards that have four, six, and as many as twelve-to-sixteen layers of dielectrics and conductors. Why increase the number of layers? Having more layers increases the capability of the board to distribute power, reduce cross-talk, eliminate electromagnetic interference, and support high-speed signals. The number of layers used for a PCB depends on the application, the operating frequencies, pin density, and the requirement for signal layers.

With a two-layer stack-up, the top layer—or layer 1—works as a signal layer. A four-layer stack-up uses the top and bottom layers—or layers 1 and 4—as the signal layers. In this configuration, layers 2 and 3 serve as the planes. A pre-preg layer bonds two-or-more double-sided boards together and works as a dielectric between the layers. Six-layer PCBs add two more copper layers with the second and fifth layers serving as the planes. Layers 1, 3, 4, and 6 carry signals. 

Management of layers and signal information on your circuit board can be strenuous

Moving on to the dielectrics in a six-layer board, inner layers two and four make up the core while the prepreg consists of dielectric layers one, three, and five. Because the material has not fully cured, the prepreg material remains softer than the core material. PCB manufacturing processes apply heat and pressure to the entire stackup and melt the prepreg and core so that the layers can bond together.

Multi-layer boards add more layers of copper and dielectric to the stackup. In an eight-layer PCB, seven inner rows of dielectric bond the four plane layers and the four signal layers. Ten- and twelve layer boards increase the number of dielectric layers, retain the four plane layers, and increase the number of signal layers.

Using Cadence’s powerful and innovative suite of design tools enables your designs to go as many layers deep as necessary. OrCAD PCB Designer will be sure to map your layout appropriately, allow for the constraints necessary to plan your board in all its iterations and give strong modeling utility to visualize the design while it’s coming together.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.