How to Calculate the PCB Stackup for a Multilayer PCB

PCB Stackup

PCBs can be built with a number of layers. Each layer contains copper and insulating materials, and can contain vias (drilled holes or buried). Vias connect the layers of the board and provide routes for power and signals. The PCB stackup is the arrangement of these layers, and it has significant impacts on signal integrity and EMI performance. To determine the best stackup for a particular design, you must consider various factors such as layer types, thickness, material, trace width/spacing and copper weight. These decisions can be made easier with the use of a PCB stackup calculator, such as Speedstack in Altium Designer.

The layers of a multilayer PCB are formed from core and prepreg laminates. Core is a glass-reinforced epoxy laminate with a copper plating on one side. Prepreg is a similar product that has been heat treated to bond the core layers together and to a copper cladding on the other. Both laminates are available in a wide range of thicknesses, with different dielectric constants, loss tangent values and thermal conductivities.

For maximum performance, it is important to have a symmetrical PCB stack-up with parallel signal and ground planes. A symmetrical layout also helps reduce the overall height of the pcb, making it more compact and affordable.

How to Calculate the PCB Stackup for a Multilayer PCB

There are a few other objectives that the designer must keep in mind when planning the stackup. These include interplane capacitance, parasitic inductance, shielding and electromagnetic interference (EMI). To minimize these issues, signal layers should be adjacent to the power and ground planes, and signal traces should have a narrower separation between themselves than non-signal layers. This reduces radiated energy and improves noise immunity.

In order to achieve these goals, the designer must work with their fabricator to ensure the PCB will have a proper impedance structure. This is achieved by etching and plating the proper width of the traces, keeping their distance from each other within certain limits, and ensuring that the layers are properly aligned and stacked.

The PCB stackup configuration is also impacted by the fabrication method used to create the board. For example, a 4-layer PCB could be produced in a single lamination cycle using a through-hole configuration. However, a high-density interconnect (HDI) design requires multiple lamination cycles and is usually more expensive than a through-hole design.

It is important to communicate with the manufacturer at an early stage in the design process to understand their capabilities. This will help to ensure the stack-up is compatible with mass production and that their manufacturing processes are capable of meeting the design requirements for track widths, spacing, drill sizes and other critical specifications. It will also help to avoid the potential for costly mistakes in the design and layout stages that could result in EMI distortion, signal delay or other critical problems during the production phase.