How to Design Stackups for Electronics Boards


Design Stackups for Electronics Boards

When designing a printed circuit board (PCB) you need to make tradeoffs between the number of signal layers, the placement of power planes and grounding layers and interplane capacitance. These tradeoffs are dictated by the operating frequencies and the sensitivity of the signals to interference. The goal is to arrange the signal layers, power planes and grounding layers in such a manner that signal integrity rules are complied with and the board meets the power delivery and EMI control requirements.

Generally speaking, a PCB is made up of a core laminate and a series of prepreg and component laminates. The core laminate is a sheet of cured dielectric material that is sandwiched between two layers of copper. The prepreg, also known as B stage, is a layer of woven fiberglass cloth that has been coated with a resin system to serve as the “glue” for the components and the drilled holes for vias. Choosing the optimum layer thickness for each material is critical as the insulation and adhesion properties of different materials vary.

The conventional four-layer PCB stack-up shown in Figure 2a does not offer any significant improvement in EMI control or signal isolation. The stack-up in Figure 2b is marginally better from the EMI standpoint, but is still not good enough. Both of these stack-ups require careful routing of the traces to keep them well away from the islands of poured power and ground.

How to Design Stackups for Electronics Boards

A six-layer electronics board is more appropriate for these applications. It allows you to have two centered stripline signal layers and one centered power and ground pair. This is a much more suitable stack-up for EMI control, signal isolation and power delivery.

However, there are times when a hybrid construction is required to meet specific design requirements for both RF and digital signals. In these cases, it may be necessary to use a combination of different materials in order to meet signal integrity, heat dissipation and cost.

When this is the case, a standard PCB fabrication drawing should be created that lists the different layers, their names, types and thicknesses. This should also include the dielectric material specifications, controlled impedance requirements, manufacturing tolerances and testing information.

In addition, a fabricator must be specified for each layer. This will allow the fabricator to ensure that the proper materials are used for the desired purpose.

When using a hybrid construction it is important that the design engineer communicates with the fabricator so the proper layers of each material are chosen. It is also essential that the design engineer understands the fabricator’s process and capabilities. This will help them provide accurate dimensional and routing data to the fabricator for the most efficient fabrication possible. A properly designed hybrid PCB can result in a more efficient and cost effective solution for high-speed, high-frequency signaling, EMI control and other electronic assembly challenges. Please contact us with your PCB design needs today. We offer quick turnaround times and exceptional customer service!

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