Layer sequence is one of the most important parameters in PCB design. Signal, power and reference layers must be in the correct sequence to achieve the optimum of electrical properties and manufacturing costs. Copper thicknesses as well as the characteristics and thicknesses of the Insulation material used (prepreg, FR4 etc) will have a direct effect on the electrical behaviour of the PCB.
This is especially true for RF designs and those with DEFINED IMPEDANCE tracks, where the correct buildup will ensure that the required impedance values are achieved (within the given tolerances). With SEMI-FLEX the buildup allows the PCB to be bent for the purpose of installation and for IMS the focus of the buildup is on thermo management.
To produce a multilayer PCB the manufacturer places laminates cores and prepregs on top of each in what is known as the lay-up and bond process.
Cores are rigid base material laminated with copper on one or two sides. Prepregs (pre-impregnated) is an epoxy resin sheet that is not yet fully cured. Only when under high pressure and temperature does the resin become adhesive and flows to bond to the other cores and prepregs thus laminating them together.
Rules for the buildup of multilayers
The first important rule for the layer buildup is that non-adhesive layers (cores) must have at least one adhesive layer (prepreg) between them. Whereas several adhesive layers can be placed on top of each other. However, there are many more rules for the construction of multilayers.
This set of rules is mastered by the Eurocircuits layer build-up editor. The eC-Smart tool offers PCB designers over 940 predefined buildups. Depending on the required thickness of the PCB, the number of layers and the structure, the editor displays predefined buildups. The interactive tool shows you which factors influence the price and helps you to choose the most cost-effective layout.
We show you how to use the layer structure editor here – The Buildup Editor User Guide
Click on the video below to see how we make a 4-layer PCB.