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.