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Beyond Design: Stackup Planning, Part 1

The PCB substrate that physically supports the components, links them together via high-speed interconnects and also distributes high-current power to the ICs is the most critical component of the electronics assembly. The PCB is so fundamental that we often forget that it is a component and, like all components, it must be selected based on specifications in order to achieve the best possible performance of the product. Stackup planning involves careful selection of materials and transmission line parameters to avoid impedance discontinuities, unintentional signal coupling and excessive electromagnetic emissions.  

The complexity of electronics design is undoubtedly going to increase in the future, presenting a new set of challenges for PCB designers. Materials used for the fabrication of multilayer PCBs absorb high frequencies and reduce edge rates, thus putting the materials selection process under tighter scrutiny. Ensuring that your board stackup and impedances are correctly configured is a good basis for stable performance.

So where do we start? Over the years, I have found that many engineers and PCB designers do not understand the basic structure that makes up a substrate. We all know that multilayer PCBs consist of signal and plane layers, dielectric material and soldermask coating, but there is a lot more to it.

The most popular dielectric material is FR-4 and may be in the form of core or prepreg (pre-impregnated) material.  The core material is thin dielectric (cured fiberglass epoxy resin) with copper foil bonded to one or both sides. For instance: Isola's FR406 materials include 5, 8, 9.5, 14, 18, 21, 28, 35, 39, 47, 59 and 93 mil cores. The copper thickness is typically 1/3 to 2 oz. (17 to 70 µm).

The prepreg (B-stage) material is comprised of thin sheets of fiberglass impregnated with uncured epoxy resin which hardens, when heated and pressed, during the PCB fabrication process. Isola’s FR406 materials include 1.7, 2.3, 3.9 and 7.1 mil prepregs that may be combined to achieve thicker prepreg.

To read this article, which appeared in the June 2015 issue of The PCB Design Magazine, click here.