Beyond Design: Stackup Planning, Part 1


Reading time ( words)

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.

Share


Suggested Items

Julie Ellis: Communication and Fabrication Knowledge Critical for Designers

11/19/2018 | Andy Shaughnessy, Design007 Magazine
Field Application Engineer Julie Ellis of TTM sees it all: good designs, bad designs, and everything in between. Her classes on proper DFM techniques are always a big draw. She taught at the inaugural AltiumLive in 2017 and was back at this year’s event. I caught up with Julie and asked her to discuss some of the things she covered in class. As she points out, many issues could be eliminated if designers communicated with their fabricators and had a better understanding of how PCBs are manufactured.

Altium Designer Increasingly Used for High-Speed Design

11/05/2018 | Andy Shaughnessy, Design007 Magazine
I recently met with Mark Forbes, the director of technical marketing at Altium, during the AltiumLive event in San Diego. We discussed Mark’s class on MCAD/ECAD collaboration and the success of AltiumLive, as well as the growth of Altium users who design high-speed PCBs with Altium Designer.

Karl-Heinz Fritz on Cicor’s DenciTec Technology

10/24/2018 | Nolan Johnson, I-Connect007
In a recent interview, Karl-Heinz Fritz, VP of technology at Cicor, discusses the business, DenciTec technology, the impact of tariffs on trade, and applications for 3D printing and additive manufacturing, including potential new opportunities for PCB designers.



Copyright © 2018 I-Connect007. All rights reserved.