Wednesday, February 06, 2013 | Lee W. Ritchey, Speeding Edge
This article originally appeared in the November 2012 issue of The PCB Design Magazine.
The practice that has worked in the past of allowing the fabricator to design a PCB stackup no longer works. Speeds have increased to the point where signal integrity and power delivery considerations make it necessary to employ far more discipline in the choice of materials and arrangement of the layers in the stackup – both of which are outside the skill set of virtually all PCB fabricators. A common part of the vendor selection process has been to submit each design for a quote to several fabricators and make the choice based on price only. This is very often a fatal method of vendor selection because the lowest bidder often takes shortcuts in order to achieve the lowest price and/or may not possess the skills necessary to manufacture quality PCBs of the given complexity.
Among the demands placed on stackup design are:
• Providing enough signal layers to allow successful routing of all signals to signal integrity rules.
• Creating copper thickness in planes and signal layers that meets the conductivity demands of signals and power and, at the same time, be reasonable to manufacture.
• Providing enough power and ground layers to meet the needs of the power delivery system.
• Specifying dimension trace widths and dielectric thicknesses that allow impedance targets to be met.
• Ensuring that the spacing between signal layers and their adjacent planes is thin enough to satisfy crosstalk needs.
• Specifying dielectric materials that are economical to manufacture and readily available.
• Avoiding the use of expensive techniques such as blind and buried vias and build up processing if possible.
• Providing for prototype manufacture in one factory or country and production manufacture in another factory or country.
Information needed to design a successful PCB stackup is scattered among many documents and specifications, many of which are not in the public domain. This document is intended to bring all of the necessary information into one document to improve the design process.
How a Typical Multilayer PCB is Built
In order to understand the choices that must be made when designing a PCB stackup, it is useful to review how a typical multilayer PCB is fabricated. Figure 1 is a diagram showing the components that make up a six-layer PCB. The manufacturing method shown in the figure is referred to as foil lamination. This refers to the fact that the two outer layers begin as sheets of foil copper with no images etched into them. This is the most cost-effective way there is to manufacture a multilayer PCB and should be the objective of the stackup design process.