Standard Board Thickness and Your Layer Stack
I’ll confess that I have a love for 1990’s sitcoms. If Jerry Seinfeld were a PCB designer, he would probably ask “what’s the deal with the 1.57 mm board thickness?” It’s a fair question after all, and it makes you wonder why many standard values (e.g., 50 Ohms impedance on high speed transmission lines) are used in PCB design and other areas of engineering.
There are good reasons why these standard values have found their place in PCB design, and these values have little to do with industry standards. Regarding board thickness, the reason is largely historical, although most manufacturers have tailored their fabrication capabilities to a variety of other thicknesses.
Board Thickness: A History Lesson
Lee Ritchey nicely sums up the history of the apparently standardized 1.57 mm board thickness. Without repeating everything the great Mr. Ritchey says about board thickness, we can briefly summarize the reasons this value became something of a standard within the industry.
When electronic devices were transitioning to transistors and integrated circuits, boards were built using breadboarding on plywood workbenches, where the top layer of a plywood sheet was replaced with a material called Bakelite. If you are familiar with plywood, then you know that the thickness of a single ply in plywood is 1/16 in., or 1.57 mm.
This thickness became something of a standard when board-to-board connections became necessary. Early board-to-board connections used rack units with edge connectors, and these edge connectors needed to match this standard thickness. Nowadays, instead of Bakelite, we use materials that allow etching and plating, such as epoxy laminates like FR4.
If you take a moment to think, you’ll realize there is no reason for this thickness to be preferable over any other thickness other than to accommodate the layer count and copper weight that is used in a particular board. Many manufacturers opt for this board thickness with lower layer counts because that’s the way it’s always been, or in order to accommodate a standardized edge connector. Looking throughout the industry, many routing standards (e.g., PCIe add-in cards) specify this value for board thickness to this day.
As boards have become more complex, and as copper weight and layer counts have increased for different applications, boards tended to become thicker. Two popular board thicknesses are 2.36 mm and 3.18 mm, which just happen to be 150% and 200% of the 1.57 mm board thickness.
DFM and Board Thickness
If you start looking at manufacturer websites, most manufacturers offer fabrication to a variety of thickness values. Although the 1.57 mm value is something of a required capability for any manufacturer, many manufacturers have adapted their capabilities to fabricate boards with various multiples of this thickness.
Different manufacturers can fabricate boards with different layer counts and copper weights, and many will offer their set of “standardized” board thicknesses. Manufacturers will also limit the allowable board size to specific values for different stackups. Other manufacturers will simply limit the panel size to specific dimensions. You should always check your manufacturer’s capabilities before designing your layer stack in order to keep costs in check and ensure manufacturability.
Choosing the right layer thickness, and thus the total board thickness, depends on a number of factors. When working with impedance controlled boards, the thickness needs to be carefully controlled and will depend on the desired trace width. The weight of your board, component compatibility requirements, and required connectors are also significant factors. You’ll need to balance all of these alongside satisfying your manufacturer’s capabilities in order to ensure your board is fully manufacturable.
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