Issue link: https://iconnect007.uberflip.com/i/1514189
54 DESIGN007 MAGAZINE I JANUARY 2024 know that dimensions across core materials are quite predictable and the datasheet infor- mation is good to rely on. But suddenly, "it depends" when a foil is laminated onto pre- preg, or two cores are laminated with prepreg with the Z-axis dimensions. Datasheets and Interpretations When you look at a stack schematic as in Figure 1, the labels showing core, prepreg, and the foils are separate from the prepreg mate- rials. But to the uninitiated, it is not so clear what is pressing into what and which dimen- sions are fixed. e schematic view of a stackup is a very clear illustration of the content of the stack, but it is not to scale, and more importantly, it does not show graphically where the traces impress into the prepreg, nor visually show the plated thickness on drilled or double-plated thick- ness on sequentially laminated traces where needed. Figure 2 shows the schematic overview of the stack alongside a proportionally scaled view so you can see just how much the pressed stack varies from the schematic stack. I have intentionally overlaid the propor- tional stack on the right of the schematic stack to show how the copper presses into the pre- pregs in contrast to the copper on the core side where its Z-dimension across the core is fixed. is illustration shows that new designers oen wish to get accurate data from the mate- rial datasheet. e fact that PCBs are laid-up cores, followed by prepregs, cores, prepregs, and then foils means that the isolation dis- tance between the copper on a core and the next core will depend on the amount the cop- per traces impress into the prepreg that flows during the heat and pressure of the press cycle. Stackup tools can make a good prediction of this, given the copper density of the tracking layers, but there will always be a degree of vari- ation from fabricator to fabricator, depending on their press equipment and settings. is doesn't make the information on isolation dis- Figure 1: Schematic view of a stackup.