Device-Specific Power Delivery Network (PDN) Tool 2.0 User Guide

ID 683293
Date 8/24/2021
Public
Document Table of Contents

1.2.2.5. Cap_Mount

The Cap Mount tab calculates the capacitor mounting inductance seen by the decoupling capacitor.

Note: Power rails on different layers have different mounting inductances. For the best results, run the PDN separately for each layer set.
Figure 17. Cap Mount Tab

The capacitor mounting calculation is based on the assumption that the decoupling capacitor is a two-terminal device. The capacitor mounting calculation is applicable to any two-terminal capacitor with the following footprints: 0201, 0402, 0603, 0805, and 1206. Enter all the information relevant to your layout, and the tool provides a mounting inductance for a capacitor mounted on either the top or bottom layer of the board. Depending on the layout, you can choose between VOE (Via on End) or VOS (Via on Side) to achieve an accurate capacitor mounting inductance value. Generally, VOS can have lower mounting inductance due to a smaller via pitch. Also, X2Y cap can be considered as a solution for a space-limited design.

If you plan to use a footprint capacitor other than a regular two-terminal capacitor or X2Y capacitor for decoupling, you can skip the Cap Mount tab. In this case, you can directly enter the capacitor parasitics and capacitor mounting inductance in the Library tab (under the Custom field in the Decoupling Cap section of the library). As with the other tabs, you can save the changes made to the tab, restore the changes, or restore the tab back to the default settings.

You must pay special attention to the via lengths for the capacitors. Via inductance comprises a substantial portion of the PDN impedance.