Mastering Parasitic Extraction in PCBs with EMWorks

Introduction

In the realm of modern electronics, printed circuit boards (PCBs) play a vital role in connecting and powering various components. However, as PCB designs become more complex and high-frequency signals traverse their intricate pathways, an often-overlooked factor can significantly impact performance—parasitic elements. To shed light on these hidden effects and enable optimized PCB designs, engineers rely on the power of electromagnetic finite element analysis (FEA) tools for accurate parasitic extraction. In this blog, we delve into the fascinating world of parasitic extraction and explore how the EMS for SOLIDWORKS, an electromagnetic FEA tool, aids in this process.

Understanding Parasitic Elements

A PCB (Printed Circuit Board) structure refers to the arrangement and organization of components, electrical traces, and other elements on a printed circuit board. It encompasses the physical layout and configuration of the PCB, including the layers, traces, pads, vias, and other elements that make up the circuit board.

Parasitic elements refer to unintended components and effects that emerge within a PCB due to its physical structure and interconnections. These elements include capacitance, inductance, and resistance, which can cause signal degradation, crosstalk, and unwanted resonances. These elements can have a significant impact on the performance of the PCB, especially at high frequencies. By identifying and quantifying these parasitic effects, engineers can make informed design choices and minimize their impact.

Let's explore the diverse types of parasitic elements commonly encountered in PCBs:

Capacitance

Capacitance arises from the electric field between conductive elements in close proximity, such as traces, pads, and planes. It can lead to coupling between signals, resulting in crosstalk and signal degradation. Parasitic capacitance can also cause unwanted resonances and affect the impedance characteristics of traces and components.

Inductance

Inductance is associated with the magnetic field generated by current flowing through conductive elements, such as traces and vias. It opposes changes in current and can cause signal delays, impedance variations, and unwanted coupling. Parasitic inductance can lead to voltage drops, affect signal integrity, and contribute to electromagnetic interference (EMI) issues.

Resistance

Resistance is the inherent property of conductive materials that restricts the flow of current. In PCBs, resistance arises from trace and via materials, as well as component leads and interconnects. Parasitic resistance can cause voltage drops, and power losses, and affect signal integrity, particularly in high-speed designs.

Mutual Inductance

Mutual inductance occurs when the magnetic fields of two or more adjacent conductive elements couple together. It can induce unwanted coupling and affect signal integrity, especially in closely spaced traces or in power delivery networks.

Skin Effect

At high frequencies, the skin effect becomes significant, causing the current to concentrate near the surface of conductors. This effect increases effective resistance and alters the current distribution within the conductive elements, impacting their performance.

Dielectric Loss

Dielectric loss refers to the dissipation of energy within the dielectric materials used in PCBs. It can result in power losses, signal attenuation, and affect the performance of high-frequency circuits.

EMWorks Solutions

EMS for SOLIDWORKS, an electromagnetic FEA tool developed by EMWorks. Inc., provides a powerful framework for simulating and analyzing the behavior of electromagnetic fields within PCBs. These software packages utilize numerical methods, such as the finite element method (FEM), to solve Maxwell's equations and capture the complex interactions of electric and magnetic fields.

To be able to extract the parasitic parameters using EMS for SOLIDWORKS, the following steps should be followed:

Model Creation

The first step in parasitic extraction using EMS involves creating a detailed 3D model of the PCB. This entails accurately representing the physical geometry, including traces and components. 

Material Assignment

To simulate real-world behavior, appropriate material properties must be assigned to different regions of the PCB model. Dielectric constants, conductivity, and magnetic permeability are key parameters that influence electromagnetic interactions. By inputting accurate material data, EMS ensures a reliable simulation of parasitic effects.

Mesh Generation

Generating a high-quality mesh is crucial for capturing fine details of the PCB structure accurately. EMS for SOLIDWORKS offers flexible meshing capabilities, allowing engineers to control element size and quality. Fine-tuning the mesh ensures precise calculations while maintaining computational efficiency.

Boundary Conditions

Defining proper boundary conditions is essential for an accurate simulation. Engineers must set up appropriate excitation sources, such as voltage or current sources, and specify ground connections or reference planes. These boundary conditions mimic real-world scenarios and help simulate the desired electrical behavior of the PCB.

Solver Settings

Configuring the solver settings is a critical step in the parasitic extraction process. Engineers select numerical methods, convergence criteria, and simulation parameters such as frequency or time domain. Optimizing these settings ensures accurate results within a reasonable computational time frame.

Simulation Run

With the model, materials, mesh, and solver settings in place, it's time to run the simulation. EMS for SOLIDWORKS solves the electromagnetic field equations, providing insights into the distribution of electric and magnetic fields within the PCB. This comprehensive analysis accounts for the effects of parasitic elements, unraveling their impact on signal integrity and performance.

A Basic Case Study

A PCB structure shown in Figure 1, contains two copper traces on a 4-oz FR4 PCB square board and a 5 mils thickness copper Ground. One copper trace will be present on the top layer, while the other trace will be on the bottom layer. These copper traces are used to route electrical signals between different components and provide connectivity on the board.

The most common type of PCB material is FR4, which is a flame-retardant fiberglass epoxy laminate. FR4 is widely used due to its excellent electrical insulation properties and mechanical strength.

Fig. 1. SOLIDWORKS Model of the PCB Structure Used for Simulation

Parasitic Capacitance Calculation

To compute the parasitic capacitance of the PCB structure the Electrostatic module of EMS is invoked. This type of analysis helps in understanding the electrical behavior of the PCB and identifying potential parasitic effects that may affect its performance. 

To account for the capacitance of a given conductor:

• Assign the electrical properties to the relevant components in the model. This includes specifying the dielectric constant (εr) for the PCB material and the conductivity of the copper traces.
• Define the boundary conditions for the analysis. Typically, you would assign a floating boundary condition, including the ground plane, in EMS.  

As a result, this PCB structure has three floating conductors, i.e., the left and right traces as well as the ground plane. 

After the analysis is complete, you can extract the capacitance values from the simulation results. The EMS capacitance results are shown in Figure 2.  

Fig. 2. Parasitic Capacitance Computed by EMS

Parasitic DC Inductance and DC Resistance Calculation

To compute the Parasitic DC resistance and Parasitic DC inductance of the PCB structure, you would select EMS Magnetostatic analysis if you're interested in the behavior of inductance and resistance at DC or low frequencies.

To do that, proceed with the following steps:

• Assign the electrical properties to the relevant components in the model. This includes specifying the conductivity of the traces and other conductive elements in the PCB structure.
• Define the boundary conditions for the analysis. This typically involves specifying the appropriate current paths or voltage sources for the relevant components or conductive surfaces: The copper traces are modeled as coils.

The computed Parasitic DC Inductance & DC Resistance are shown in Figure 3 and Figure 4 respectively.

Fig. 3. Parasitic DC Resistance Computed by EMS

Fig. 4. Parasitic DC Inductance Computed by EMS

The distribution of the electric field and the Potential inside the PCB structure are shown in Figure 5 and Figure 6 respectively.

Fig. 5. The Electric Field Distribution inside the Structure

Fig. 6. The Potential Distribution inside the PCB Structure

Parasitic AC Inductance and AC Resistance Calculation

In addition to DC inductance and resistance calculation, EMS is equipped with AC Magnetic and eddy current capabilities which are used to compute the AC resistance and AC loop inductance for the PCB structure at hand at the frequencies: 1Khz, 2Khz, 5Khz, 10 KHz, 20 KHz, 50 KHz, 100 KHz, 200 KHz, 500 KHz, and 1 Mhz.

To compute Parasitic AC Inductance & AC Resistance:

• Assign appropriate material properties to the different parts of the geometry
• Define the boundary conditions for the analysis. This involves specifying the excitation source: The two traces are modeled as coils.  

Figure 7 shows the AC resistance for frequencies from 1 KHz to 1 MHz computed by EMS and compared to Reference [1].  Whereas Figure 8 shows the AC inductance results and comparison for the same frequency range.

Fig. 7. AC Resistance Computed by EMS and Compared to Reference [1]

Fig. 8. AC Inductance Computed by EMS and Compared to Reference [1]

Parasitic elements are intrinsic to PCBs and can significantly impact their performance. By understanding and effectively managing these parasitic effects, engineers can optimize their designs, enhance signal integrity, reduce crosstalk, and improve the overall reliability of the PCBs. EMS for SOLIDWORKS provides valuable insights and simulations, allowing engineers to quantify and mitigate the impact of parasitic elements, leading to more robust and efficient PCB designs.

References

[1] Jingen Qian, “RF Models for Active IPEMs”, MS Thesis in Electrical Engineering,  Virginia Polytechnic Institute and State University, January 31, 2003.