Impedance vs Resistance: Which is Most Important for PCB Layout?

November 4, 2020 Cadence PCB Solutions

Key Takeaways

  • Why is impedance important for PCB layout?

  • Why is resistance important for PCB layout?

  • Effectively using impedance and resistance for your board layout.

Trying to decide which should be first

Which one is first?

Some of the best entertainment you can have is to watch children eat. I recall spending many meals staring at my daughter, barely bigger than the bib she had on in her high-chair, picking up one item and dropping it just to grab something more appealing.  She appeared to be critically considering which food is the best and should be the first to be eaten. I derived a good deal of joy from observing her as she was obviously oblivious to the fact that both foods would eventually join and work together for her body’s nutrition. 

Similarly, when designing the layout for a circuit board, it can be easy to settle into a routine where certain parameters are given more emphasis than others. This is even more commonplace when the parameters are dependent upon or variations of each other. For example, resistance is or can be used to express virtually all other electrical parameters in a circuit. Nevertheless, this important parameter is often not considered when laying out a PCB. Impedance, on the other hand, which is derived from resistance, is a common design factor. 

Let’s explore this issue by looking at how impedance is used to create board layouts and how resistance affects the design process. This will allow us to compare impedance vs resistance to determine which is more important for laying out your PCB. 

How Impedance is Used for the PCB Layout

The PCB layout design process can be broadly divided into the following activities:

Major PCB Layout Tasks

  • ☐ Material selection tasks

    • ☐ Selecting layer material

    • ☐ Defining layer thicknesses

    • ☐ Choosing solder mask type

  • ☐ Placement tasks

    • ☐ Determining where to place components

    • ☐ Determining locations for drill holes and vias

  • ☐ Routing tasks

    • ☐ Setting trace copper weights

    • ☐ Setting trace lengths and widths

    • ☐ Defining trace paths

    • ☐ Defining via types

  • ☐ Design Checks

    • ☐ DFM compliance

    • ☐ PDN analysis

    • ☐ Thermal analysis

Of the major layout tasks listed above, material selection and routing tasks are most reliant on impedance parameters. For example, it is common practice to define a fixed impedance that in turn defines a dielectric constant,  which is used to define and select material types for the layer stackup, as shown below.

Example of 6-layer fixed impedance stackup

Fixed impedance stackup

For trace routing, impedance is also a major factor. This is especially true for high speed RF signals where changes in impedance can create signal integrity issues; such as noise.  Therefore, impedance is indeed important for PCB layout design, but what about resistance?

The Impact of Resistance on the PCB Layout

There are some cases where impedance can be analyzed without including the resistive or real element. For example, when the reactance is much larger and resistance can be treated as negligible. However, in actuality, resistance plays a role for all electrical circuits as it impacts the level of current flow. For PCB design, the transmission lines or traces are relatively short and copper, which has one of the lowest resistivities, is most often the material used. Therefore, real power losses are typically not an issue; however, eddy current or I2R losses due to changing fields are a concern. 

For board layout design, resistance is most often taken into account when performing design checks; such as power distribution (PDN) and/or thermal analysis. These analyses are typically done by simulating the board’s operation with desired inputs and with environmental conditions similar to what will be nominal for the assembled PCB or PCBA deployment. Thermal analysis, in contrast to PDN simulations, has a primary goal to determine how heat is distributed along the surface and throughout the board. This is important to ascertain what thermal dissipation and/or distribution techniques and devices need to be implemented to ensure your board’s manufacturability and operation.  Thermal analysis and PDN may be performed separately (by different software tools, for example) or using a single advanced tool; such as Cadence’s Celsius Thermal Solver

The importance of resistance for power distribution and thermal analysis is based on the fact that changes in heat significantly alter material properties; such as resistance. Now, we know that both impedance and resistance are important for PCB layout design. However, is one more important than the other?

Impedance vs Resistance or Impedance and Resistance for Board Layout?

As explained above, it is not a question of which is important: impedance or resistance, for PCB layout design. Obviously, both are important, as illustrated in the table below. 

IMPORTANCE OF IMPEDANCE VS RESISTANCE FOR PCB LAYOUT

PCB Layout Tasks

Impedance

Resistance

Material Selection

X

 

Placement

   

Routing

X

X

Design Checks

 

X

The table indicates which major PCB layout tasks are principally dependent upon impedance and/or resistance. From this perspective, neither of these parameters can be conclusively stated to be more important than the other; therefore, for good PCB layout design you should utilize both where they are impactful.  

Irrespective of whether you are laying out your board or performing any other board design tasks, the best option is to utilize the industry’s most comprehensive PCB Design and Analysis platform. With Cadence’s Allegro PCB Editor you have all the functionality needed to design boards to any degree of complexity, while simultaneously evaluating design quality aspects; such as DFM. Additionally, dedicated tools for performing PDN and thermal simulations are integrated for easy incorporation into your design package.  

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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