Impedance Calculations and PCB Stackup Design in Altium Designer

There are three demands placed on stackup design: controlled impedance, crosstalk control, and the need for interplane capacitance. Impedance in your traces becomes a critical parameter to consider during stackup design for high speed PCBs, but the stackup will also influence crosstalk susceptibility and sheet capacitance between ground and power planes. Instead of using a homemade tool to work out the impedance of each trace in your PCB, Altium Designer incorporates PCB impedance calculations as part of your design rules. You’ll be able to manage the impedance throughout your board based on the stackup you need for your next PCB.

ALTIUM DESIGNER

A unified, heavily rules-driven PCB design platform with built-in PCB impedance calculations.

In the early days of circuit board design and fabrication, logic circuits were so slow that the only concerns were making connections between logic or discrete parts and providing a path for DC power to each part. All a designer needed to do was provide enough signal layers for all wires and enough copper in power paths to deliver DC power with minimum of sag or droop. The glass cloth in the laminate and prepreg did not matter, nor was the resin system important. The goal was the lowest priced PCB that would stand up to the soldering process.

Today, ICs are fast enough that problems such as reflections and crosstalk are prominent in any high speed board. ECL was the first logic family that made these problems prominent. Few of the early engineers designing with TTL and CMOS had any understanding of how to design a PCB with controlled impedance. In the past, engineers would simply demand the fabricator deliver PCBs with a known impedance, usually 50 Ohms.

Since impedance control is so important, designers need to perform impedance calculations and PCB stackup design simultaneously. There are calculators on the internet that can give you some accurate results at specific frequencies, but they cannot account for dispersion throughout the signal bandwidth. They also cannot account for complicated trace geometries and coupling in real boards. If you have an impedance calculator integrated into your design tools, you can quickly determine impedance for any trace geometry as you create your PCB.

You can control the impedance of all these trace by designing the right stackup and trace geometry.

Controlled Impedance with the Right Stackup

Since all modern digital systems are effectively high speed devices, impedance control is now a priority. The impedance of the traces in your PCB depends on their geometry, material, and arrangement. Their configuration within the board also affects the impedance seen by signals in your traces. Edge-coupled, embedded, and offset trace arrangements all have variations in their impedance values, and you’ll need design software that can account for the impedance of these different geometries.

A number of equations are used to define and calculate various design parameters in PCBs, just like any other area of engineering. The equations for trace impedance must account for everything from the substrate dielectric properties to the geometry of your traces. The most cited formula is the IPC-2141 formulas for microstrip impedance:

IPC-2141 microstrip impedance formula

This formula is known to produce up to 7% error in certain frequency ranges, which is unacceptable for ultra-high speed applications and in multilevel signalling. The trace impedance determined with Wadell’s equations is more accurate, reaching less than 1% error. Here, you need to account for the distance to the nearest ground plane in your PCB stackup, which you can specify in your PCB design tools.

How Your Stackup Helps Control Impedance

The figure below shows an example of the tradeoffs between routing layers and power plane capacitance for a 10-layer PCB (bold lines are plane layers, P = prepreg, L = laminate). The stackup on the left side has six signal layers, but it only has one pair of planes closely spaced. This is good for routing space, but not so good for power delivery if there is a need for interplane capacitance. The stackup on the right has only six routing layers, but it now has two sets of plane pairs. This is good for interplane capacitance, but not as good for routing space.

Two example stackups for 10-layer PCB

In both cases above, all the signal layers are mated with planes across pieces of laminate except the two outer layers. When the power and ground planes are closer together, they provide more interplane capacitance, thus the two planes act like a large decoupling capacitor. If the outer layers are to be used for controlled impedance traces, allowance must be made for the fact that their tolerance will not be as good as the buried signal layers.

Getting your stackup just right means that you need software that lets you fully customize your layer stack, right down to selecting the appropriate dielectric constant for your board layers. A stackup manager that links to material library helps greatly as you can choose from common board materials when planning your stackup. You can select the right thickness for your layers and define the best trace arrangement for your board.

Once the number of power planes, ground planes, and signal layers have been determined for a given design arranging them in such a way that signal integrity rules are complied with and power delivery needs are met is a series of tradeoffs. If there is a need for interplane capacitance it will be necessary to arrange the layers so ground and voltage planes are spaced close to each other.

• The capacitance between plane layers determines PDN impedance and your trace impedance.

  Learn more about interplane capacitance, power integrity, and signal integrity.

• Your stackup affects your trace impedance, and you can take advantage of this to control impedance throughout your board. Learn more about your layer stackup and how it affects trace impedance.
• If you’re not familiar with the IPC formulas or Wadell’s formulas for trace impedance, you can use these to calculate impedance with high accuracy in standard and advanced laminates. Learn more about trace impedance formulas and calculators.

Defining rules and constraints for a high speed PCB in Altium Designer

Impedance Calculations and PCB Stackup Design

The value of H in the above equation accounts for your layer spacing in your stackup, which plays an important role in determining the impedance seen by your signals, as well as some other important aspects of high speed circuit board design. The layer arrangement and separation in your stackup affect three important aspects of high speed PCB design:

• Interplane capacitance: This is critical for maintaining stable power in high speed devices. Placing power and ground planes on adjacent layers with thin separation provides high interplane capacitance, as mentioned above.
• Parasitic inductance: The loop inductance for traces determines susceptibility to inductive crosstalk. When the layer spacing is smaller, the loop inductance is also smaller.
• Coupling between traces: Traces will couple inductively and capacitively, and the layer spacing plays a role in determining this level of coupling. This, in turn, determines the values of even, odd, and differential impedance.

The dielectric constant of your substrate also determines your trace impedance, as shown in the above equation. The dielectric properties of your substrate also need to be included in your impedance calculations and PCB stackup design. This is because the dielectric constant varies with frequency, which will affect coupling, crosstalk, and trace impedance.

Accurate Impedance Calculations vs. Frequency

A question that often arises is, at what frequency impedance calculations should be performed? Modern ICs have rise times well below 1 nanosecond. It is the rise time that is important when calculating impedance. For binary signalling, the knee frequency harmonic of a 1 nanosecond edge is about 0.35 GHz. Therefore, to have an impedance that is properly matched at this frequency, the dielectric constant value used in this calculation should be the 0.7 GHz value. Getting the right impedance requires dispersion data built into your impedance calculation tools.

One of the reasons that PCB fabricators do not get impedance calculations right is that they often use the typical value of dielectric constant listed on the data sheet which is usually the 1 MHz value. If the dielectric constant varies with both frequency and resin content where does one obtain the correct values? The best PCB design software integrates these capabilities using data in a materials library, which will account for dispersion in the dielectric.

Because of these complications involved in accurate impedance calculations and PCB stackup design, your impedance calculator needs to consider trace geometry and dispersion simultaneously. This is why the best PCB trace impedance calculators are built on an integrated 3D field solver. This type of impedance calculator provides the most accurate results for any geometry.

• Your stackup affects your trace impedance, and you can take advantage of this to control impedance throughout your board. Learn more about your layer stackup and how it affects trace impedance.
• Any impedance calculation needs to account for dispersion in the dielectric substrate in order to produce accurate results at any frequency. Learn more about dispersion in FR4 and other substrates.
• If you must terminate certain elements in your PCB, there are a number of standard termination methods that you should use. Learn more about integrated field solvers in your PCB design software.

Impedance calculations and PCB stackup design for these traces takes a calculator with an integrated field solver.

Unifying Signal Integrity Tools in a Rules-Driven Environment

Working with software that includes impedance calculations as part of your design rules allows you to automatically check that the impedance in your traces is properly controlled. You won’t have to manually check every single trace. Instead, you can specify the calculation you need for your trace arrangement and substrate, and your design tools will automatically check your routing to ensure controlled impedance throughout your printed circuit board layout.

The rest of the PCB design industry still separates your critical design, management, and verification features into separate programs with different workflows. When you need to control the impedance of single-ended or differential traces in your PCB, you’ll have to use multiple design tools that each focus on a different aspect of your final PCB. Working with design software that separates tools into different windows reduces your productivity, making it difficult to address what should be simple design issues.

Your design tools, design rules, component models, and simulation packages should be placed in an integrated environment with a consistent workflow. Features in this environment are all accessible from a single program, and new features can be easily added to your design software as extensions. This is exactly the type of design environment you’ll find in Altium Designer.

Defining trace geometry and impedance control tolerances as design rules in Altium Designer

Impedance Calculations and PCB Stackup Design in Altium Designer

Impedance calculations can be done by hand, but you’ll get the most accurate results for any frequency and geometry when your impedance calculator is integrated with your PCB design tools. This type of unique integration of multiple design tools is only possible when you work in a fully unified design environment. Altium Designer is the only PCB design software package that runs on a heavily rules-driven design engine, and all your design verification features interface with your design tools in a single interface.

• At the core of Altium Designer is a rules-driven design engine. This unique software architecture allows all your design tools to communicate with the same data model. Learn more about the rules-driven design engine in Altium Designer.
• Your layer stackup determines your trace and PDN impedance throughout your board. Your layer stack manager should allow you to customize your layer arrangement and define the impedance required for your application. Learn more about the advanced layer stack manager in Altium Designer.
• Impedance calculations, propagation delay, reflections, and crosstalk are easy with the integrated field solver in Altium Designer. Learn more about Simberian’s integrated field solver in Altium Designer.

Instead of scouring the internet every time you need help, you can get access to all the design support you need directly from Altium. Everyone that uses Altium Designer can access the AltiumLive forum, podcasts with professional designers, a thorough knowledge base, and webinars provided by industry experts. Altium doesn’t leave you to design alone and provides all the support you need.

If you’re tired of working with separate design tools in different programs, you need to use Altium Designer’s unified circuit board design platform. You can access all your important design tools for high speed PCB design and signal integrity analysis in a single program. Instead of guessing at best trace geometry for your next PCB, Altium Designer can help you get it right the first time with integrated impedance calculations and PCB stackup design tools.

Altium Designer on Altium 365 delivers an unprecedented amount of integration to the electronics industry until now relegated to the world of software development, allowing designers to work from home and reach unprecedented levels of efficiency.

We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. You can check the product page for a more in-depth feature description or one of the On-Demand Webinars.