Field Solvers for Transmission Line Analysis: How Similar Are They?

Printed Circuit Design & Fab Online Magazine - Field Solvers for Transmission Line Analysis: How Similar Are They?:

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AppCAD Version 4.0.0

AppCAD is one my personal favorites for RF calculations.

Link to AppCAd Home Page and Download: AppCAD:

click on image to view

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TxLine 2003 - Impedance Calculator

One of my favorite Impedance Calculators is TxLine 2003.

This calculator is quite good, the CBCPW calculation is the best I have found.

Here a screen shot of the microstrip calculation.

For microstrip calculations, the Effective Dielectric Constant is derived from a combination of the resin, glass and air. It does not include the effects of soldermask. However it good enough to get you within 50 ohms +/- 5%.

Phase Constant

Phase Constant sounds like something complicated, until you realize it's nothing more than angular rotation of phase in degrees of a sine wave for a selected unit of length.  

In the screen shot above I intentionally manipulated the Dielectric Constant (Er) while keeping the electrical length set to 360 degrees until the phase Constant equaled 60 degrees. 

As shown the Physical length of the microstrip at 1GHz wtih a dielectric constant of 5.62213 equals 60 degrees per inch. So 6 inches = 360 degrees (6 x 60 = 360). Pretty simple math.

Critical Length 

We have often heard about the term critical trace length for high speed and RF designs. Most often you hear numbers like 1/7 or 1/10 of a wavelength as being the critical trace length.

The critical trace length is simply the length at which you must use a good transmission line to send transmit and receive signals and on your PCB.

Digital circuit designers are familiar with Rule of Thumb #1 Bandwidth of a signal from its rise time.

Differential Impedance . . . finally made simple

Eric Bogatin


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The Critical Length of a Transmission Line

"In this article, we show that if the length of the interconnect, in inches, is longer than 2 x the rise time of the signal, in nsec, its transmission line properties are important, and the board should be  designed with controlled impedance, and a termination strategy implemented." 

Link to Full Article PDF:

Dr. Eric Bogatin 

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Nickel-Gold Plating Copper PCB Traces

Source: Polar Instruments Application Note AP171

"Recently a number of customers approached us because impedance controlled tracks displayed higher losses than expected. This has been evident as the impedance test trace has sloped upwards across the tested area. While on fine line traces it is quite normal to see a gently sloping trace, the level of slope (and hence loss) was much higher than predicted — so much so that Polar decided to investigate further.  Eventually we discovered that the entire length of the trace had been nickel plated in the cases we investigated. While nickel is acceptable on short lengths of pad to accommodate gold plating, plating the whole trace length is generally not a good idea. This application note explains the effect that nickel will have on high frequency transmission lines."

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Microstrip Propagation Times - Slower than we Think

UltraCad - Microstrip Propagation.pdf

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RB/CPW Pitch (Gap)

RB/CPW Line Width and Pitch (Gap)

Pitch is the center to center distance between two lines of the same width.



For RB/CPW calculation:

Pitch minus line Width equals Gap. Pitch - Width = Gap (copper to copper clearance).

Using the example shown above (Pitch) 0.0336" minus (Width) 0.0196" = (Gap) 0.014"


That's It !

Microstrip and Stripline Design and Rise Time

MT-094.pdf (application/pdf Object)

When to use Controlled Impedance Traces - Analog Design

Why Effective Er is not the same as Bulk Er

EffectiveEr.pdf (application/pdf Object)

50 Ohm Two Layer RF Prototype - CPWG

Fast and cheap two layer RF Prototype Board:

50 Ohm CPWG Tips for 062 Boards - Steve Hageman

Controlled Impedance

Controlled Impedance Articles:

Which Impedance Calculator is Right ? Randy Clemmons

What is Characteristic Impedance - Eric Bogatin

The How and Why of Obtaining Accurate Impedance Calculations - Lee Ritchey

Resistive Nickel Losses - AP17 Polar Instruments

Optimizing Impedance Discontinuities of SMD Pads  Altera - AN-530

Online Trace Impedance Calculator Links:

cgi-wcalc - Dan McMahill

EMC LAB INFO

Circuit Sage

Mantaro Impedance Calculators  Java - PCB Related Calculations

Desktop Trace Impedance Calculator Links:

AWR TxLine 2003 (Free) Excellent Tool

Rogers MWI-2014 Software (Free)

Saturn PCB Toolkit (Free) - PCB Design Calculator

UltraCAD Design Inc.  (Free / Buy) - Douglas Brooks
ICD has a Stackup Calculator and Offers a 14 Day Evaluation

VIA Impedance:  

Designing Controlled-impedance Vias  Thomas Neu - Texas Instruments

The Poor Man’s PCB Via Modeling Methodology  Bert Simonovich's Design Notes

Which Impedance Calculator is Right ?

Regarding controlled impedance calculators. What I learned in the IPC CID+ Class is that most impedance calculators only get you in the ballpark and that the only calculation that counts is the one that your fabricator uses to meet the design requirements.

Every fabricator has a slightly different process.  Some use more subtractive (Etching) processes while others will use more additive (plating) processes.  

Polar Instruments impedance calculator seems to be very popular with many fabricators. However, in addition to the calculator's results the fabricators apply their own correction factors based on experience.

You can send the same design with the same stack up and material call outs to a 2 or 3 fabricators and they will come back with different results and recommendations.

I typically use the Saturn PCB Toolkit to estimate trace widths, then consult with the fabricator when precise (+/-10%) controlled impedance lines are required.

There are plenty of free impedance calculators on the the internet, however you should consult with your fabricator if you need controlled impedance traces.

For FR4 materials the dielectric constant (also known as Er or Dk) can vary greatly from ~3.8 to 4.7.  And the Dk number in the datasheet for the material is the neat resin number. The neat resin number is the epoxy only Dk, before factoring in the fiberglass woven material.   

The final Dk and is a function of the resin type, glass type, and the ratio of resin to glass.  Also the glass weave can have a significant influence on controlled impedance of FR4 traces.

Here are three of the most popular glass weaves used in FR4 laminates.

 







That's it !

Trapezoid Traces - Etch Factor 1:1

Pattern Etching Process

First the etch resist is applied to define the copper patterns on the laminate.

The etching solution starts dissolving the copper foil on the laminate which is covered by resist.

The resist creates protected areas which are not etched away.  The etching process turns what was a solid copper area in to the desired pattern (traces and pads).

It takes time for the etching solution to work its way down through the copper. The upper portion of the copper is exposed to the solution longer than the copper at the base of the trace.

The etching process is timed controlled. The copper area is dissolved from the top down by the etching solution.

When etching reaches the base of the trace as shown in After Etching Process image the board is removed from the solution and rinsed to prevent over etching.

During the etching process the upper portion of the trace has been exposed to the solution longer than the bottom edge of the trace, thus creating the trapezoid trace shape.  

During the etching process the solution undercuts the resist as shown in the After Etching  Process image.

Blue is the Etching Solution.
Orange is the Copper Foil.
Green is the Laminate.


Before Etching Process

 

After Etching Process

 

The Etch Factor = Y Divided by X.

The Etch Factor is typically 1:1 (X=Y) which reduces the total cross section area of the trace by the thickness of the copper squared.

Properly calculating the cross section of small traces can be important if you need to determine how much current the trace can handle without rising above a given temperature.

The Free Saturn PCB Toolkit Calculator has an option which includes the Etch Factor Compensation for precise Conductor Cross Section Calculations.

Saturn PCB Toolkit
 
Randy