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 !

How to be a Circuit Board Designer by Jack

Jack Olson has some excellent PCB Design Advice.  I especially like the Thermal Expansion Animation and CAD to CAM Sections.

How to be a Circuit Board Designer by Jack Olson

Z Axis Thermal Expansion Animated

CAD to CAM 

Thank you Jack for sharing.
Randy

Intertronics PCB Repair Guide

Comprehensive PCB Repair Guide

Download Intertronics PCB Repair Guide

RoHS Boards - 50% Reduction in Reliability

According an Article posted by Testing Methodologies, RoHS assembly temperatures have placed stresses on PCBs to the point that we are experiencing a 50% reduction in reliability of properly designed boards.

Full Article: FindArticles / Technology / Printed Circuit Design & Manufacture / May, 2008

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