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