What is PCB Etch Factor?

PCB Etch Factor or etch compensation is an etching process modification used by the PCB manufacturers to decide the extent and accuracy of etching. It helps the designers and manufacturers decide the minimum etching allowance on the track width. During etching, unwanted copper is removed gradually from the PCB surface to realize the desired conductive track. For instance, if a PCB design requires a track width of 0.25 mm, then the manufacturer has to initiate etching if the track width is more than 0.25 mm. The amount by which the track width is increased is called etch factor. It is proportional to the copper weight required to be removed. More copper to be removed indicates a higher value of the etch factor. 

No etching process gives perfect straight cuts, they always create undercut, an etching defect, which can be seen in the figure below.

In mathematical terms, an etch factor is the ratio of track width to the amount of undercut. The PCB CAD designer has to provide enough spacing in the design so that a manufacturer is able to utilize that space to start etching with a little wider track. The minimum spacing in the design must meet the manufacturer’s decided minimum space.