Shawn,

Etch factor is determined by many factors - etching chemistry, metallic plating or photoresist defining the copper to be removed, spray pressure in the etcher, top or bottom of the board travelling horizontally through the etch machine, pH of the etching solution, ability of the shop to time the board travel (slower means over etch, faster means underetch), etc.  

Etching is something of a chemical art, rather than a precise mechanical operation like drilling.  

I suggest you work with your fabrication shop process engineers.  They know what etch factors they can produce, and the straighter you want your copper conductor walls, the more care (hence higher price) your fabricated board will become.  Most any etch factor is possible - artwork line width compensation, tuning the etcher for your specific boards, running the boards fast twice and flipping the boards top to bottom between runs, etc.  However, each of these steps costs time and productivity, and you will pay for it. 

There is one sister industry to PWB board fabrication that makes a living only etching metal sheets - photochemical machining.  They etch from both sides making relatively staight sidewall parts, in 1 to 50 mil metal thickness, in various metal alloys, and compete with die stamped flat parts. They have the ultimate knowledge of etch factors.  

Denny Fritz



-----Original Message-----
From: Upton, Shawn <[log in to unmask]>
To: TechNet <[log in to unmask]>
Sent: Wed, Jun 20, 2012 11:52 am
Subject: [TN] Etch Factor


Ok, got my specifications, looking at IPC-2221A Figure 10-3, for conductor width 
see section 10.1.1 also, pages 77&79); and IPC-A-600H 3.2.1 (see page 86).  It 
ooks like "etch factor" drives how much of a trapezoid one has on any given 
race, although outer layers (since they are plated) may have overhang.
What I don't see is any qualitative numbers: ie, an etch factor of y +/-z is 
ypical for outer layers, vs w for inner layers.  Are these numbers variable 
rom vendor to vendor; and have to be called out on any given FAB drawing, since 
here isn't a generic number to be pointed at?
Shawn Upton, KB1CKT
est Engineer
llegro MicroSystems, Inc
[log in to unmask]
03.626.2429/fax: 603.641.5336

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