From IPC T-50:

Etch Factor (term - 54.0452)
The ratio of the depth of etch to the amount of lateral etch,
i.e., the ratio of conductor thickness to the amount of
undercut. (See Figure E-2.)

Sorry that only Wee Mei will probably see the attached graphic from IPC T-50. 

Anyway, an etch factor of 1 is basically horrible.  Any good shop should be able to etch down faster than the lateral undercut of the etchant.  This is particularly true for photoresist or tin defined spaces to be etched, and alkaline etchants.  Acid etchants - cupric chloride with photoresist (cupric attacks tin) should be basically comparable to alkaline for a higher etch factor in most cases.  Nickel/gold electroplate (not common these days) is more tricky with alkaline etchant.  An electrochemical reaction is set up between the copper and nickel so that the etch factors are not as good.  

In short, fabricators should be able to provide an etch factor of 2 or 3 with some able to etch to a factor of 4.  

Denny Fritz









-----Original Message-----
From: Lum Wee Mei <[log in to unmask]>
To: TechNet E-Mail Forum <[log in to unmask]>; Dennis Fritz <[log in to unmask]>
Sent: Wed, Jun 20, 2012 8:09 pm
Subject: RE: [TN] Etch Factor


I happen to be looking for the same query as Shawn - what is the acceptable or 
omfortable etch factor number? I remember someone telling me many many years 
go that this etch factor is 1 but I have forgotten whether it should be greater 
han or less than.
Thanks and regards,
wee mei~
-----Original Message-----
rom: TechNet [mailto:[log in to unmask]] On Behalf Of Dennis Fritz
ent: Thursday, June 21, 2012 4:00 AM
o: [log in to unmask]
ubject: Re: [TN] Etch Factor
Shawn,
Etch factor is determined by many factors - etching chemistry, metallic plating 
r photoresist defining the copper to be removed, spray pressure in the etcher, 
op or bottom of the board travelling horizontally through the etch machine, pH 
f the etching solution, ability of the shop to time the board travel (slower 
eans over etch, faster means underetch), etc.  
Etching is something of a chemical art, rather than a precise mechanical 
peration like drilling.  
I suggest you work with your fabrication shop process engineers.  They know what 
tch factors they can produce, and the straighter you want your copper conductor 
alls, the more care (hence higher price) your fabricated board will become.  
ost any etch factor is possible - artwork line width compensation, tuning the 
tcher for your specific boards, running the boards fast twice and flipping the 
oards top to bottom between runs, etc.  However, each of these steps costs time 
nd productivity, and you will pay for it. 
There is one sister industry to PWB board fabrication that makes a living only 
tching metal sheets - photochemical machining.  They etch from both sides 
aking relatively staight sidewall parts, in 1 to 50 mil metal thickness, in 
arious metal alloys, and compete with die stamped flat parts. They have the 
ltimate knowledge of etch factors.  
Denny Fritz

-----Original Message-----
rom: Upton, Shawn <[log in to unmask]>
o: TechNet <[log in to unmask]>
ent: Wed, Jun 20, 2012 11:52 am
ubject: [TN] Etch Factor

k, got my specifications, looking at IPC-2221A Figure 10-3, for conductor width 
ee section 10.1.1 also, pages 77&79); and IPC-A-600H 3.2.1 (see page 86).  It 
oks like "etch factor" drives how much of a trapezoid one has on any given 
ace, although outer layers (since they are plated) may have overhang.
hat I don't see is any qualitative numbers: ie, an etch factor of y +/-z is 
pical for outer layers, vs w for inner layers.  Are these numbers variable rom 
endor to vendor; and have to be called out on any given FAB drawing, since here 
sn't a generic number to be pointed at?
hawn Upton, KB1CKT
st Engineer
legro MicroSystems, Inc
[log in to unmask]
3.626.2429/fax: 603.641.5336
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