Typical rule of thumb for any good fabrication process. 1mil per side per oz of copper. Anything under 1oz is again typically the weight of copper and over 3oz gets a little more calculating.
1oz 7mil trace to get 5 minimum gives the process the biggest window possible. IPC +/- 20% rule would be 5.6 finished which fits fine.
Thank you, Jim Mahoney
Mass Design Inc.
Applications Engineer
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dennis Fritz
Sent: Thursday, June 21, 2012 7:21 AM
To: [log in to unmask]
Subject: Re: [TN] Etch Factor
Are you looking for etch factors for innerlayers of a multilayer board, or for the outer signal layers of a board?.
Inner layers - you have to use the full copper thickenss required by your design - one ounce (35 micron), half ounce, two ounce, etc. Photoresist defines the conductor spaces. I would expect a minimum etch factor of 2.
Outer layers are made with the thinnest foil practical - say 3, 5 or 9 micron. Then, pattern electroplating builds up most of the copper conductor required. Copper electroplating is contained by the photoresist used - say 25 micron or 50 micron. So, you can see that the copper conductor is say 25+5 microns high = 30 microns. Etching through the starting foil of say 5 microns, should give an etch factor of at least 6, nearly vertical. This process may be called "semi-additive" by some, and essentially vertical sidewalls are possible. Are you specifying the copper foil thicknes on the outer layers of your design?
Consider that with an etch factor of 1 in 35 micron foil, since the etchant undercuts from both sides of the circuit trace, it is impossible to make a trace smaller than 35+35 = 70 microns. However, we know that circuit traces far smaller than 70 microns are possible, so fabricators have solved much of the etch factor problem.
In short, the etch factor is very dependent on the manufacturing process used by the fabricator.
Denny Fritz
-----Original Message-----
From: Lum Wee Mei <[log in to unmask]>
To: Dennis Fritz <[log in to unmask]>; TechNet <[log in to unmask]>
Sent: Thu, Jun 21, 2012 12:50 am
Subject: RE: [TN] Etch Factor
Dennis,
Thanks. What ratio is the acceptable norm?
Thanks and regards,
~wee mei~
From: Dennis Fritz [mailto:[log in to unmask]]
Sent: Thursday, June 21, 2012 11:10 AM
To: Lum Wee Mei; [log in to unmask]
Subject: Re: [TN] Etch Factor
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.)
-----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 comfortable etch factor number? I remember someone telling me many many years ago that this etch factor is 1 but I have forgotten whether it should be greater than or less than.
Thanks and regards,
~wee mei~
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dennis Fritz
Sent: Thursday, June 21, 2012 4:00 AM
To: [log in to unmask]
Subject: Re: [TN] Etch Factor
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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