Cupric system are great for inner layer since this chemistry efficiently
removes copper and has generally a lower etch factor/side wall attack
than you will see at the slower ammoniacal rates.  Curpic is by far easy
to control, but a properly controlled alkaline system does have
advantages with some design types.  Cupric with external layers is not
compatible using a tin resist and the external layers are much more
prone to have design flaws creating the acid trap issue.  Your CAM
system should be affective in the detection and correction of these
types of layout issues.        


Jeffrey Bush
Director, Quality Assurance and Technical Support
VERMONT CIRCUITS INCORPORATED 
           76 Technology Drive - POB 1890 
              Brattleboro, Vermont 05302
                Voice - 802.257.4571 ext 21
                    Fax - 802.257.0011
                       <http://www.vtcircuits.com/> 
                           

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Wednesday, September 13, 2006 10:02 AM
To: [log in to unmask]
Subject: Re: [TN] Etchant traps

Thanks a bunch Joe.  That helped answer some of my questions.  Does
anyone out there actually have a picture of the results of an alkaline
etch actually creating this "trap" problem?

I use cupric chloride, and I have a calibration pattern of "square
spirals" at varying line and space.  The corners of these tell a lot
about how the cupric chloride etches in different situations:  The
amount of over-etch is monotonic with the area of bare dielectric
immediately adjacent to the photoresist-protected copper.  This matches
my experience with circuits others have built for me:  Isolated pads
tend to get over-etched the most.

Wayne Thayer

>>> [log in to unmask] 9/12/2006 10:50 pm >>>
Greetings Wayne,

I realized that I did not really respond fully to your message of
yesterday.

You are right in your thinking and observations based on current best
practices and it is a detail that should be corrected/clarified/brought
 current. I
was recounting experiences that likely don't hold well today  with
current
chemistry and equipment.

My experience was primarily been with copper ammonia alkaline etchants
beginning in the early 1970s and etching equipment not up to today's
quality.

We observed occasions where the sharp corners when fluid exchange was
not
good and copper etching irregular where this occurred.

One of the curious things about alkaline etch chemistry is that the
etch
rate of the chemistry increases with metal concentration which is the
genesis of
the suggestion based on the observations at the time.

As you suggest, design tools are not likely to make acute angles
without
permission. Again, the more important reason today remains the  one
mentioned
earlier and related to simple mechanical issues and  electrical
performance.

A very good article on alkaline etch by Karl Deitz can be  found at
the
following link if you have  interest.

_http://www2.dupont.com/Imaging_Materials/en_US/assets/downloads/techtal
k/TT02
05.pdf#search=%22%22etch%20rate%22%20%22ammonium%20chloride%22%22_
(http://www2.dupont.com/Imaging_Materials/en_US/assets/downloads/techtal
k/TT0205.pdf#sear
ch=""etch%20rate"%20"ammonium%20chloride"")

Thanks for your observations and comments.

Kind regards,
Joe


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