I agree with Randy. I have used the dichromate etchant and
had no trouble detecting post separation and copper cracking
when it occurred. I have seen inexperienced operators who
so overetched microsections with ammonium hydroxide peroxide
microetch that the sections could not be reground to find
the electroless copper interface to do an evaluation!
Training of microsection personnel and constant monitoring
is extremely important. The problem with the etching
variability over time with ammonium hydroxide peroxide will
show different results depending on where in that time
period the etching is being done. I believe Randy's way of
making etch when needed then discarding is the most
reliable. I also recommend microsections be audited
frequently in order to make insure the quality of the work
of these inspectors. You can't spend too much time
monitoring this area.
NOTE: The shop I where I worked dropped dichromate in an
attempt to get all chrome out of the shop.
Regards,
Dave Sullivan
Rockwell Collins, Inc.
[log in to unmask]
______________________________ Reply Separator _________________________________
Subject: Microsection Etchants
Author: [log in to unmask] at ccmgw1
Date: 3/31/97 1:23 PM
I have noticed numerous posting on the best microsection etchant to use.
There are merits and disadvantages in the etchants mentioned to date. I
specifically would like to discuss the sodium dichromate and the ammonium
hydroxide & hydrogen peroxide.
The ammonium hydorxide & hydrogen peroxide is 1 of the more unstable
copper etchants that does an excellent job of microetching. The most
attractive attribute of this etchant is in the hands of a less skilled or
naive person, the etchant will not overetch the surface to the severity
of a chromate etchant. (Not say that only less skilled people use this
etchant) I define 'instability' as variable etch quality over a short
period of time (hours). At Merix, we make enough etchant to microetch
the work at hand and discard the remainder.
The soduim dichromate etchant is a stable etchant (last for months) that
is more agressive than the ammonium hydroxide & hydrogen peroxide
etchant. The aggressive nature of this etchant requires greater skill by
the user to prevent the surface from being overetched which hides cracks
and separations. I used the sodium dichromate etchant at my prior
employer for 15 years and was able to detect separations and cracks.
While I worked at this employer, we published papers on our ability to
detect separations and our concerns that separation was being shipped.
This leads me to believe the type of etchant is not so important as the
user of the etchant. One of my favorite BC cartoons is a caption that "If
you don't know what it does; don't screw with it". The skill and
knowledge of the user is as impotant as the etchant itself. IPC-MS-810
lists the types of etchants that are recommended for etching of
microsections. Whatever etchant is chosen for use, the key is to create
the desired etch quality of the surface to make wise decisions on
quality. The primary qualities are a faint definition of the electroless
line, strike platre line (as applicable), and the copper grain
boundaries. When these boundaries become wide and/or dark in appearance,
sensitivity is lost for detecting separation and cracks.
Lastly, the primary reason that the sodium dicromate etchant popularity
declined is the the chrome content and EPA rulings on chrome. The issue
over chrome was a key reason that the etchant was dropped from the test
methods and military use.
Happy microsectioning,
Randy Reed
Merix Corporation
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