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March 1997

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From:
"Reed, Randy" <[log in to unmask]>
Date:
Mon, 31 Mar 1997 10:33 -0800
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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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