Homebrew PCBs

I am etching general pcb's and also etching pcb's 
for MEMS devices.  The MEMS uses copper pads as small as 
possible so I am attempting to get down to 0.125mm (0.005")
and am trying the laminator+toner transfer with treated
paper and "GreenTRF" process.  Others do this for SMT
according to online sources so I am hopeful it will work.


In researching alternatives (especially flexi materials)
 I ran across a white paper at Dupont.  
This paper references a resin which reduces
undercut.  Anyone hear about this before?


Tech Talk 
Fine Lines in High Yield  (Part CXLI) 
Ferric Chloride Etching of Copper 
Karl H. Dietz  (for CircuiTree Magazine,  April, 2008) 


http://www2.dupont.com/Imaging_Materials/en_US/tech_info/techtalk.html



Quote:


Ferric chloride has the reputation of giving a more favorable etch factor (less lateral etch) 
than cupric chloride. I don't have data to this effect, and engineers whom I talked to and who 
had experience with both processes in PWB fabrication did not have solid results to corrobo- 
rate this, in part because equipment parameters also affect the etch factor and processes in 
different equipment sets are difficult to compare. It is not obvious what component in the 
ferric chloride etchant might act as a banking agent to protect the etched side wall of a 
copper feature from further lateral etching. "Dragon's Blood", a red naturally occurring resin 
has been used successfully to protect side walls from more lateral etching.