Getting the aqueour LPI solder masks to hold up to the electroless
Ni/immersion Au baths is tricky at best; however, there are a number of
things that can be done to increase your odds, such as:

1. Apply the mask on fairly thick (at least 1.0 mils over the circuit
traces), Some people even double coat the LPI masks for better results in El
Ni/Au.

2. In terms of processing the LPI solder mask, factors that help include:
     A. Do a good preclean on the copper surface (microtech, mechanical scrub
- pumice is best -  and good thorough dry.
     B. Apply the mask to the PWB as soon as possible after preclean (within
one hour) for optimum adhesion to the copper.
     C. Overexpose the mask as much as possible to build up the sidewall (the
weakest point of the mask). Stouffer step 21 guide reading of clear metal 12
or higher would be recommended.
     D.Increase the cure time by 50% (for example, instead of 60 minutes at
300 deg F, use 90 minuters).
    
3. In terms of processing the boards through the electroless Ni/Au process,
factors that may help include:
     A. Subject the panels to the El Ni/Au process as soon after the final
cure of the mask as possible (within one hour).
     B. The El Ni/Au process usually consists of about 15 different
processing steps (including rinses) that have a time duration in tanks of
usually > 2 hours. Make sure these steps are followed closely and that the
panels are subjected to the various steps as short a time as possible (do not
store a rack of panels in a solution - even a rinse tank).
     C. The electroless Ni is the real "killer" because of the high
temperature (usually > 180 deg F) and long time (usually 20 minutes or more).
Try to minimize this step in terms of temperature and time.

I've seen a number of different LPI masks from different suppliers that have
survived the electroless Ni/Au process; however, I've seen a lot of failures
too. On the other hand, if you really control a number of areas, you will
probably have success.

Hope this information is helpful. Please contact me if I can be of further
assistance.

Regards,

Larry Fisher
Dexter Electronic Materials
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