In a message dated 3/7/02 12:46:50 PM Eastern Standard Time, [log in to unmask]
writes:

<< At what stage the electroless nickel and immersion gold process is
 applied, is it after the solder mask application or before? I believe
 normally it is after the solder mask application but I want to verify
 it. I have heard that chemistry involved is very nasty if left on the
 board. If I plug or mask via holes from one side, I want to make sure
 that there is no residue left of that nasty chemistry. Can it be cleaned
 if mask from one end? Any comments? >>

Ken,

Yes, ENIG is normally applied after solder mask.  Some shops have made boards
doing the ENIG before mask - a process with pros and cons.  Pro - it avoids
attack on the solder mask.  Cons - Solder mask can have adhesion issues to
gold compared to copper, and obviously, you use more gold to plate all the
circuitry rather than just the pads in the ENIG last process.

The Electrolesss Nickel solution is quite hot in some processes - 160 to 190
degrees F.  This tends to soften the mask and allow chemistry to "soak in".
All the plating chemistry for ENIG is aqueous based - so long rinses in water
- especially DI should clean up the board.  Of course, chemistry that has
"soaked in"  will take a while to leach out again.

If you mask from one end, this just makes the solution trasfer from hole wall
to rinse water take longer.  You are not getting fresh water into the hole
barrel to dilute (rinse away) the contaminants.  Would not suggest single
sided mask down the ENIG line.  Remember, all the solutions are aqueous, and
the gold bath will try to "clean" the Electroless nickel solution out the
holes too.  Result - contaminated gold bath that plates too thin or shuts
down.

Denny Fritz
MacDermid, Inc.

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