Pete,

Thanks for your explanation. I wish that your conclusion were much more
widely known and understood. Here's my summary of what I have learned
about ENIG from what you have shared. 

*    Black pad is a result of the board fabricator letting the
phosphorus concentration in the electroless nickel plating bath get too
low.

*    Too low a phosphorus concentration in the nickel allows excessive
chemical attack of the nickel during subsequent immersion gold
deposition.

*    This condition can be, and now usually is, prevented by the
fabricator monitoring the bath phosphorus concentration and keeping it
within the specified range.

*    As confirmation that they are controlling the electroless nickel
plating process properly, fabricators now regularly report the
phosphorus concentration to the bath chemical manufacturer.

*    Brittle interface fracture is a separate characteristic ENIG
failure mode.

*    When liquid solder wets and reacts with the nickel to form an
intermetallic compound, the phosphorus is excluded, increasing its
concentration at the interface between IMC and solder.

*    Susceptibility to BIF develops when solder is present as a liquid
for too long. The concentration of phosphorus in the interfacial layer
increases enough to become an easy path for crack propagation.

*    Unlike black pad, occurrences of susceptibility to BIF are usually
caused by the user allowing solder to remain molten too long.  

*    BIF susceptibility can also be caused by the fabricator allowing
not too little but too much phosphorus to deposit in the electroless
nickel. Hence fabricators must keep the phosphorus concentration - in
the bath and in the plated nickel layer - between upper and lower
limits. 

I wonder how many board users who have been "bit" by black pad or BIF,
and who because of their bad experience have prohibited procurement of
boards with an ENIG finish, will reconsider their ban now that the
causes are better understood and fabricators control the process better.
I suspect that since there are good alternative finishes most will not. 

Incidentally, all users of BGAs may still get ENIG. Some BGA
manufacturers use it as the under-bump metalization, and none identifies
the UBM in the product specification. Those fabricators who use organic
solderability preservatives over copper for UBM create a separate risk -
complete dissolution of the pads during repeated ball melting. This
latter risk is of particular concern to those who want to replace SAC
balls with SnPb (or vice-versa) before component placement. Let's hope
that the fabricators of the ENIG-finished boards used for BGAs
understand black pad and BIF, and that the fabricators of OSP-copper
finished boards for BGAs deposit enough copper.  

The board finish that is gaining market share most rapidly is immersion
silver. The liabilities and prevention means of this finish are  

*    Susceptibility to tarnishing (reaction with hydrogen sulfide in the
air) during storage - easily preventable for long times by storing the
board in an airtight plastic bag. The diffusion rate of H2S through bag
plastic is negligible.

*    "Champaign voids" (numerous tiny bubbles from co-deposited organic
compounds that volatilize and accumulate at the solder interface during
soldering, and in excess weaken the connection) - preventable by using a
bath chemistry that limits the amount of volatile organic co-deposits.

 

Gordon Davy 


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