Jim -

    IPC-TR-476 (1977) and IPC-TR-476A (1997) are outstanding documents when it
comes to explaining the causes of  dendritic growth, intermetallic growth,
electromigration, or whatever else you chose to call it.  In general this
condition is dependent on the presence of some form of contaminate, moisture,
and an electrical current.  With this combination, a small conductive path can
form from the negative to the positive conductor.  Note that this condition is
different from tin whiskering, which is the formation of crystalline whiskers
from pure tin.

    Obviously, a good LPI solder mask is in order, to minimize conductor
exposure on the bare board.  Accepted practice for avoidance generally
includes having the product scrupulously clean after solder and test has been
completed, then applying a respectable coat on all conductive surfaces.
Conformal coating all of the conductive areas under a BGA, other than
Parylene, could be a real challenge, unless one of the dead soft coatings like
Solithane 113/300 were vacuumed into the area.

Call me at 256.882.4536 if I can be of assistance.

Regards - Kelly

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################