Patrick:
     
     How fresh is the paste being used? Also, how wet has the weather been 
     during the time of this phenomenon?  In other words, has the paste had 
     the chance to absorb moisture before screen printing?
     
     One mechanism for solder splatter MIGHT be that during the preheating 
     operation, entrapped moisture in the paste could rapidly expand and 
     "explode" from within the deposited paste, especially if a damp 
     surface layer of paste is spatulated into the jar before placing it on 
     the screeen printer.  This rapid expansion of moisture COULD send 
     solder balls flying to nearby areas on the board (e.g. gold fingers).  
     They probably result in melted spots of ~0.005" - 0.010" in diameter 
     and are probably located on the end of the finger nearest the 
     deposited solder paste, right?
     
     An experiment you could run to prove or disprove the theory is:
     
        1) Tape the fingers of a several boards with Kapton tape.
        2) Paste and reflow one set of boards with paste that has been 
        allowed to sit open for a period of time to collect moisture from 
        the air.
        3) Paste and reflow another set of taped boards with fresh paste 
        from the supplier.
        4) Carefully inspect the gold finger area under a microscope and 
        see how many solder balls are present on the tape for each set.
     
     The N2 environment should have no direct affect on the phenomenon you 
     are seeing.  In either case, the gold fingers would be protected while 
     the data is being collected, so the product quality would NOT be 
     compromised.
     
     When or if you find the root cause of the splatter, I would appreciate 
     it if you would let me know the results.
     
     Good luck!
     
     Bill Fabry
     Plantronics, Inc.
     [log in to unmask]
     
     
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Subject: [TN] ASSY, solder residues on  gold fingers
Author:  "TechNet Mail Forum." <[log in to unmask]> at INTERNET