I'm a little confused by some of the process steps. 
     
     Image and gold plate (assuming also nickel underplate). Is the entire 
     circuit pattern gold or just the bond sites? If the entire pattern is 
     gold , then why cover with resist? I would think the application of 
     resist over gold and subsequent stripping would contribute to 
     contamination. Also, look into eliminating the silk-screen, if 
     possible. No telling what's being re-deposited on the gold when you 
     cure the legend ink. Another thought, what are you applying the Entek 
     56 to? If the pads are gold and the circuits covered with soldermask, 
     what purpose does the Entek serve?
     
     LPISM may be a factor as well. Have seen excellent success with Taiyo 
     and Enthone masks. We don't do Probimer here, so it's hard to offer 
     advice.
     
     As to ET pins, they'll cause some failure with respect to bond sites, 
     but they aren't contributing to the orange stain. Have your customer 
     identify critical bond area, then test to an area of the pad outside 
     of that critical area. Judging by the dimensions of the pad, they 
     ought to be able to accomplish this.
     
     Regards,
     
     Tom


______________________________ Reply Separator _________________________________
Subject: FAB: organic contam. on wire-bond gold
Author:  [log in to unmask] at INTERNET_GATEWAY
Date:    11/27/96 3:11 PM


     
     HELP - What's the secret to making wire-bondable gold?
     
     Our ppm failure rate on wire bond sites is a little too high for our 
     customer.  Smallest wire-bond pad is about .014"x .050" and wire 
     mat'l is Al (generally higher failure rates on smaller pads).  The 
     highest percentage culprit has been described as an [arguably] 
     visible "orange stain" - very thin with a Carbon/Oxygen signature.  
     Board is double-sided with the following route: Drill, panel plate 
     Cu, image & plate gold (DuPont 9020), strip resist (RR3), image 
     (cover gold), cupric etch circuitry, pumice w/brushes, LPISM 
     (Probimer-65), solvent develop, bake & UV cure, legend, ET, Entek 56. 
      Gold parameters: 83-85 knoop, Ni content: 30-40ppm, As,Cu,Pb,Co, all 
     less than 3 ppm, Gold conc. is on low side: Au strike: 0.15-0.25 
     tr.oz./gal, Au Plate: 0.4-0.6 tr.oz./gal @ ~2 ASF.
     
     Questions: 
     - What are the most common contributors to wire-bond failures? 
     - Is LPISM residue one of them?
     - Are aqueous developable LPISMs generally more successful?
     - What are the, say, 3 most critical parameters to control at deep 
       gold?
     - How critical is gold surface topography (roughness)? 
     - Are ET pin dents a failure cause?
     - Can pumice operations be detrimental to the gold surface? 
     - Is low Au concentration necessarily a liability?
     
     Any input will be greatly appreciated.
     
     J.Felts
     PC World, Toronto
     
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