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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