Mike,
I experienced something like a number of years ago. We thought too that the bubbles or skin was
organic, but it was not. The story is long, too long. Roger Massey helped me to find the reason.
Anyway, the bubbles was indeed millions of small blisters in the gold finish, and the eye interpreted
it like organic bubbles. I'll ask famous Steve to wallpaper a pic or two. U/S or cleaning or plasma..nothing removed the "bubbles". It wasn't until we made SEM/EDAX/XPS as we understood what it was. But still we did not understand why. The gold "skin" did not adhere correctly to the underlying nickel, so, when we performed heavy wire aluminium wedge bonding or fine wire gold ball bonding, the bonds lifted oftenly. We really got frustrated. On top of that, we found that copper had diffused quite through the nickel layer, and that the gold bubbles or skin appeared only where we had copper in the nickel surface. Can't happen, you say. So said Roger too at the beginning, but he found (clever guy!)the source in guise of temporarily aggressive process water etc. If he reads this, he may add some of his golden grains. The rest of the story is that we rejected all the boards, the board supplier got my report, they did some adjustments, and the phenomenon never reappeared.
(Report 22, my thumbnail.)
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Mike Wolf
Sent: den 2 juni 2005 13:43
To: [log in to unmask]
Subject: [TN] Residue on bondable gold?
We are experiencing some type of residue on PCB's with gold as a final
metal finish. The residue adversely affects wire bonding to the gold.
The residue is present on both in-house fabbed boards and outside vendor
fabbed boards. It appears to be related to the soldermask in that there
are recognizable patterns (bubbles maybe) in the soldermask, that are
visible in the residue on the gold.
An aggressive plasma cycle loosens the residue (does not remove) but at
same time attacks the FR4 & Smask. No amount of solvent cleaning
removes the residue. Nor does ultrasonics.
Our in-house fab is electro- Copper, Nickel, & Gold, I'm pretty sure
the outside vendors are using ENIG.
We vacuum bake all our bds as a precaution to avoid die attach
bleedout. Could this bring to the surface problems we might not
otherwise see?
Has anyone else seen this type of residue? Know what it is or how to
get rid of it/prevent it?
Michael Wolf
EH&S Manager
EVI Technology LLC
7138 Columbia Gateway Drive
Columbia MD 21046
Phone # 443-542-2826
Fax # 410-290-1925
[log in to unmask]
www.evitechnology.com
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