| Hi folks , | Lately we are seeing a lot of rawcards with black spots on the pads - most of them have Au finish(immersion gold) . We did a lot of inspecton after wave solder and found that only (fine pitch) QFP are affected the most. Our inspection /testing process is to use the dental pick (90 deg. angle touching the lead to pad joint) on the leads. | u-section reveals that the contamin looks like tar or oxide. | Does anyone have any experience with this lately ? | | Thanks in advence Kc. | It does get ugly, doesn't it? Where in the micro-section metallographic structure do you see the "spots?" Do they appear to extend all the way from the resin surface or the copper surface? What is your thickness specification for Au/Ni?
Likely it is not water staining you're seeing but too thin a layer of Nickel metallizsation (well under 100 millionths"). If this is the case, the Ni barrier is not capable of preventing the copper from "leaching" (copper plating thickness is about 1.2 additional mils on top of a foil thickness of about .7 of a mil) through to the gold and causing the appearance.
Copper, if not prevented by a sufficiently thick Ni barrier, will form an intermetallic with the gold and rapidly and excessively oxidize on the pad surface. Also, the Ni, when containing high phosphorous content (acceptable amounts not yet known) in the plating solution, will form an intermetallic with the Ni and oxidize possibly providing the evidence you see.
Now that you've x-sectioned, other metallographic analysis is advisable. Consult your independent test laboratory, or one you have internally, to find the best technique to determine exactly what the "tar" is really.
There are some big time experts capable of providing in depth analysis and advice on this subject. They reside at your board supplier's chemical suppliers. Often they participate in the IPC TechNet. Lately, as might be understood due to increased gold useage, there has been much concern and talk about immersion gold and attendant issues/problems. Much of this has been discussed and is archived.
Earl Moon
reply »