Steve,

Just to clarify my comments a bit more.

I understand that the area you are speaking of is for wirebonding only.
However, since the entire assembly is electroless Au plated, is there
somewhere else where a wiping contact is being made [outside of the bonding
area]?

A good example from hybrid days is a hermetic coaxial connector.  Needs to
be mated/de-mated many times on the outside and wirebonded to on the inside.
First apply the underlying Ni.  Then, plate the center contact with
electroless [hard] Au all over + soft gold all over, making it wirebondable
on the inside and of reasonable durability on the outside.  [did not have to
be concerned with solderability]

This is exactly what you have - and it has been around for a long time.
Possibly someone is cutting & pasting from another not-so-applicable
document...?  Doesn't hurt to ask them what they are up to.

Myself, given 150 µinches of Ni under 50 µinches of pure soft gold, I would
nearly be in heaven bonding to it.  I only wish I could have had that on
some boards I've dealt with.  Like being on a nice warm sunny beach with my
beautiful wife in one hand and a nice cool drink in the other - it would not
get much better! 

Depending upon prior processing conditions, they could likely get away with
no more than 20-30 µinches and not even be aware of the reduction in soft
gold thickness.

Sometimes it is also a good thing to look at the amount of area actually
being plated.  If it is a small area, the cost differential between 30 & 50
µinches may not be all that great.  If you were plating an entire 18 x 24"
panel - well then, I wouldn't mind having the extra 20 µinches to help pay
the bills.

If not making a contactor somewhere else on the assy, I too do not
understand why the e-less Au is needed.  The 150 µin of Ni will do a great
deal to prevent any Cu diffusion.  

As far as corrosion goes ... if the environment is THAT bad, why are you
placing a bare [or glob topped, etc] die there?

Be interesting to hear what kind of story you get back.

Steve Creswick
http://www.linkedin.com/in/stevencreswick




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