For thermosonic gold ball bonding, you should have pure, soft gold. The
problems that you are seeing are probably due to the materials added to
harden the gold. Thallium, lead, nickel, copper, chrome, or titanium can
all decrease bondability.
Talk to your board vendor, and see if they can plate pure soft gold for the
wire bonding.
There is an article in Volume 20, Number 3 IMAPS Journal regarding
thermosonic gold ball bonding to immersion gold, electroless nickel.
Perhaps this metallization is a possibility. It is more difficult than
bonding to soft pure gold, but it can be done.
Dave Anderson
Medtronic, Inc.
Opinions are my own, and do not necessarily reflect those of my employer.
>>> <[log in to unmask]> 05/29/98 08:08am >>>
Everybody, good day.
Does a woodpeck prefer hickory or limetree? And does a automatic wire
bonding machine prefer hard gold or soft gold? From the metaphor to the
problem: Our board supplier is a very good one, I think, but our special
headache is not his, at least not yet. We design with Teflon-on-Brass
plates (US-made). On the Teflon (in fact Teflon mixed with ceramic and
glass etc) there is a Copper layer. This copper layer is etched and
Nickel plated. Finally you put electrolytic Gold on. The process is
such, that both the Nickel and the Gold becomes too hard for gold ball
wire ultrasonic bonding. Wanted hardness is 100 HV. The hardness from
supplier X is 450HV. The question is: what is possible to do in the
board processing steps? I know that the chemistry of Nickel-and Gold
baths is a mysterious world, current density plays a role, heat
treatments also and a lot of other things. But we don't have the time to
experiment weeks and months. Is there someone with a woodpeck's
experience?
Ingemar Hernefjord
Ericsson Microwave Systems
Sweden
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