Agree 100%, heavy wire U/S bonding with alumina wire is so powerful that most kinds of debris, even nickeloxides, will be crushed and pushed aside, bringing fresh nickel for making those microjoint (microwelds). So, you can heavy wire bond directly on nickel without any gold if you want. 25micron gold ball bonding will not work this way. In that case the several-times-harder-nickel will not match with the soft gold wire, the mechanical waves will not meet correct material 'answer' and therefore you get poor bonds. Either use 2-5 micron pure gold on the nickel, or palladium/gold. We have tried both and it works fine. In the equation, you must consider also the kind of dielectric you use. If the conductor is on FR-4, Teflon or on other organic material is not trivial ingredient. I suppose Harmans Bonding Bible have most answers, get one if you don't have it.
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: Creswick [mailto:[log in to unmask]]
Sent: den 4 oktober 2000 04:35
To: [log in to unmask]
Subject: [TN] ENIG
Scott,
First, one must know what bonding technology the Customer is using. Is it
ultrasonic aluminum of thermosonic gold?
If it is aluminum, then the gold is effectively only there to protect the
nickel - to provide a good, clean, nickel surface to bond to (with the 3-5
uinches of gold efectively excluded from the bond). The exception being if
someone has put hard gold on the surface, then it may have been better to
leave the gold off altogether.
If the application is thermosonic gold (where the ultimate goal is a gold to
gold bond), then PURE, SOFT gold of a MINIMUMm of 10-15 uinches is normally
required. If the board is scheduled to see multiple bakes, or elevated temp
storage prior to wirebonding, increases in gold plating thickness are very
helpful. I believe the KEY critical parameter here is PURE gold - such
elements as thallium, cobalt, copper, etc co-deposited in the gold plating
as grain refiners, etc. can extremely quickly ruin a bondable surface. A
'pore free' plated film, if there actually is such a creature, also
facilitates the long term preservation of the plated surface, and its
subsequent bondability.
There have been many other posts concerning adhesive resin bleen, solder
mask bleed-out etc. Naturally, these also may have an effect.
Possibly, some of the more 'plating inclined' individuals out there can add
a few comments. From a User's perspective, if you can find and nurture a
good plater, your life becomes tremendously more forgiving.
Steve Creswick - CTS RF Integrated Modules
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