APeder01,

One source of information that you might want to check out is the July 1996 
issue of PC FAB magazine.  This issue has many articles related to different 
surface finishes currently being used.

Here are answers to your other questions:

1.  Is "electroless gold" the same as "immersion gold?"

	The answer is both yes and no depending on who you are addressing.  Specific 
terminology for electroless plating are sometimes not the same from person to 
person and region to region.  It is best to clarify the term with the person 
before starting a discussion to prevent any misunderstanding.
	Example:  Immersion Gold is also called
			Self Limiting Gold
			Metal Replacement Gold
			Electroless Gold
			Flash Gold

2.  How thick can we expect the gold to deposit?

	Most Immersion gold will stop plating at approximately 8 microinches.  You 
will find some people say that the thickness is closer to 10 and maybe 15 
microinches but I believe along with many others that 8 microinches is more 
accurate.
	There are some modified Immersion Gold baths that claim to be able to plate 
up to 20 micro inches but these "juiced" up bathes tend to have spotty plating 
thickness, and are not 
reliable.
	There are Autocatalytic Gold (electroless process) baths which will plate 
thicker gold deposits.  In theory the process will plate forever as long as 
the bath is in balance.  But for most wire bonding applications 15 to 30 
microinches for aluminum wire and 50 to 70 microinches for gold wire are 
typical.

3.  Is the gold deposition process a self limiting one?

	Yes.  The Immersion Gold solution replaces the Electroless Nickel with gold 
until all the Nickel sites are sealed, at which point the reaction stops.

4. How much gold is required to adequately keep the nickel solderable?

	Typically the gold is plated to the maximum of approximately 8 microinches.  
This to insure that the electroless Nickel is completely sealed to protect 
from it from passivating.
	Lately I have evaluated some Simm Chip Modules from off shore that have as 
little of 1 to 2 microinches of gold over the nickel.  Some modules were 
successfully assembled in the  1st pass, while other required various degrees 
of rework.


I hope this helps you with your evaluation and if you have other questions, 
call me at (714) 842-1987 or (714) 842-4853.

Cheers

Frank Kurisu
Aurelon, Inc.
Huntington Beach, CA
----------
From: 	APeder01
Sent: 	Friday, July 12, 1996 6:55 AM
To: 	[log in to unmask]
Cc: 	APeder01; MCHAN
Subject: 	FAB: Electroless Nickel - Immersion Gold

     My company is in the process of evaluating the use of Electroless 
     Nickel - Immersion Gold as a finish, for both SMT and mixed technology 
     PWBs.  Is this a good plating system to evaluate??  Is there any 
     "conventional wisdom" that we're missing?
     
     Secondly, some questions about the gold deposition.
     1. Is "electroless gold" the same as "immersion gold?"
     2. How thick can we expect the gold to deposit?
     3. Is the gold deposition process a self limiting one?
     4. How much gold is required to adequately keep the nickel solderable?
     
     Any other comments regarding this topic is appreciated.

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