Eddie,

ENIG is the most called out surface finish after HASL and OSP. Billions of 
pads have been plated with the finish and experienced asssemblers will tell you 
that it is their ffinish of choice.

ENIG's market share continues to increase at the expense of ESP and HASL.

The finish offers the following advantages:
-  It is solderable
-  It is aluminum wire bondable
-  It is the best electrical contacting surface  
-  The nickel strengthens the plated thru hole
-  Excellent shelf life , does not tarnish or discolor
-  Nickel is a diffusion barrier and does not allow the dissolution of the 
underlying copper into the solder when going over the wave solder 
-  It is easy to inspect before and after assembly

The key is to understand that when one uses ENIG the solder joint formed is a 
Ni/Sn solder joint and not a Cu/Sn one. Ni/Sn requires higher assembly 
temperatures to form the IMC. (A minimum of 20 seconds at 220oC, with the thermal 
profile adjusted accordingly)

The immersion gold thickness called out should be minimum 2- 4 uins just 
enough to preserve the nickel surface until such time as it is assembled (solderd 
to). refer to IPC 4552 Specification for ENIG.

Excessive gold could damage the underlying Nickel during immersion 
deposition. The gold dissipates into the solder. 

With proper understanding of the uniqueness of this finish, all problems 
attributed to it can be easily avoided and contained.

After all, HASL, OSP , Immersion silver, immersion tin, electroless 
palladium, elctrolytic Ni/Au, are not 100% reliable and trouble free. They simply do 
not get the press that ENIG gets.

George Milad
Uyemura International Corp
Tech Sales NE
Chairman IPC Plating Committee
(516) 901-3874 Mobile
(860) 793-4011 Office


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