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Tue, 05 Dec 95 12:07:11 EST
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De: Denis Dionne                                                               
*** Renvoi de la note de 12/04/95 16:42                                        
Non-classifi(c)e / Unclassified                                                  
Objet:   Immersion gold plating                                                
                                                                               
If you are talking about an Immersion Gold layer over Nickel I can             
offer the following information:                                               
                                                                               
Fine pitch and micro ball grid array devices require a flat and uniform        
site for attachment, that can be acheived with electroless nickel and          
immersion gold plating.                                                        
                                                                               
Some advantages of this type of plating is:                                    
-uniformity of thickness                                                       
-long shelf life                                                               
-coating of electrically isolated features                                     
-no flux required (but can be used)                                            
-encapsulation of features                                                     
                                                                               
The predominant disadvantage is higher cost than other finishes.               
                                                                               
As you mention, there is possibility of embrittlement with gold                
finishes but it seems that testing as shown that a 3% gold weight              
content should be respected to avoid such problems.  This is shared            
by several authors.                                                            
                                                                               
The immersion gold thickness should be thin enough so as to provide            
adequate protection but not cause any embrittlement problem.  A thicker        
gold coating can be achieved with electroless gold if there is a need for      
wire-bonding.  We currently process both type of plating.                      
                                                                               
Two recent articles of interest to you would be:                               
                                                                               
Reflow soldering to gold, Electronic packaging and production                  
V 35 N 6 Jun 1995                                                              
                                                                               
Impact of PCB surface finish on SMT assembly process                           
National Electronic packaging & production conference v 2 1995 p 697-702       
(good references...)                                                           
                                                                               
Please feel free to contact me for additionnal information.                    
                                                                               
                                                                               

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