If you desire to tin lead plate the fingers, probally the whole board would be 
tin/lead plated.  The problem with tin/lead plate for an SMT board is the 
soldermask.  One of the main advantages of HASL is you can use soldermask in 
conjuction with it much easier.  This is because with HASL the soldermask is 
put on prior to HASL.  For a Tin/Lead plated board the soldermask is put on 
after tin/lead plate.  Thus normally on a Tin/Lead plated board if you use 
soldermask the soldermask will be over tin lead.  This is usually considered a 
problem for several reasons, the main of which is that soldermask doesn't like 
to stick to tin lead during reflow at the assy house.

To put soldermask on a tin/lead plated board properly you need to first strip 
the tin lead from the circuitry you want covered with soldermask.  This 
selective strip process can be done by many PCB suppliers, but at a premium, 
usually about 5% of board cost. 

The plus side of a tin/lead plated surface is much more consistant and level 
than HASL.  A example of when it is used is board for TAB components.  TAB 
devices require a very controlled amount of tin/lead on the TAB footprint.  
Most assembly houses needing to bond TAB components to boards require tin/lead 
plated boards with selective striping of tin/lead where the soldermask is.

Another disadvantage of tin/lead plated board is it is slightly more difficult 
to etch a tin/lead plated board.  For tin/lead plated boards the tin/lead 
serves as a etch resist.  The tin/lead is typically about a half a mil thick.  
This leads to a shadow effect at etch.  HASL boards usually use a much thinner 
tin plating as a etch resist with less shadow.

Surface finish for contact fingers and for component mounting is such a 
critical part of a PCB I wonder if the IPC would consider (if they haven't 
already) making a Surface Finish specifation listing all of the available 
types of finishes with their advantanges and disadvantages.
Hope this helps,

[log in to unmask]
Dell Computer

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FROM: BRUCE HOUGHTON  27/552/844/TOR                                           
*** Resending note of 05/11/95 11:12                                           
Phone  (416) 448-5701     Tie 778-5701                                         
Subject: Tin/lead plating vs HAL                                               
Doug,                                                                          
Tin/lead plated (and reflowed) tabs over nickel will give a very               
consistent and controlled thickness of solder over the tabs.                   
HASL is difficult to control for both thickness and uniformity.                
Tin/lead plate and reflow tabs may cost a little more.                         
     
Bruce Houghton                                                                 
Celestica, Inc                                                                 
=========================================================================      
> Date: Thu, 11 May 1995 06:47:38 EDT                                          
> From: [log in to unmask] (MR DOUGLAS C JEFFERY)                             
     
-- [ From: Douglas C. Jeffery * EMC.Ver #2.10P ] --                            
     
     
I recently had a customer inquire about the advantages or disadvantages        
of using tin/lead plating over HAL for a connector application (SIMMs).        
We have been 100% HAL for several years and I couldn't really answer           
his question other than to say we don't have a tin/lead bath...So..Are         
there any thoughts as to one method being preferred over the other?            
(note. the question is about the insertion tabs not component                  
attachment, and he currently is not using ni/au).                              
     
Thanks for any insight.                                                        
     
Doug Jeffery                                                                   
Electrotek, Inc.                                                               
[log in to unmask]                                                            
414-762-1390