But if you do choose to use a different alloy other than what is on your current PWB or assembly print, it would have to be qualified and approved by your customer in Korea. You can't just use a different alloy because it accommodates the existing board design.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
Sent: Monday, November 11, 2013 4:24 PM
To: [log in to unmask]
Subject: Re: [TN] SnBi solder and SAC solder balls

Guy
To address your last question first: No I don't believe that mixing Sn/Ag/Cu with Bi/Sn will produce the possibility of a reduced melting temperature intermetallic zone forming in your solder joints analogous to those reported when mixing Sn/Pb with SAC. (Sn/Pb/Bi MP around 100C). The lowest melting range combination pair of constituents is already present (Sn/Bi at 138C).

For your other queries: Welcome to Conjecture World!
We can note
1) That the properties of Sn/Bi have been found to be improved by the addition of silver.
2) That "reinforced SAC" or "enhanced SAC" alloys have been developed by the addition of small alloying additions of Bismuth and other stuff.
Patented alloys which have enhanced thermal cycling abilities, temp withstand etc from these additions are available from certain suppliers, and are reasonably well documented and have been in use in high rel apps (auto
etc) for some years.

So that sounds kind of favourable. what will actually happen will depend on the actual alloy mix you form, how homogenous it is, the stresses that the assembly will see in service and the service life you are looking for. You might be able to puzzle something out from phase diagrams but you have a lot of variables, I can understand binary diagrams,(2D) and cope with ternary
(3D) but  I can't do 4D or more. Not many people can - do four dimensions that is. You need a proper metallurgist or to talk to someone who has gone before you. In that respect I did get the link in the NASA report given by Dave to work by a little jiggery pokery (it doesn't seem to work direct). I did a quick scan through. It's longer than I can cope with at this time of night, but it looks as though it might be helpful. 
It's below

 
https://tdglobal.ksc.nasa.gov/servlet/sm.web.Fetch/TEERM_LFE_JTR_FINAL.pdf?r
hid=1000&did=41742&type=released 

Sorry not to be more specific, maybe others with more direct experience can be more helpful.

Regards 
 
Mike 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Monday, November 11, 2013 1:30 PM
To: [log in to unmask]
Subject: Re: [TN] SnBi solder and SAC solder balls

Hi Guy - The NASA DoD Lead-free Project did test SAC solders with SnBi surface finishes so that information may be helpful (not exactly what you are looking for). The project results can be found on at:

http://www.teerm.nasa.gov/NASA_DODLeadFreeElectronics_Proj2.htm


Honeywell and Celestica have also published an investigation on tin/bismuth solders - check the APEX proceedings for that information.


Dave Hillman
Rockwell Collins
[log in to unmask]



From:   Guy Ramsey <[log in to unmask]>
To:     <[log in to unmask]>
Date:   11/11/2013 07:19 AM
Subject:        [TN] SnBi solder and SAC solder balls
Sent by:        TechNet <[log in to unmask]>



We have a customer in Korea using Samtec ?Solder Charge? connectors.  The
alloy is SAC38 ( 95.5, 3.8, 0.7). 

They want to stay Pb-free. 

 

Our 60 layer, 480mm by  585 mm by  6.5 mm thick board, with  6100+ 
embedded
capacitors cannot survive the Z axis expansion seen at SAC reflow.  Test
samples have survived 10 reflows at 210°C. 

 

Has anyone tried to qualify the performance of SAC solder with SnBi solder
paste?  Does it have the same problems associated with mixing SAC and SnPb
paste? 

 

Thanks, 

Guy


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