Hi Guy-
I suspect your customer only wants the joints to be as reliable as if the whole assembly was soldered with SAC305. And as long as your service temperatures are at least 40C below the melt, the SnBi should be able to meet this bar. On the other hand, if you were to switch to SnPb, then the customer might demand a higher bar!
By the way, I've found that pre-flowed solder, such as the product SIPAD puts out, allows reflow at about 10C less than with paste (much much less surface area the flux needs to clean). You might want to consider something like that if you need to keep SnPb below 210C.
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Tuesday, November 12, 2013 12:57 PM
To: [log in to unmask]
Subject: Re: [TN] SnBi solder and SAC solder balls
Oh I been Googling!
The long and short of it is, no-one can know without understanding the whole solder system.
My gut tells me the result wouldn't be unreliable in an environment with thermal excursions below 100C, and service life less than 5 years, but Murphy was an optimist.
The solder charge combination of SAC38 from Samtec's part and the mix of SnBi solder paste, with process variation during reflow, make a very complex and difficult system to optimize and model. Better to stay SnPb and order the part with SnPb solder charge.
Some of the aging properties of SAC305 are a little frightening for long
service products.
Guy
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Monday, November 11, 2013 6:15 PM
To: [log in to unmask]
Subject: Re: [TN] SnBi solder and SAC solder balls
I remember something about internal oxidation SAC plus Bi. Can't remember
exactly what impact on reliability. You might want to goggle it.
Original Message
From: Mike Fenner
Sent: Monday, November 11, 2013 5:26 PM
To: [log in to unmask]
Reply 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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