Hi Peter! The original intent of implementing the Sn62 solder alloy was
exactly as you described - using the 2% silver content of the alloy to
alter the solder joint dissolution characteristics (the idea that the
introduction of Sn62 alloy was a plot by evil metallurgists to takeover the
electronics industry solder processes is just a myth started by Doug
Pauls!). The use of SAC solder alloys families would seem to be a
improvement for reducing the "leaching" effect due to the 1-4% silver
content the alloys possess but also keep in mind that the melting
temperature is going up by approximately 34C. We can't revise the laws of
physics and the temperature impact outweighs the benefits we get from the
increased silver contents. And yes, double sided reflow just adds another
layer of complexity to the issue. Pbfree soldering has a much tighter
process window than SnPb soldering. And your headaches probably don't stop
there - depending on the alternative termination finish choice you might be
trading leaching concerns for tin whisker concerns. Isn't Pbfree soldering
fun!

Dave


                                                                           
             Peter Barton                                                  
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             02/01/2007 03:05                                      Subject 
             AM                        [TN] Soldering to Pt/Ag             
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               Peter Barton                                                
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Many thanks to all who responded to my question on the above subject, a
couple of which confirm my own theories.

It seem that parts with Pt/Ag (and for that matter others with a Silver
content) terminations are susceptible to Silver leaching from the component

metallisation into the bulk solder and that this is exacerbated by higher
process temperatures.

Now I am not a metallurgist so correct me if I am wrong - we used to employ

62Sn/36Pb/2Ag solder on hybrid assemblies to mitigate this type of problem
back in the good old non-RoHS days. The theory being that the 2% Ag in the
solder slowed the dissolution rate. As we are now using a solder with 3.5%
silver should this not improve the situation, or do the higher process
temperatures used just cancel this out?

Dave Hillman has said that using these parts with SAC305 CAN be done with
careful attention to the soldering process profile. We have done our best
to
minimise the TAL and the peak temperature, mindful of getting all the other

components on the assembly reliably soldered also. The problem parts
concerned are on both sides of a double sided assembly and those on the
side
that is reflowed twice are definitely much worse so I guess this confirms
that they are not really suitable.

Bit of a problem really as there is not an alternative part. Oh well  - As
a
responsible contractor we will now inform our customer and await their
response.



-----------------------------------------------------------------------
Peter Barton
Senior Process Engineer
ACW Technology Ltd
Dinas Isaf West
Tonypandy
Mid Glamorgan. CF40 1XX  Wales

Tel: 01443 425275 (direct)
Fax:  023 8048 4882
International Tel : +44 1443 425200
International Fax : +44 23 8048 4882
Website/URL:  www.acw.co.uk

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