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February 2007

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From:
"Wenger, George M." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wenger, George M.
Date:
Thu, 1 Feb 2007 08:49:39 -0500
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Besides the increased temperature for Pb-Free you also hav
Dave,



Besides the increased temperature for Pb-Free you also have an increased

Sn concentration (95% for Pb-Free and only 60% for near-eutectic SnPb

solder) which can "suck-up" lots of Ag so it shouldn't matter if the Ag

concentration is 1% or 35). 





Regards, 

George 

George M. Wenger 

Senior Principle FMA / Reliability Engineer 

Wireless network Solutions 

Andrew Corporation, 40 Technology Drive, Warren, NJ 07059 (908) 546-4531

[log in to unmask] 





-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman

Sent: Thursday, February 01, 2007 8:31 AM

To: [log in to unmask]

Subject: Re: [TN] Soldering to Pt/Ag terminations



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



             <[log in to unmask]>



             Sent by: TechNet

To 

             <[log in to unmask]>         [log in to unmask]



 

cc 

 



             02/01/2007 03:05

Subject 

             AM                        [TN] Soldering to Pt/Ag



                                       terminations



 



             Please respond to



              TechNet E-Mail



                   Forum



             <[log in to unmask]>



             ; Please respond



                    to



               Peter Barton



             <[log in to unmask]>



 



 











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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