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Subject:
From:
"MacFadden, Todd" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, MacFadden, Todd
Date:
Tue, 16 Aug 2011 15:32:41 -0400
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Hi folks,

We specify a thickness range for ImmAg on our PCBs of 6-16 microinches (0.15-0.4 um). A supplier is requesting to use a range of 6-20 u". These boards will be assembled with silver-bearing solder paste (SAC305). Should I be worried about silver embrittlement (i.e., the weakening of the solder joint due to excessive presence of Ag-Sn IMC platelets in the bulk) at this increased thickness range?

I know there was much discussion about this issue during the latest rev of IPC-4553A, in which the upper limit of ImmAg was set at 16u". I found the thread below between Werner and Dave Hillman in the archives, in which Dave suggests no embrittlement concern with ImmAg, although the thickness is not specified in that discussion (which took place before the IPC-4553 revision).


Does anyone have any thoughts on this?

Thanks in advance,
Todd

P.S. I couldn't find any Tech Reports on the IPC 4-14 Committee page, so perhaps the study Dave mentions below was not completed?




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

From: "David D. Hillman"

Date: Tue, 27 Jan 2009 10:45:30

To:

Subject: Re: [TN] SnPbAg solder and Immersion Silver PCBs





Hi Werner! I don't believe that there is an silver embrittlement issue

with the use of ImAg PCBs. Rockwell Collins has been using ImAg PCBs for

over 10 years with no issues of silver embrittlement. We have also

conducted several lead-free assembly studies using the SAC305 solder alloy

and have found no issue with silver embrittlement. And, the IPC 4-14

Plating committee did not find any silver embrittlement issues with a test

assembly that was excessively silver loaded - ImAg PCB finish and silver

in the solder paste - and we did not find any evidence of silver

embrittlement. The 4-14 committee will be publishing that study as an IPC

Technical Report in the near future after completing the necessary

committee reviews. As a general rule of thumb, I think silver

embrittlement is on a much lower risk register than gold embrittlement.

However, I have seen the PLCC study data and it indeed was a case of

silver embrittlement so I think that each situation needs to be understood

to avoid having an embrittlement issue.



Dave Hillman

Rockwell Collins

[log in to unmask]<http://listserv.ipc.org/scripts/wa.exe?LOGON=A2%3DTECHNET%3B2b12bd15.0901>



-----------------------------------------------



Werner engelmaier

Sent by: TechNet

01/27/2009 10:30 AM

Please respond to

TechNet E-Mail Forum



Subject

Re: [TN] SnPbAg solder and Immersion Silver PCBs





 Hi Kevin,

That "thin (?) silver finishes...prevent the dissolution of the component

lead into the solder joint" is a Red Herring.

All you may be getting is AG-embrittlement which is just as bad as

Au-embrittlement. It was just such 'silver finishes' on TI PLCCs that

caused those PLCCs to fall off PCBs during bed-of-nail testing in 1982 at

IBM-Austin and resulted in the formationof the IEEE Compliant Lead Task

Force.



Also, how would an Ag-finish prevent the formation of IMCs with the lead

metal?


Werner






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