I agree with Dave. Our first internal specification for immersion silver when there was only one (AlphaLevel) on the market was 2 to 10 microinches. We used that specification because the plating always fell in that range. When other immersion silver chemistry came on the market we found that several gave plating thickness above 10 and we had to change. We changed our internal specification from 2 - 10 to 2 - 20. We didn't just arbitrarily double the upper limit. We did our home work and actually did lots of reliability testing with various immersion silver thickness. Even the ones we tested up to 30 to 40 microinches gave acceptable results but we decided to be conservation an use 2 to 20. The other thing you need to be aware of is that the immersion silver thickness does vary depending on features size. You could be measuring 13-16 microinches on large ground planes and when you measure very small features 0.8mm pitch pads the immersion silver thickness could be in the 22-26 microinch range.
Yes 22-26 is certainly above our 2 to 20 but we view the 2 to 20 as a guideline and not a hard, fast, pass/fail range. As long as our copper features on a board have a silver color (and that happens around 1-2 microinches) and they don't exceed 30 microinches we use the boards. We are not overly concerned about the low end because if the boards solder there isn't a reliability risk and at the upper end our data shows we don't have a reliability risk at 40 microinches so we decided to be conservative and reject any thickness >30 microinches. If you follow the IPC-4553 guidelines you should have no problem.
To address the second part of your question we using the 2 - 20 microinch specification, which I explained above is in actuality 2 - 30, with all of our assembly processes including SAC305 and SAC405. So I think you should have no problem with 2 - 20 microinches.
Regards,
George
George M. Wenger
Senior Principal Reliability / FMA Engineer
Andrew Corporation - Wireless Network Solutions
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 Office (732) 309-8964 Mobile
E-mail: [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Tuesday, August 16, 2011 3:46 PM
To: [log in to unmask]
Subject: Re: [TN] Immersion Silver PCB Finish - Acceptable Thickness
Hi Todd - using a range of 6-20 with SAC305 should still be acceptable in
terms of not having a silver embrittlement situation. Look in the
IPC-4553A, Appendix 6 and the IPC Technical Report 586 for the data used
by the 4-14 Committee to establish the range used in the 4554A
Specification.
Dave Hillman
Rockwell Collins
[log in to unmask]
"MacFadden, Todd" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
08/16/2011 02:32 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"MacFadden, Todd" <[log in to unmask]>
To
<[log in to unmask]>
cc
Subject
[TN] Immersion Silver PCB Finish - Acceptable Thickness
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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