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December 2008

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Subject:
From:
"Wenger, George M." <[log in to unmask]>
Reply To:
(Leadfree Electronics Assembly Forum)
Date:
Sat, 6 Dec 2008 22:44:25 -0500
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My guess is that I've been in the solder joint / failure an
Bob,



My guess is that I've been in the solder joint / failure analysis /

reliability world a  similar amount of time and the one think I've

learned that I can pass along is 



"Never Say Never".



Regards,

George

George M. Wenger

Andrew Wireless Solutions

Senior Principal FMA / Reliability Engineer

40 Technology Drive, Warren, NJ 07059

(908) 546-4531 [Office]  (732) 309-8964 [Cell]

[log in to unmask]

 



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

From: Leadfree [mailto:[log in to unmask]] On Behalf Of Bob Landman

Sent: Saturday, December 06, 2008 1:04 PM

To: [log in to unmask]

Subject: Re: [LF] Pro's and Con's of using Immersion Silver



A person I know who works at CALCE (www.calce.org) has spent a lot of

years in studying solder joint reliability.  She told me the other day

that she thinks that IAg is better than ENIG because ENIG has "black

pad" problems and I should switch to IAg.  



Here are several article on ENIG problems



http://circuitsassembly.com/cms/content/view/5042/95/

 

According to George Milad and Gerard O'Brien of the IPC Plating Process

subcommittee, "Black pad is a low-level nickel corrosion defect formed

during the immersion gold deposition step. It takes a compromised nickel

surface and prolonged dwell in the immersion gold bath for it to occur."

Black pad is well known to fabricators and EMS suppliers, and as Milad

and O'Brien added, the subcommittee "was convinced that with good

definition and a proper specification, the black pad phenomenon could be

virtually eliminated."



-this is not a problem IF you have a decent board vendor.  We have never

seen black pad nor has our assembler except for one time (and that

vendor is no longer in business).

 

The other problem is a weakness in the nickle coating as is described in

this article

http://www.empf.org/empfasis/feb04/0204help.htm



When an ENIG surface finished PWB undergoes soldering, the solder must

adhere to the underlying electroless nickel plate. This is because the

immersion gold is so thin that all of the gold dissolves into the solder

upon soldering. The solder bonds directly to the electroless nickel. 



If that nickel surface is contaminated in some way, the resulting joint

will be weak and the locus of failure will be just below the surface of

the nickel, leaving both nickel and phosphorous (a normal constituent of

electroless nickel) on both the pad and the failed solder joint surface.





The EDAX analysis of the failed solder joint showed the presence of

nickel at both the fracture surface of the solder joint and the pad from

which the solder joint had lifted (see Figures 4-1 and 4-2) This is a

positive indication that contaminated nickel was the cause of the

failure. 

 

Our assembler (who I have used for several decades) has not had a good

experience with IAg.  A client gave them boards with silver saver paper

between each board (SMT both sides). They ran half the boards (which ran

fine) then sealed the rest in a dry seal bag.  Six months later they ran

the rest of the boards (tried to) and the results were terrible.



Gold has been successfully used for decades (before SMT). Tektronix and

HP both used gold plated PCBs and they were serviceable decades later

(when repairs were needed).  Silver in the ceramic terminal strips in

Tektronix scopes (I'm a former Tek and HP Field Engineer) would tarnish

and require cleaning to be used.



If a board needs to be serviced or modified at some time in the future,

or the runs of boards are not completed when the boards are delivered, I

would never use IAg and that is why we use ENIG.



Bob Landman, President

Senior Member, IEEE PES, Reliability Society

H&L Instruments, LLC

Electro-Optics for Industry & Science

34 Post Road, PO Box 580

North Hampton, NH 03862-0580

(tel) 603-964-1818 (fax) 603-964-8881

www.hlinstruments.com



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