Nothing is ever that simple, but I KNEW that someone would come back with a load of
stuff on intermetallics and so on, so the the overall Technet answer would be
complete. MAybe a Whoops from me in trying to be brief, basically I made these
assumptions - which I should have spelled out:
The Sn96 was specified for better temp withstand - which is not novel
That the Au board finish was to avoid Sn/Pb plate or HASL because of the Sn96, and
the Au plate would actually be the usual standard thin submicron Au over Ni.
But the component mix would otherwise be regular finish.
That's kinda close to the prefered scenario in your post. I know that from work in
the past Sn96 is thought to be too inflexible and not good in life testing, but
believe that its had somewhat of a reappraisal following more recent work. It is in
use in some automotive and hi rel applications as outlined above.
Finally to look at the Lead free literature because although Sn96 is a long
established alloy and is extensively referenced, it has been re-evaluated in this
work alongside Sn63 and the proprietary mixes. Running on from that was the thought
that pretty well all the lead frees are high temp so if that is the intention, and
Sn96 is NBG then some of the alternatives are spelled out and so on, but by now the
what ifs and maybes are getting too many to cover.
Anyhow, overall this would allow an informed judgement on the specific application
compared to the generalities I addressed, avoiding the "can't solder to gold"
syndrome.
Mike Fenner
BSP, OX15 4JQ, England
T: +44 1295 722 992
M: +44 789 999 7715
F: +44 1295 720 937
-----Original Message-----
From: Paul Klasek <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 25 August 1999 02:18
Subject: Re: [TN] SN96 solder...
>Mike I doubt it's that simple ;
>while you have a relatively good feel one way (more dispersed negative
>effect) ;
>it's the Ag factor which is very similar in it's embritteling effect to Au
>(being less known due to less usage on bare cards and metalisation).
>Apart from plasticity modulus being shifted with absence of Pb (having a
>play in the recommendation) ;
>if you'd take Wassink's recommendation of no more than 4% Au in a joint you
>have :
>3.5%Ag + X%Au bare card + X%Au component metalisation (bad stories on LED's
>overplate) which gives you a snowflake in hell chance of staying under 4% on
>bottom line .
>
>The recommendation is correct Steve ; look for silver embritteling
>documentation ; more of tin dispersing effect is minimal compare to massive
>set back on the silver character ;
>unless you have less than 0.3um (metric) of gold on the nickel and NO gold
>parts ; you have a slim chance of living more than say 1-3 years (pending
>geometries) with that particular card .
>A probability to high not to be conveyed to your customer in my humble
>opinion .
>
>Let's see what Werner says ; this is as far as I know .
>Won't bother you with my piles of files .
>
>You've been testing us Mike didn't you ? ; can't believe Bob's trainee would
>miss silver in line ?
>Sorry to differ ; don't want to see buddy in a ditch .
>
>See you
>
>Paul Klasek
>http://www.resmed.com
>
>-----Original Message-----
>From: Michael Fenner [mailto:[log in to unmask]]
>Sent: Wednesday, 25 August 1999 5:21
>To: [log in to unmask]
>Subject: Re: [TN] SN96 solder...
>
>
>Standby for a welter of responses!
>Here is a simple answer.
>The Au issue for Sn96 is the same for Sn63. Except that as there is about
>50% more
>tin present per joint the amount of gold per unit of tin is less, so Au
>embrittlement
>is less likely. So basically if you reckno you could do it in Sn63 you will
>be OK in
>Sn96. For further details refer to almost anything written on lead free I
>guess.
>
>Mike Fenner
>BSP, OX15 4JQ, England
>T: +44 1295 722 992
>M: +44 789 999 7715
>F: +44 1295 720 937
>
>For product information visit www.belzona.com
>-----Original Message-----
>From: Stephen R. Gregory <[log in to unmask]>
>To: [log in to unmask] <[log in to unmask]>
>Date: 24 August 1999 17:52
>Subject: [TN] SN96 solder...
>
>
>>Hi ya'll!
>>
>>I've got a project coming where it's called out to use a SN96 solder. Not
>>having the characteristics on the top of my head, I went to a web page to
>>refresh my memory. A SN 96 alloy is 96.5% Tin, and 3.5% Silver. It's a
>>eutectic alloy with the liquidous and solidus temperature at 221-degrees C.
>>
>>The page stated the typical use was in applications where standard lead
>based
>>solders did not meet the safety or strength requirements. It also went on
>to
>>say that a SN96 was not recommended against gold plated surfaces. Guess
>what?
>>I was told the PCB is gold plated!
>>
>>So what is the issue, gold embrittlement? Undesirable intermetallics? Would
>>sure like to know something about this...
>>
>>Appreciate any info!
>>
>>-Steve Gregory-
>>
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