Eric,
I once saw a particular component that started exhibiting cracked solder
joints and the leadframe termination turned out to have been changed to a
thick silver plating over copper. It seems that with the thicker silver, an
intermetallic layer of Ag3Sn formed over a layer of Cu6Sn5 and that the two
layers separated easily. The earlier parts without silver plating did not
have such weak joints and did not fail in such a manner. I do not know if
that is the same mechanism as what Werner and his team saw.
In Sn62/Pb36/Ag2 solders, the Ag3Sn is there, but typically forms as slabs
or blocks in the bulk of the solder joint. AuSn4 also does this, but it
typically forms very large, very thin plates, and as you can see there will
be 12X more of AuSn4 than Ag3Sn for a given atomic% of either Ag or Au
material, and more intermetallic/solder surface area to boot given the
plates themselves are thinner. It is that separation of solder from the
AuSn4 plates that causes the classic gold embrittlement everyone talks
about. Not enough AnSn4 plates, no embrittlement. I've never seen this
type of failure due to Ag3Sn plates in Sn62/Pb36/Ag2 solder.
Edwin Bradley
Motorola
> ----------
> From: [log in to unmask]
> Reply To: TechNet E-Mail Forum.;[log in to unmask]
> Sent: Thursday, May 6, 1999 4:44 PM
> To: [log in to unmask]
> Subject: Re: [TN] Silver coating on connectors
>
> <<File: ATT193938.txt>><<File: ATT193939.txt>><<File: ATT193940.txt>>
> Hi Werner,
>
> Thanks for the interesting story. So, if silver forms brittle IMC with
> tin at
> about 3%, why do people use (and pay for) Sn62/Pb36/Ag2, Sn96/Ag4 and even
> Sn95/Ag5? Are all these silver-bearing alloys inherently brittle?
>
> Is there any hard evidence that the failure mode in this particular TI
> case back
> in 1982 was related to Sn/Ag IMC and not to something else -- say for
> instance
> -- electroplating defect?
>
> Eric Yakobson
> Alpha PC Fab
>
>
>
>
> [log in to unmask] on 05/05/99 03:01:20 PM
>
> To: [log in to unmask]
> cc:
> Subject: Re: [TN] Silver coating on connectors
>
>
>
>
> In a message dated 5/5/99 12:25:22, [log in to unmask]
> writes:
> >Hi Werner,
> >Sorry, I've never heard of "silver embrittlement", which is worse than
> gold
> >embrittlement before. Perhaps you could enlighten us? Also, I'm not
> quite
> sure
> >I understand how the nickel barrier layer would help to eliminate it.
> >Thanks.
> >Eric Yakobson
> >Alpha PC Fab
>
> Hi Eric,
> The industry found out
>
################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe: SUBSCRIBE TechNet <your full name>
To unsubscribe: SIGNOFF TechNet
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################
|
