Bev,
The bulk concentration (particularly when you are talking about a ball, not a joint) means almost nothing.
It is important whether the platelets are segregated at the ball surface or they are in the bulk.
Coming back to your original question, I'd have to say that all the answers you've got so far are right to some extent, but all of them should have started withqq a famous phrase "it depends" :-)
In addition, I think you should have asked whether a mixture/alloy of Sn and AuSn4 intermetallics is sokderable.
Let me explain.
If you try to solder to a surface covered with a Sn/ AuSn4 mixture (and the bulk concentration of Au in the mixture/alloy is less than 30%), then you have pure Sn in the mixture and the lower the bulk percentage of Au, the more pure Sn you have in the alloy. IF there is no segregation of the AuSn4 intermetallics at the surface, then the more pure Sn - the easier to solder to it. However, it the intermetallics is segregated at the free surface, then one would be trying to solder to it and according to the Werner's e-mail, it's a bad idea.
Regards,
Vladimir
----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Wed Jan 16 17:24:52 2008
Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold Intermetallic
Dave,
I only have 0.8 to 1.2% gold in the solder balls - as determined by XRF.
Bev
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, January 16, 2008 5:06 PM
To: [log in to unmask]
Subject: Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold Intermetallic
Hi gang! Steve - I just sent you another set of photos showing a solder
joint that has gold embrittlement. Bev - if you have a solder joint that
contains the AuSn4 microstructure phase then you have an indication that
your overall composition has exceeded the "magical" 4% limit. The
presence of the AuSn4 platelets cause the solder joint to behave in a
brittle manner when subjected to some use environments. The formation of
the AuSn4 phase isn't going to interfere with the formation of your solder
joint but they do result in solder joint integrity degradation. The
pretinning of gold plated component leads is an attempt to replace the
gold with a solder coating and lower the overall total percentage of gold
that would exist in the solder joint - the lower the overall solder joint
gold content, the lower the likelihood for the AuSn4 phase to form.
Dave Hillman
Rockwell Collins
[log in to unmask]
Steve Gregory <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
01/16/2008 03:35 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Steve Gregory <[log in to unmask]>
To
[log in to unmask]
cc
Subject
Re: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold Intermetallic
Hi Inge ol' pal!
I went back and looked on my pages, and am not sure if they're still
there. But one of these might be:
http://stevezeva.homestead.com/files/fracture_bad.jpg
http://stevezeva.homestead.com/files/fracture_bad_2.jpg
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Wednesday, January 16, 2008 3:09 PM
To: [log in to unmask]
Subject: [TN] SV: [TN] SV: [TN] Solderability of Tin-Gold Intermetallic
Not really true, the Scottish blood in my veines tells me to be careful
with money, the kyrgise blood makes me crush the table, therefore, noone
can actually be under it.
I think there are pictures from my large AuSn platelets somewhere on
Gregory's wall. If not, I can probably dig them up again from some old
report. If such crystals are abundant in a solder joint, the components
may just jump off the board when giving a mechanical shock.
/Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Werner Engelmaier /*
Skickat: den 16 januari 2008 20:30
Till: [log in to unmask]
Ämne: Re: [TN] SV: [TN] Solderability of Tin-Gold Intermetallic
Hi Inge,
As usual people are well advised not to argue with a (this) Swede; 1) he
usually knows what he is talking about, and 2) if not, he will drink you
under the table.
The IMCs formed by Sn/Pb, and Sn/Ag as well, are platellet-shaped crystals
(in cross-section they may look like needles but are always plateletts),
and I have seen no evidence that they are solderable. Solderability simply
depends on how much Sn, or SnPb, to which you wet, is present. So the
amount of these IMCs affect both solderability as well as
Au/Ag-embrittlement.
Inge, how is Gerd? I sure have not heard anything about him for some time.
Werner
**************
Start the year off right. Easy ways to stay in shape.
http://body.aol.com/fitness/winter-exercise?NCID=aolcmp00300000002489
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