Hi Guy(s), I thought this discussion shouldn't, like many others, just fade
away, but be recorded, so I copied to Dropbox/Soldering and gave this title:

"

*Correspondence about gold scavenge when using SAC and discussion on
alernatives to SAC, e.g. Indium alloys*
*Inge*


On 30 March 2013 11:55, Mike Fenner <[log in to unmask]> wrote:

> I guess that's me!
> Most points have been covered, but a few points to clarify.
>
> In these sorts of discussions it is easiest to think of indium as tin's
> lazy
> cousin.
> Tin dissolves gold rapidly. So in a tin/lead solder you tend to have hard
> tin/gold intermetallics adjacent to a lead rich layer. This will crack when
> stressed. The stress levels required can be very low. A gentle tap can be
> sufficient in extreme instances. The problem is reduced slightly and
> counter-intuitively with high tin alloys such as SAC. The more concentrated
> tin dissolves the gold even faster but as there is 50% more per joint the
> gold is diluted further.
>
> While tin dissolves gold rapidly, indium being the last cousin does so
> slowly. The difference is about 12:1
> For the same sort of reasons Indium makes solder joints more slowly than
> tin. This needs to be kept in mind when selecting /formulating fluxes for
> indium alloy soldering.
> So indium can make solder joints to gold surfaces. Note that indium does
> dissolve gold even if only slowly,  things speed up/change when they're
> hotter, so for most high indium based alloys a max operating temperature of
> 125C is recommended.
>
> Tin makes brittle /hard intermetallics with gold. Indium has a similar
> relationship with copper. Pure indium will solid state diffuse into copper,
> even at room temperature, it will do so indefinitely even at room
> temperature till all the indium has gone. The resultant intermetallic is
> hard and brittle. [The interface can end up looking as though the mythical
> tin worm has eaten it]. Pure indium is rarely used of course and bare
> copper
> is easily avoided.
>
> So when looking at soldering thick gold with tin alloys or copper with high
> indium am alloys to me the answer is: Get rid of the Sn/In or the Au/Cu.
> IF possible plate with 4-5 um Nickel and you can use any alloy you like
>
> IF you can't change the gold surface, then you have to change solder
> Getting rid of tin is easy enough, just use a Pb based alloy instead. The
> technical problem then is high melting point. (See below on Au/Sn)
> IF you need a low melting point without tin then you are usually looking at
> indium instead. There are any number of Indium lead combinations.
> Personally I prefer 60In/40Pb. This has a similar Melting range to tin/lead
> so represents a minimum change solution. Joints formed by indium/lead
> alloys
> have excellent thermal cycling properties which is another reason for using
> them.
> Gold tin is an alternative. This makes a uniform very high strength bond
> witch is difficult to break.
> The MP of 80Au/Sn is 280C. This puts reflow over 300C and most soft solder
> fluxes will thermally degrade during reflow, giving a hard to remove
> "caramelised" residue. You'll get a solder joint OK but it might be hard to
> see. Flux removal will be difficult. For these reasons nitrogen inerting is
> good, it's not necessary to get ultra low PPM as we just want to protect
> the
> flux not the joint formation. A low oxygen atmosphere will simplify
> cleaning. The high MP also gives long profile times and cooling down can
> also be an issue. At high temps most organics - substrates etc - are pretty
> delicate.
>
> Costs
> Tin is sold by the ton[ne]
> Indium by the pound/kilo
> Gold by the ounce.
>
> Enough said!
>
> Availability
> In spite of the high MP 80Au/Sn is processible by some solder companies
> pretty much as Sn/Pbs. So fine powder paste is possible and quite common.
> Solid wire is possible from a few companies, flux core is not possible.
> No one can afford bar solder in this alloy
> .
> When looking at availability of Indium alloys as solder paste we need to
> recall another interesting property of indium: perfectly clean indium will
> cold weld to itself faster than you can say superglue. This clearly has
> implications when trying to make oxide free indium (alloy) powder.....
> So very fine powder in high indium alloys is.... just lets say "difficult".
> It will almost certainly be made to order in small quantities from a very
> limited supplier base, making for relatively high prices. (But still
> nowhere
> near Gold Tin.) Check lead times also as these will vary according to the
> time of ordering. Solid wires are common to order, flux core no.
> When using indium alloys especially as pastes, all the rules about
> handling,
> shelf life etc, processing conditions/control are more true and really do
> need to be observed.
>
>
> Regards
>
> Mike Fenner
> Bonding Services & Products
> M: +44 [0] 7810 526 317
> T: +44 [0] 1865 522 663
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge Hernefjord
> Sent: Friday, March 29, 2013 3:50 PM
> To: [log in to unmask]
> Subject: Re: [TN] solder which does ot scavenge gold
>
> Hi Guy,
>  Uve already got guru's advice, so maybe I don't add much of interest. Only
> tell you that we had a nightmare with Indium soldering to gold. The result
> was rhat we banned Indium for the next coming 1,000 years.  Why? I'll brief
> you and if you want deeper insight, you should contact the old english
> Indium man. Nobody beats him, natural after half life at Indium.
>
> So what...
>
> Independent of what Indium alloy you use, the first phase when soldering
> with SAC or other high-Tin solders, is the formation of a AuIn2 layer. This
> takes only seconds. The  process eats at least 1 micrometer of gold. If the
> gold is thinner, there will not be a correct IMC and the component will
> loosen when thermomechanically exposed. So, remember: the gold that you
> solder to, must be thicker than 1 um!
>
> Furhermore, if your total solder joint is thin, let's say 15 um or thinner,
> AND you have a thick gold, there is a risk of a second IMC, namely Au9In2.
> It's not ternary, just one single phase and is known for its brittlement
> and having overall lower data than AuIn2. If the soldering top temperature
> is too long, you may even get Kirkendall voids,which makes Au9In2 even more
> brittle.
>
> I hope I remember the above correctly. I worked with tat problem about 30
> years ago!
>
> Inge
>
> PS there is a an old paper in my Dropbox under  'Soldering....' The
> headline is " Aspects on Indium Soldering...'  I have better reports, but
> don't know where they are..
>
>
> On 29 March 2013 06:27, Guy Ramsey <[log in to unmask]> wrote:
>
> > Background info:
> > We were asked to populate and assembly with a Ma-Com part, MA46H120, a
> GaAs
> > Constant Gamma Flip-Chip Varactor Diode. The data sheet says:
> >
> > Mounting Techniques - These chips were designed to be inserted onto hard
> or
> > soft substrates with the junction side down. They can be mounted with
> > conductive epoxy or with a low temperature solder preform. The die can
> also
> > be assembled with the junction side up, and wire or ribbon bonds made to
> > the
> > pads.
> >
> > Solder Die Attachment - Solder which does not scavenge gold, such as
> > Indalloy #2 (80In-15Pb-5Ag) is recommended. Sn-Pb based solders are not
> > recommended due to solder Embrittlement.  Do not expose die to a
> > temperature
> > greater than 235C, or greater than 200C for longer than 10 seconds.
> >
> > The Indalloy #2 cost $2,222.00 for 100gm or type five solder paste.
> >
> > We now have another customer asking for us to solder to thick gold. They
> > don't have a low temp requirement and asked for SnAu solder (Indalloy
> #182)
> > 80Au-20Sn. . . I shudder to think what that will cost, and I don't think
> > the
> > part, an inductor, will survive the 300C reflow.
> >
> > Question:
> > Does SAC 305 "scavenge gold"? Would it form a brittle solder connection
> on
> > a
> > thick soft gold pad?
> >
> > Guy
> >
> >
> >
> >
> >
> >
> > ______________________________________________________________________
> > This email has been scanned by the Symantec Email Security.cloud service.
> > For more information please contact helpdesk at x2960 or
> [log in to unmask]
> > ______________________________________________________________________
> >
>
>
> ______________________________________________________________________
> This email has been scanned by the Symantec Email Security.cloud service.
> For more information please contact helpdesk at x2960 or [log in to unmask]
> ______________________________________________________________________
>
>
>


______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________