Steven,

What about using Epo/Tek H44 single component gold filled epoxy?  It cures at 150C, lower than Guy's temp max. It's been decades since I had experience with EpoTek but I recall the product was highly regarded.

http://www.epotek.com/sscdocs/Datasheets/H44.pdf

Bob

Sent from my iPhone

On Mar 31, 2013, at 6:10 PM, Steven Creswick <[log in to unmask]> wrote:

> Mike,
> 
> 
> 
> You are correct about the minimal  inerting required for the In, if using
> flux.  
> 
> 
> 
> For some reason, I was thinking of the Au/Sn with respect to the inerting
> comment, even though I had essentially ruled the Au/Sn out.
> 
> 
> 
> Kindest regards,
> 
> 
> 
> Steve C
> 
> 
> 
> 
> 
> From: Inge Hernefjord [mailto:[log in to unmask]] 
> Sent: Sunday, March 31, 2013 4:14 PM
> To: TechNet E-Mail Forum; Steven Creswick
> Subject: Re: [TN] solder which does ot scavenge gold
> 
> 
> 
> Thanks Steve, formic acid it was. No, we could not handle them, but for our
> MRSI  cell II it was no problem. Its placement accuracy is +/- 5 microns!The
> machine stands on pressurised air cushions, and a 1 ton granite block
> absorbs vibrations that the cushions couldn't stop. In opposition to a
> Panasonic chip shooter MRSI is slow, but fast enough.
> 
> 
> 
> On 31 March 2013 19:43, Steven Creswick <[log in to unmask]> wrote:
> 
> Formic acid?
> 
>    .... and Mike thinks we couldn't handle 100 x 200 µm performs   chuckle
> chuckle.   :-)
> 
> Ours was the medium vac model, down to about 70-80 mT...
> http://www.sstinternational.com/prod_vacpres.html
> 
> Steve C
> 
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge Hernefjord
> Sent: Sunday, March 31, 2013 1:35 PM
> To: [log in to unmask]
> Subject: Re: [TN] solder which does ot scavenge gold
> 
> Hi again,
> 
> this seems to be a never ending story. Well, I add some from our production
> of GaAs and InGaAs MEMS like upcons. We found that the Indium oxidized very
> fast for a number of alloys, so, after a lot of experimenting, we found the
> best solution.  We used a SSEC vacuum soldering machine, put the hundreds of
> objects in the vacuum chamber, with 25 micrometers thick InSn preforms and a
> small weight on them all. Then we sprayed a "cloud" of ant's acid (forgot
> the chemical name) and put the cover on, vacuum pumped and soldered at +115C
> for 15 minutes.  Don't ask me why this and that, because I don't have all
> details in mind. If I remember right, noble Mr In helped us..
> 
> There is a somewhat useful article about the process in Dropbox under
> Soldering " Development of..Indium Soldering..."
> 
> Inge
> 
> 
> On 31 March 2013 16:05, Mike Fenner <[log in to unmask]> wrote:
> 
>> Hmm
>> Not sure there is much benefit from inerting at low temperature in
>> presence of flux.
>> Viability of performs against paste would depend on the volumes
>> involved and geography of assembly.
>> I see Guy talked about T5 paste. If T5 really is needed that implies a
>> tiny dot size and implies a small perform also which might give
>> supplier issues and handling issues in use.
>> Personally I would recheck the T5 requirement on the one hand and
>> availability on the other. [This to make sure it isn't a "price book"
>> quote].
>> Possibly other solder processes might be possible.
>> So far as non solder attached is concerned:
>> H20E is OK, but like Steve I prefer 84-1 series more, I step back to
>> take notes on wire bonding etc.
>> 
>> Best Wishes
>> 
>> 
>> 
>> Mike
>> -----Original Message-----
>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
>> Sent: Sunday, March 31, 2013 11:43 AM
>> To: [log in to unmask]
>> Subject: Re: [TN] solder which does ot scavenge gold
>> 
>> Guy,
>> 
>> A couple of additional thoughts for you.
>> 
>> Instead of paste, consider the use of performs.  Like Mike says, flux
>> will be a challenge.  Definitely use inerting and possibly a localized
>> reflow apparatus that provides a very good localized reflow
>> atmosphere.  The low temp of the proposed IN alloy, and the high temp
>> of the Au/Sn will definitely introduce speedbumps in a nice process flow.
>> 
>> If your substrate were LTCC or 96-99% alumina the CTE of the GaAs
>> would be almost a perfect match [3-7, 6-7, and 6-7 PPM/°C
>> respectively].  The slickest thing to do with LTCC is create a pocket
>> to drop the diode in [face up] and use no-loop ribbon bonds to bond
>> from diode to substrate using 0.5 x
>> 2 or 4 mil Au ribbon.   Bonding would readily be performed below your max
>> allowable temp limits.
>> 
>> Since most adhesives do not bond well to gold or solder, the potential
>> for CTE mis-match in your system [and subsequent bad connections]
>> would seem to require a thorough examination.
>> 
>> If you could use ENEPIG for a surface finish, you could
>> non-conductively bond the diode, face up, and wire, or ribbon, bond
>> the diode to the circuit.
>> Can you get away with it from a frequency response/circuit function
>> point of view?  This still presents issues from a process flow point
>> of view, but seems to be the cleanest approach to a hybrid guy.  That
>> or bumping...
>> 
>> A blessed Easter to all.
>> 
>> Steve C
>> 
>> 
>> -----Original Message-----
>> From: Steven Creswick [mailto:[log in to unmask]]
>> Sent: Friday, March 29, 2013 10:57 AM
>> To: 'TechNet E-Mail Forum'; 'Guy Ramsey'
>> Subject: RE: [TN] solder which does ot scavenge gold
>> 
>> Guy,
>> 
>> Others likely have already touched on this but the Indalloy #2 has a
>> 154°C liquidus which meets your temp limitations.  80/20 Au/Sn is a
>> 280°C eutectic which will not meet your temp limitations.
>> 
>> I suspect that 80/20 will be just as expensive as the #2.
>> 
>> Don't know what you substrate/board is, but it will likely not take
>> kindly to the temps required of 80/20 either.
>> 
>> I view a conductive adhesive as being THE last thing I would do.
>> Instead or H20E, I would definitely steer you to Ablebond 84-1
>> [anything in the 84-1LMI, LMINB1, etc series].  Much better thermal
>> characteristics!  But getting any adhesive to adhere to gold is
>> problematic!  ANY amount of substrate/board flex, and the die will pop
>> right off.
>> 
>> Generic silver glasses have too high a cure temp as well.
>> 
>> You could thermosonically flip chip bond this low I/O device to the
>> board if you could bump either the diode or the board.  Simply
>> requires one or two gold ball bonds [to form the bumps] on each diode
>> [or substrate/board pad].
>> The Au/Au bond will form nicely at 150°C + ultrasonics and about 35-50
>> gms of force per 'bump'.  Alas, you require a bondable board and a $250K
>> bonder...   Au/Au thermocompression could work to, but temps will be way
>> too
>> high.  Additionally, the face of the diode will standoff the
>> substrate/board by 25-50 µm, depending on wire size used, and actual
>> bumping process.
>> 
>> I don't like working with high Indiums either, but don't see an
>> immediate alternative.
>> 
>> 
>> Steve Creswick
>> Sr Associate - Balanced Enterprise Solutions
>> http://www.linkedin.com/in/stevencreswick
>>                         616 834 1883 <tel:616%20834%201883> 
>> 
>> 
>> 
>> -----Original Message-----
>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
>> Sent: Friday, March 29, 2013 9:28 AM
>> To: [log in to unmask]
>> Subject: [TN] solder which does ot scavenge gold
>> 
>> 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
>> 
>> 
>> 
>> 
>> 
>> 
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