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March 2013

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Subject:
From:
Inge Hernefjord <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Inge Hernefjord <[log in to unmask]>
Date:
Sun, 31 Mar 2013 19:34:30 +0200
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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
>
>
>
> -----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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