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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