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Subject:
From:
Steven Creswick <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steven Creswick <[log in to unmask]>
Date:
Sun, 31 Mar 2013 06:43:17 -0400
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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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