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October 2005

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Subject:
From:
Leo Higgins <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Leo Higgins <[log in to unmask]>
Date:
Fri, 14 Oct 2005 08:27:36 -0700
Content-Type:
text/plain
Parts/Attachments:
text/plain (177 lines)
We manufacture an IC package using a Cu carrier upon which an Au/Ni/Cu/Ni/Au
stack is plated to create wire bond contacts and a die attach pad.  The ICs
are assembled on the matrix sheet with high temp (150 - 175C) for die attach
and wire bonding, followed by transfer molding at about 170-180C, followed
by 5 hours of post mold cure at 175C.  Then the copper carrier is etched
off, and this etching process cleans the exposed Au pads somewhat.  Cu
diffusion into the Au layer from the Cu carrier is low and has been
characterized with scanning Auger depth profiling.  Under some conditions
the Au pads can develop a darker color, but solderability remains excellent.
The solder paste cleans the Au surface of any oxides, rapidly dissolves the
Au and any Cu that has diffused into the Au, and wets very nicely to the
underlying Ni. The mechanisms described below do not appear sound.


Best regards,
Leo

Director of Applications Engineering
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas     78704

ph     512-383-4593
fx      512-383-1590
[log in to unmask]
www.asat.com


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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Ryan Grant
Sent: Friday, October 14, 2005 9:23 AM
To: [log in to unmask]
Subject: Re: [TN] Direct Immersion Gold


Interesting.  Is the brown color the diffused and oxidized copper?  Does
the high affinity of Tin and Gold lift the oxidized copper atoms within
the gold matrix into pseudo solution with the bulk solder?  Even if the
oxidized copper is not molecularly bound (wetted) to the tin atoms, if
it is completely surrounded, it would appear as part of the bulk solder
(or at least until it precipitates out).  That would turn the exposed
copper gold matrix layer into a sacrificial layer.  So in other words,
even though the solder can't wet to the oxidized copper on top, it would
displace that copper by lifting the gold with it and access un-oxidized
copper underneath.  Is that what is happening?

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gerard O'Brien
Sent: Thursday, October 13, 2005 5:56 AM
To: [log in to unmask]
Subject: Re: [TN] Direct Immersion Gold

Ryan - it has to do with density of the deposit. If you perform a
porosity
test on DIG, the results mimic that of 30 microinches of gold rather
than
the 2-4 microinches that is actually there. I have been testing in real
time
DIG and using a wetting balance have a data set over 900 days of storage
-
non protected office environment. If I compare my ENIG longterm test to
DIG,
the DIG solders better at 899 days, wets faster and produces a higher
wetting force. The underlying Cu (oxide) I guess is easier to solder to
than
a partially passivated nickel layer.
After 900 days it does not look very nice - rather brown in color but
boy
does it solder.


Regards


Gerard O'Brien
Photocircuits.


-----Original Message-----
From: Ryan Grant [mailto:[log in to unmask]]
Sent: Wednesday, October 12, 2005 10:34 AM
To: [log in to unmask]
Subject: [TN] Direct Immersion Gold

Hi Techies,

Can anyone explain what direct immersion gold is?

My supplier gave me samples of several different board finishes, of
which two groups were direct immersion gold from two different
suppliers.  The gold over copper was 5uin and 7uin, which I was
surprised the copper hadn't already diffused into the gold.  I told him
I had no interest in direct immersion gold...but, since he gave me the
samples, I might as well test them with the samples that I am interested
in (immersion silver).  First, I baked the boards at 125C for 48 hours
to kill the solderability.  Then they were reflowed twice to kill
solderability, and checked for wetting.  Again, I expected the copper to
completely diffuse through the gold, and/or the solderability to be
dead.  To my surprise, the gold was still there and solderability was
still good; whereas immersion tin and OSP showed evidence of dewetting.
(ENIG and immersion silver still had good wetting too).

So can anyone explain why the copper and gold didn't diffuse into each
other?

Thanks,
Ryan

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