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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