Rick,
We use an internal specification of 3-16 micro inches of Immersion gold
over 150 +/- 100 micro inches of electroless nickel. This internal
specification was based on work at my former employer when they were
making cell phones. That earlier work was presented at the 1st Annual
IPC National Conference on Surface Finishes and Solderability in MN
about 6 years ago. We also did a lot of work measuring thin deposits of
immersion gold. You can measure extremely thin deposits of immersion
gold. We've measured deposits as thin as a few tenths of a micro inch
and correlated our XRF measurements using Auger depth profiling.
Although our internal specification requires a minimum of 3 micro inches
of immersion gold, once in a while we do get boards with 0.5 to 1.5
micro inches of immersion gold. This usually occurs from new vendors we
haven't dealt with before. Once in a great while we'll also get boards
from an established vendor that have extremely thin immersion gold (this
doesn't happen frequently but it does happen). We always inform our
suppliers when this happens and issue a CAR. We don't consider thin
gold a reliability issue because the gold dissolves and we solder to the
underlying electroless nickel. We do believe that thin gold reduces
shelf live but to the best of my knowledge there isn't any "good" shelf
life data available for various thickness immersion gold plating. If
the thin immersion gold solders "well" (i.e., no defects, low contact
angle, physically strong, and metallurgically sound) it protected the
underlying nickel long enough to form the solder joints and the joints
should be reliable. If the thin gold solders "poorly" it didn't protect
the underlying nickel long enough (or the underlying nickel was poorly
plated to start). I don't think anyone can provide you with
quantitative data that lets one know the shelf life for various
thickness immersion gold deposits. I've had thin deposits that soldered
well after several months and I've also had thin deposits that didn't
solder well the day we got the boards. The best thin to do is get boards
from good vendors, require at least 3 micro inches of immersion gold,
measure what you get, and only use thin deposits when you have no other
choice but also check how they solder.
Regards,
George
George M. Wenger Andrew Corporation
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [log in to unmask]
-----Original Message-----
From: Tikusis [mailto:[log in to unmask]]
Sent: Friday, January 03, 2003 7:59 PM
To: [log in to unmask]
Subject: Re: [TN] 'Thin' Immersion Gold Finish?
Rick,
Are you sure you have the correct standards to measure a thin deposit of
immersion gold? You can not use hard gols
standards on your x-ray fluorescence to measure immersion gold. Also,
electroless nickel must be measured with
electroless nickel standards. Call your x-ray fluoresence equipment
manufacturer for help, and also toalk to the
immersion gold supplier for advice on measuring such a low thickness.
Our experience is that immersion gold is
difficult to accurately measure. Also, a deposit of 3 to 9 microinches
should be visibly good coverage. 1 to 1.5
microinchs should be visually poor coverage, which is reason for
rejection. If the coverage looks visually good, then
I would doubt that it is really as low as 1 to 1.5 inches thick.
Rick Thompson wrote:
> We have an overseas board supplier that is supplying an immersion gold
> finished board that is only 1-1.5 microinches of gold over 100
microinches
> of nickel. I believe this may be too thin a gold layer for adequate
> protection of the nickel but don't have any objective evidence for
this.
> Can anybody give me any feedback or other information sources that
might
> support this? IPC-2221 specifies approximately 3-9 microinches of
gold
> which is what we generally require, but I'm looking for additional
objective
> evidence for requiring thicker gold deposits.
>
> Thanks in advance.
>
> Rick Thompson
>
> Sr. SMT Process Engineer
> SMTEK International, Inc.
> +1 (805) 532-2800
> [log in to unmask]
>
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