Hi Steve,
Found this from our own Richard Stadem posted in Circuitnet, and should
answer all your questions:
To provide a flat pad, nickel is plated to the copper at a thickness of
approximately 150 microinches (not 150 microns). Nickel oxidizes fairly
quickly, so a very thin layer of gold (3 to 5 microinches) is subsequently
plated over the nickel (not underneath as others have said) to prevent it
from oxidizing. ENIG is preferred by some for that reason; the
solderability remains excellent for years.
The purpose of the nickel in ENIG is to provide a barrier between the
copper pads and the gold. If the nickel is skipped and the gold is plated
directly to the copper, a reaction will occur and over a very short time
the copper will eat through the gold and can even appear on the surface as
an "orange pad". You cannot solder to this pad without physically removing
these oxides first.
Chemical stripping can be done, but not without some collateral damage to
other features, including the solder mask and the unprotected via walls. It
is also very difficult to clean away 100% of the stripper, some always
remains behind.
During the soldering process, 100% of the 3 to 5u inches of gold on the top
is immediately dissolved into the molten solder; no solder joint is formed
with the gold. The underlying nickel is then exposed, and some of the
nickel will dissolve into the molten solder, but no-where near all 150u
inches of it. Your intermetallic layer is then made of the dissolved nickel
into the solder alloy; you solder to the nickel barrier.
The rate of dissolution of the nickel into molten tin/lead solder is 10
times slower than the rate of dissolution of copper into that same solder.
At reflow temperatures, it takes a little bit longer to form a good IMF
with nickel than it does with copper.
Consider then, attempting to solder a PCB that has some pads with no
nickel, some with, and some that may or may not have nickel under the gold.
For those pads that meet the IPC 4552 standard, the gold will be
immediately absorbed, and if the nickel is at least 100 u inches thick, no
copper oxides will have formed, and the nickel should form a good IMF with
the molten solder.
For those pads that never had benefit of nickel, but are only exposed
copper with a thin OSP coating that was probably applied several days after
the problem was discovered and is subsequently oxidized to the level of a
1910 Lincoln penny, well, the chances of forming a good IMF between the
solder and the copper is sort of like maybe, but probably not.
When soldering, as others here have stated, some IMFs are going to form
more than others, and while some IMF is usually enough, the fact is at
least SOME of those pads that APPEAR to have plated over just fine and are
actually skipped or nearly skipped and yet are covered with gold with some
degree of nickel oxide barrier under the gold, who knows? You simply cannot
solder to an oxide, once formed. Flux will not remove these types of oxide
barriers effectively.
With ENIG, a fabricator has to keep many, many process parameters under
perfect control before, during, and after nickel plating, and even more so
when doing the final gold plating. It is quite obvious that some or many
things went wrong with this lot. So Tony Lentz and Jerry Karp gave you good
advice; scrap the entire lot and tell the fabricator to replace them, and
make sure the fabricator does not take the bad lot back and rework them and
sell them to you later.
Look for another fabricator who has some proven experience in ENIG plating.
They have to run a very tight ship to plate ENIG properly (and any other
finish as well, for that matter). A really good IPC-certified board shop
can show you complete process control metrics, and the IPC has excellent
standards for performing a PCB fabricator audit.
[image: image]*Richard D. Stadem*
<http://www.circuitnet.com/experts/panel_294.html>
*Advanced Engineer/Scientist*
*General Dynamics*
Richard D. Stadem is an advanced engineer/scientist for General Dynamics
and is also a consulting engineer for other companies. He has 38 years of
engineering experience having worked for Honeywell, ADC, Pemstar (now
Benchmark), Analog Technologies, and General Dynamics.
On Mon, Jan 27, 2020 at 3:45 PM Dwight Mattix <[log in to unmask]>
wrote:
> Without the Ni barrier, the copper will diffuse through the gold rendering
> it unsolderable.
>
>
> -----Original Message-----
> From: TechNet <[log in to unmask]> On Behalf Of Steve Smith
> Sent: Monday, January 27, 2020 2:32 PM
> To: [log in to unmask]
> Subject: [EXT] [TN] Is Nickel really needed under the gold plating on
> copper?
>
> Hi All,
>
> I was asked today if nickel is really needed under the gold for plating on
> copper. I have understood it that the layer of nickel provides a mechanical
> backing for the gold layer and that it was also needed for the soldering
> process. Is this correct and is it the only reason it is used?
>
>
> My regards,
>
> Steve Smith
> Design Drafting Group Manager
> Staco Energy Products
>
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--
Steve Gregory
Kimco Design and Manufacturing
Process Engineer
(208) 322-0500 Ext. -3133
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