Kevin,

 Add to that the monitoring of the ENIG. If the process is not controlled
you could end up with black pad or other soldering problems.

Bob K.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Glidden, Kevin
Sent: Wednesday, August 15, 2012 12:01 PM
To: [log in to unmask]
Subject: Re: [TN] PCB finish: ENIG vs. hard gold

Thank you, everyone, for your responses.  To summarize (and ensure my
understanding is correct):

"Hard Gold" is essentially electroplated gold that is co-deposited with
other alloying metals (nickel and/or cobalt) to make it "hard".  This
plating is primarily used for gold-finger edge card contacts, and not
soldering.    Many responses indicated that due to the alloying metals,
these surfaces are not at all solderable.

The distinction is that "soft gold" or "gold flash" [which may be  what the
PCB supplier actually meant to say] that is also electroplated can typically
be soldered to, but has some key disadvantages to ENIG, such as:

- the ENIG immersion gold process is self limiting and as such results in a
uniform plating thickness, whereas the electroplated gold final thickness is
dependent on many factors such as plating chemistry cleanliness, geometry,
current density, processing time, etc, etc.  The concern is that too much
gold can cause gold embrittlement of the solder joints (weak solder joints)
as the gold itself "dissolves" into the solder joint mass during the
soldering process.    The max thickness recommended is 4 to 8 microinches
(vs typical 3-5 microinches for ENIG), which requires strict process
control.

- the ENIG process results in better uniformity, whereas the electroplated
gold has a higher porosity, which in turn can result (particularly at
thinner deposit thickness) with less protection for solderability, reduces
shelf life, etc.

- the ENIG process results in higher purity gold, as it is chemically
induced.  The electroplated gold typically has a lower purity as other
conductive metals can co-plate in solution.

- the ENIG immersion gold process results in gold plating over the pad edges
as well as the lands, whereas the elctroplated covers only the lands and
would leave bare copper on the pad edges.

Based on these points and the responses, it seems ENIG is the better and
safest choice.


From: Glidden, Kevin
Sent: Tuesday, August 14, 2012 12:12 PM
To: [log in to unmask]
Subject: PCB finish: ENIG vs. hard gold

Hi everyone,

We have a PCB supplier who is offering "Hard Gold" as an alternate to ENIG
finish, due to PCB size and processing capabilities.  The application is
Class 3 (mil/aero).  The circuit is thin FR4 and subject to SMT reflow w/
Sn63Pb37 (non RoHS) solder alloy, and moderate flexing in end application.

Can anyone offer info on the differences between the two finishes, and/or
their suitability?  Is there an IPC spec (or specs) that outline "hard
gold"?

Thanks,

Kevin Glidden


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