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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