ENIG Surface Industry Standards and Common Practices

ENIG Surface Industry Standards and Common Practices
Per IPC-4552, there should be 2-5 micro-inches of immersion gold applied over 120 to 240
micro-inches of electroless nickel.

Nickel Thickness:
Condition 1: Nickel thickness results below the range (Thickness < 120 to 240 micro-inches)
may result in gold peeling and solderability issues.
Condition 2: Nickel thickness results above the range (Thickness > 120 to 240 micro-
inches) may result in nickel cracks and solder joint failure.

Immersion Gold Thickness:
The immersion gold is only there to prevent the nickel from oxidizing. The gold absorbed
into the solder joint is adding no benefit. The immersion gold is a porous surface. 
Condition 1: Gold thickness results below the specified range (Thickness < 2 to 5 micro-
inches) may result in insufficient oxidation resistance for the nickel. 
Condition 2: Gold thickness results above the specified range (Thickness > 2 to 5 micro-
inches) may result in an attack on the nickel itself. The nickel may corrode and ultimately
result in black pad if aggressive enough. The thicker the gold, the greater the risk for black

Potential Root Causes of ENIG Surface Discoloration:

1. Manufacturing nonconformities related to the age of the galvanic bath. ENIG surface
becomes vulnerable to reactive components.  This root cause is related more to the
faulty metallization.
2. The gold may not be thick enough. Galvanic reaction in the water could be resulting
in a dramatic increase in nickel-oxide thickness.
3. Baking the boards after a wash process in a standalone oven can cause oxidation on
the surface of the ENIG fairly easily.
4. Either the gold layer is too thin or not homogenous so diffusion effects of
chemical/water into the nickel surface could occur leading to oxidation.
5. Low P content in the Ni layer.  The P is in the Ni to prevent corrosion.  The washing
may be accelerating the corrosion of the Ni.  In this case, a standard ENIG
evaluation for low P or black pad needs to be conducted.
6. The ENIG layer sometimes reacts with chlorides (from flux) in the loaded cleaning
agent bath resulting in oxidation, i.e. oxidation reaction as in the black pad effect.

Troubleshooting the Discrepancies on ENIG Surface:

1. Conduct a standard ENIG surface evaluation to determine high or low Phosphorous
(P) content:
a.  In the Ni layer 
b.  In between the gold / nickel
Rich P content will result in Nickel Oxide
2. Check Nickel-Oxide formation on the very layer of ENIG by using:
a. AES (Auger Electron Spectroscopy) and look for Nitrogen and Oxygen signals 
b. TEM (Transmission Electron Microscopy) and look for Nitrogen Oxide layers or
SIMS (Secondary Ion Mass Spectroscopy) and look for Nitrogen
c. SERA (Sequential Electrochemical Reduction Analysis) and look for presence
oxides and intermetallic 
3. Investigate if a plasma treatment step with either Hydrogen or Oxygen would
improve the result. If Oxygen helps, then the issue is related to the organic residue.
If Hydrogen helps, then the issue is more of an oxidation reaction.