ENIG Solderability Issues

We are facing solderability issues on a particular lot of electroless nickel - immersion gold plated circuit boards. The nickel thickness measured with an XRF is around 400 microinches.

Spec calls out for 130 microinches nickel and 3 - 5 microinches gold.Assuming there is no porosity on the gold. Is there any explanation as to why we are seeing solder wetting issues? Profile seems to ber unning properly. We are using lead-free SAC 305.

R.D.

Expert Panel Responses

"The nickel thickness measured with an XRF is around 400 microinches." That is impressive thickness for electroless nickel. Are you certain it is electroless and not electro-nickel? SAC305 prefers the former far more than the latter.

Robert "Bob" Lazzara
President
Circuit Connect, Inc.

Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.

My first check would be for any contamination on the circuit board. I have in the past had issues when there was a film of contamination (possibly silicon form the bags) causing wetting issues. A manual hand cleaning resolved the issue.

Do you other lots have as much Ni? Are there any other differences between the lots?

Also, when you say the profile seems to be running properly, did you verify with a MOLE or DATApaq? Ovens are built better today, but it is always a good practice to verify your profile on a regular basis.

Walter Pierowski
Process Engineer
Esterline Interface Technologies

Mr. Pierowski is currently a Process Engineer for SMT and Electrical Assembly with 20+ years of experience. A majority of the experience was based in screen/stencil printing for electronics manufacturing including application support for SMT, LTCC, HTCC and thick film.

The specifications you describe, 130 microinches nickel and 3 - 5 microinches gold, fits the description for electroless nickel, immersion gold (or ENIG), which is defined in IPC 4552. But you don't have ENIG, you have something else altogether and I could not even predict what would happen if trying to solder to 400 uinches of nickel.

Richard D. Stadem
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.

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 pad.

Potential Root Causes of ENIG Surface Discoloration:

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.
The gold may not be thick enough. Galvanic reaction in the water could be resulting in a dramatic increase in nickel-oxide thickness.
Baking the boards after a wash process in a standalone oven can cause oxidation on the surface of the ENIG fairly easily.
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.
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.
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:

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
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
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.

Umut Tosun
Application Technology Manager
Zestron America

Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".

Reader Comment

This is also the solderability issue I am encountering. Thank you for the responses. A chemical analysis is yet to be conducted, because some PCB charges have a less "shiny" ENIG finish. This could be due to contamination, oxidation, porosity, or as I assume in my case, improper storage and baking.

Luc Mitrea, Wittenstein SE

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