|Ask the Experts|
July 15, 2020
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.
|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.
Circuit Connect, Inc.
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.
Esterline Interface Technologies
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.
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.
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:
Application Technology Manager
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