Contact Resistance issue - Gold plating on PCB

[Alex.Nelson]

Hello everyone,
I have spent 3 months now trying to resolve high contact resistance issue on my supplier PCB.
Does anyone have experience with ENIG (immersion Nickel+immersion Gold) plated contacts on PCBs?

-My "golden samples" plated in California do not show resistance creep after I put bords through reflow oven (260C max for 10sec, lead free temperature profile, convectional oven, gas is your regular air). The resistance increase from 20 Ohms to 30 Ohms in this particular design.
-My Asian sourced boards are measured 20 Ohms before reflow and !5000! Ohms after reflow due to unknown reasons.
- When the resistance goes above 200 Ohms, LEDs on my boards basically do not light up. Circutry design.

Facts I know:
- Gold thickness is within spec on all boards (between 2 and 4mil, IPC4552 standard)
- Oxyden is found on Nickel layer below Gold on boards after reflow (Auger test)
- Supplier does not measure copper and nickel impurities in gold bath
- Suppiler does not measure Palladium concentration in activation bath
- Nickel surface morphology is unknown yet, I have not tested this yet
- Gold is naturally porose at ENIG thicknesses. Arial SEM does not show any anomalies on the gold surface
- Resistance drops from 5000 Ohms back to 20 Ohms if I rub the contact surface with my finger, PVC eraser, etc.

Your thought are highly appreciated.

Alex.

 

[Murphys Law]

Only issue I've work on on ENIG was 'black pad' where on BGAs, the substrate would separate from the solderball after assembly.

Basically, a complex intermetallic would form that was crystaline and would make it brittle. http://www.smtnet.com/library/files/upload/ENIG-Black-Pad-Problem.pdf

I don't think this is related.

Did you bake your Asian boards before assembly? They maybe outgassing.

 

[Murphys Law]

One other thing - Are you using N2 during board assembly?

 

[Alex.Nelson]

Thanks for your input!

I use regular convection ovens, no additional gasses, such as N2, just air.

I sort of checked the outgassing hypothesis, when I sent boards for FTIR and found nothing on the surface of the gold. At some point I though CO, or CO2 from solder paste, or organics from FR4, but again, found nothing.

I also checked the black pad. You can usually detect it by measuring P content in Ni and near IMC. The concentration of P and distribution gradient. That did not play out. The P was in spec. (6-11%)

I spent like 10K on different testing by now And I am upset that I can not find the root cause

Alex.