Electronics > Manufacturing & Assembly

Strange Oxydation / Black deposit on ZIF connectors after PCB assembly

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I currently have connectors with black deposit on connectors (populated on ENIG finish PCB) on an EMS assembled PCB.
I explored a lot of possible causes but I've yet to figured out why.

The issue is most probably not related to the connector manufacturer, since connectors from the same reel mounted on the second board type boards are as clean as possible (even after a longer 1.5 year+ storage). The connector manufacturer kindly checked (optical inspection) the issue on some sample boards and think about possible cleaning process issue during PCB assembly, but it's not clear if it's really the case, they are not sure.

The issue only touch board type A and not board type B with the same connector (storage is the same, connector reel is the same, process is roughly similar with top+bottom reflow for A but connector is populated on pass 2 and top only for B whichs is way smaller). Those boards are conformal coated boards with a cleaning process before the coating (the conformal coating used is relatively soft and is non-aggressive and never causes issues on any other boards in any other situation). The connectors are not coated with varnish at all. The issue has been seen mainly on one small batch (almost every board), hopefully we never assembled it and on some long term stored reference board of a very old batch, nothing ever seen on other batch.

Does anyone has already encounter such an issue ?

If anyone has idea to help finding the root cause of the issue, I'll appreciate the help.

1. Top view lock closed

1. Top view lock opened (no black deposit on soldered part)

1. Inside view

2. Top view closed

2. Top view opened

2. Inside view

3. Clean connector top view

3. Clean connector inside view

Is there continuous current flowing through those pins, something like battery backup? 

I have seen something like this happen on battery backed RAM pins over a long time, or on pins that carry current during operation in bad environments.
Maybe because the current causes heat and condensation over time, leading to oxidation? I am just guessing.

No, there is no continious current flowing through those pins. The final device is sleeping (there is no power at all on those pins when it's sleeping) then it's working for a very short time, then it' sleeping, ...
And the oxydation/black depostit came also on brand new boards (never yet assembled in the product maybe stored some time).

Thanks for the idea, I thougth about some forme on fretting / or corrosion due to vibration with too low current, but it would not explain it on the new boards juste stored.

Possibly a metal sulphide film due to exposure to hydrogen sulphide or other sulphur containing compounds e.g. sewer gas or proximity to decaying organics, or contamination of your post-soldering cleaning process.  To understand why it doesn't  affect the rest of the board, you'd need to know the exact plating process for the connector pins, which the manufacturer is unlikely to be willing to divulge.  It may be something similar to the black pad problem with ENIG, which is due to contamination and corrosion of the nickel layer. 

Scanning electron microscope mass spectrometry could determine the composition of the black layer and provide further insight, and companies specialising in failure analysis of electronic components can provide that service, but you probably wont like the cost!

The spotty appearance is something one might expect if an item were not completely rinsed -- like water spots on glassware.  Why black?  Nothing is certain, but zinc contamination can cause such discoloration in nickel plating, but the pins are not nickel plated ( https://www.sciencedirect.com/science/article/abs/pii/S0301479721000864#:~:text=The%20release%20of%20zinc%20from,spots%20on%20the%20coated%20pieces ).  Maybe some alkalinizing/neutralizing agent (amine) in the washes?  Unless you know what the black is, any guess about its chemistry is most likely wrong.


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