Strange Oxydation / Black deposit on ZIF connectors after PCB assembly

[otter42]

Hi,

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

[g00se]

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.

[otter42]

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.

[Ian.M]

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!

[jpanhalt]

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.

[otter42]

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. 

Very interesting thanks, if it can be related to black pads, it's my though, we also had such issues...

Well in fact I do have some black pad problem with ENIG pads on this PCB (A) but not on the second one (B) (I have some pretty big pads for various soldering need at product assembly level) on the same board, not all of them. In the past I had a really faulty batch were the board was properly assembled, varnished by the EMS but it was impossible to solder wire on pads for product assembly without scratching them.

It's not particularly correlated with black deposit on the connector :
- I've black deposit on connector without problem on ENIG PADs
- I've black deposit on connector + black ENIG pads
- I've black ENIG pads only issues
It's however difficult to know if the pad pads are related to hand manipulation because on older batch I do only have some boards hanging arround for test or expertise, not the properly stored one.

For the connector material :
- Phosphor Bronze (contact)
- 120u Tin over 30u Nickel (platting)
- Solder pin seems to be brass

To understand why it doesn't  affect the rest of the board,

Except for some big ENIG exposed PADs for wire soldering during product assembly and some for ground contact, all the board is covered with a conformal coarting (not hard one something relativly soft an clearly not agressive) varnish by the EMS. That may change a lot of thins.

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!

It's already among my listed task & idea to find someone capable of doing that and get the cost, it may or may not be too much, I've no idea of how much it would cost...
That what I said a few weeks ago, we need to know what's the black deposit composition and then we know were it can come from.

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.

Thanks a lot it's very interesting.

It's our main hypothesis an issue with the cleaning process (it's a water based process distilled water + detergent) of the EMS production line, or at the drying stage (they blast manually some air on the panel, then it goes in an oven to dry).

It could be either the nickel from the ENIG finish (I think when the issue was too big, it was because there was too much nickel on the bare PCB) that goes in the cleaning water solution... or the tin from the connector plating leaving letting exposed the under Nickel platting : what do you think ?

I do have access to some chemistry ressources I may ask.

[Jwillis]

Nickel(III) oxide Ni2O3 (Black) is create  by the oxidation of pure carbonyl nickel powder. Not something that happens naturally. Both Nickel(I) and Nickel(II) are green. Nickel(II) is the principle Oxide of Nickel which can occur naturally.
Many connectors are Silver plated. Silver plating provides the best conductivity but is susceptible to oxidation. Silver oxide is a very dark brown to near black.   

[otter42]

Nickel(III) oxide Ni2O3 (Black) is create  by the oxidation of pure carbonyl nickel powder. Not something that happens naturally. Both Nickel(I) and Nickel(II) are green. Nickel(II) is the principle Oxide of Nickel which can occur naturally.
Many connectors are Silver plated. Silver plating provides the best conductivity but is susceptible to oxidation. Silver oxide is a very dark brown to near black.   

For the connector material :
- Phosphor Bronze (contact)
- 120u Tin over 30u Nickel (platting)
- Solder pin seems to be brass

Hi, thanks.
Those are not silver plated, maybe other thing on the board (for example the solder paste can have some silver in it like 3% or 1.5%, but I don't have its characteristic only the EMS knows that).

[jpanhalt]

Nickel(III) oxide Ni2O3 (Black) is create  by the oxidation of pure carbonyl nickel powder. Not something that happens naturally. Both Nickel(I) and Nickel(II) are green. Nickel(II) is the principle Oxide of Nickel which can occur naturally.
Many connectors are Silver plated. Silver plating provides the best conductivity but is susceptible to oxidation. Silver oxide is a very dark brown to near black.

Just a small clarification:  While formation of metal oxides is called oxidation, oxidation is a term that refers to loss of electrons by the metal.  In valence bond theory, the valence increases.  Often , discoloration of metals like silver is due to oxidation by substances such as H2S in the environment.  Nickel(II) is not always green.  From Wikipedia:

The precipitation of solid black nickel sulfide is a mainstay of traditional qualitative inorganic analysis schemes, which begins with the separation of metals on the basis of the solubility of their sulfides. Such reactions are written:[6]

Ni2+ + H2S → NiS + 2 H+