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.