Nickel coated Aluminium blackening issue!

[var10]

We have a d38999/20ff35sa connector -

http://www.digikey.com/product-detail/en/D38999/20FF35SA/AD38999/20FF35SA-ND/2063569

that fits in our anodized aluminium enclosure. The PCB sits inside the enclosure and the inside is epoxy'd. We epoxy it by placing the enclosure upside down with the connector screwed to the enclosure and pour epoxy thru the two holes at the bottom of the enclosure. To stop epoxy from leaking between the connector flange and the enclosure, before filling with epoxy we pour some peelable solder mask (2211-8SQ) latex to stop the epoxy from flowing out through the connector and enclosure faying faces. Once we peel of this solder mask the areas where we had the solder mask has turned black. I am pretty sure that its the solder mask and probably thinking of replacing it with peelable silicone. I have a feeling that it is just removing the nickel coat on aluminium connector and leaving a black discolouration mark. I would be interested in knowing what is the effect on the aluminium? could it be corrosive? (we potentially have removed a layer to stop corrosion of aluminium - this is obvious) but will this accelerate corrosion? has the solder mask caused any oxidation? Like to know because there are some units in the field and fix if the issue is very critical as they can be exposed to very humid conditions and salt fogs.

V.

 

[CoryPad]

The data sheet for 2211-8SQ mentions that it darkens during curing. It seems unlikely that a soft polymer with low adhesion strength is capable of removing a nickel surface layer from the aluminium connector. Nickel oxide and sulfide are both black, so a reaction product is a more likely possibility. I do not think there is a corrosion risk to the aluminium connector, but it is a possibility.

 

[IRstuff]

If an adhesive is able to remove a coating, then there's an issue with the coating. There used to be a "Scotch tape" test for optical coatings, which required the coating to survive a stick and peel from a piece of Scotch tape.

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[var10]

I don't think its just the adhesive that is removing the coating. May be a chemical reaction is removing or changing the colour of the coating. It worries me because could that potentially also cause corrosion?

 

[CoryPad]

Possible, but seems unlikely.

 

[tbuelna]

From the photo it does not appear the nickel plate has been degraded. Do you have any QA requirements for the epoxy encapsulating process where a discoloration of the connector nickel coating surface after removal of the masking would be cause for rejection?

You could always perform a test on a sample connector using the same masking compound and epoxy sealant. And then do some destructive testing on the test specimen to determine if the darkening is just cosmetic or a more serious condition.

 

[var10]

@tbuelna - I am performing those tests now. We started noticing this after one month so I plan to leave it for few weeks and then do salt fog and corrrosion tests. We did not notice the colour degradation right away. It started appearing after few weeks.