lead free board/chem-film chassis corrosion

[DefenseMechE]

We recently had an electronics box go through saltfog testing. It is a vented enclosure so during the test the corrosive atmosphere was present inside the box. There was significant corrosion where the board mounts to the aluminum chassis. The chassis is chromate, class 3 (low elec resistance). Any thoughts as to correcting this issue? We obviously can’t just paint it since it is a ground path for EMI suppression and conduction cooling.

 

[mcgyvr]

I'd look at zinc plating the aluminum chassis versus conversion coating. How long of a salt test? Most chromate conversion coatings "should" pass 96 hour testing if done properly.

 

[DefenseMechE]

Yea, this is the first time I have seen a failure like this. The chassis survived accept where the board attached to the chassis. I need to dig deeper. I am wondering if at some point they removed the coating for lower resistance for EMI reasons. I've never used zinc on aluminum, I will look into it- thanks. I will also update with any more findings.. I was not the lead on this design, so there are a few things I don't know (one being how long the test was)

 

[CoryPad]

Please provide more details regarding the materials used for the joint members.

Zinc electroplating on aluminium is not likely to work.

 

[DefenseMechE]

Cory- I have that exact question in with the TPM, and am waiting an answer.

 

[mcgyvr]

my head is obviously somewhere the sun is not shinning today. I was thinking how we use zinc plating on areas of a "steel" chassis that is masked for grounding. Gosh I even wrote aluminum chassis in my reply... Time to go home

 

[DefenseMechE]

Aluminum is chromated, class 3 per MIL 5541

Board is gold(IAW B48896G) over low stress nickel plate (AMS-QQ-N-290)

 

[KENAT]

So the board mounts direct to the Al chassis, there's no standoff or similar?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[DefenseMechE]

Correct. The Gold plating is in direct contact with the Chromate over the AL. Most miltiary electronics are configured this way since they have little/no convection cooling- it's all through the chassis. I am begining to think the pictures I saw of the corrosion is just salt getting between the board and the AL..not actually erosion.

 

[KENAT]

I was going to say some kind of crevice type corrosion or something.

On the limited salt spray testing I've done it can be tricky trying to work out visually what is aluminum oxide and what may be condensed salt.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[CoryPad]

If the aluminium is in contact with gold or nickel during salt spray exposure, galvanic corrosion will occur with the aluminium part corroding preferentially.

I can't think of a great solution. Perhaps you can isolate the aluminium electrically from the gold/nickel with a thermally conductive ceramic?