Someone asked me "how bad can the resistance get due to corrosion on an RJ45 ethernet connector?". These are the 8 pin crimp-on connectors, sometimes with gold plating on them. When you use them in a "power over ethernet" situation, corrosion resistance can make the connection fail.
I asked some of my fellow consultants in the Oregon IEEE consultant's group, . and got these replies:
How fast can a thing oxidize? Fast! Oxidization is slow fire.
Connection resistance gets crappier (increases) with time. Or the connection can weld itself shut, making the "connector" crappy, instead. It all depends upon the plating, the environment ( wet\dry, acid\base, hot\cold, shake\still ), and altitude ( pressure\radiation ).
I've done a lot of enviro testing. In the basement of bldg 50 @Tek, and for other employers. I've seen connections get crappy in a matter of minutes in certain environments. Or even seconds ... after a flash-over at high altitude, for example.
Low-level analog signals need good conductivity through connectors. Duh. Tek used to specify *at least* 80 micro-inches of gold on all contact surfaces.
Copper can migrate through thin gold.
The demand for these good connections is waning, as is an understanding of *what is* a good connection. Costs are rising. I was recently unable to locate both male and female gold-plated DB-25 pins from a single source. The ones I did find were only good for a few insertions\extractions. The market has moved to cheapo tin platings for PC ports. Lifetime in weeks or months, not years. Or not even specified. The move to lead-free soldering is complicating things.
Old parts won't solder in a no-clean process. Nor will old PCBs. Oxidation needs Reduction; not available in "no-clean". [...]
Everyone's got a Standard. The (now obsolete) ANSI/IPC-D-275 Standard for circuit boards and PCB assemblies has much info about type, quality, and uses of gold. [...]
Vibration has an effect on the kind of plating you need. The fresh metal-to-metal contact area you get when inserting a connector is protected from oxygen exposure (in normal conditions!) unless the contacts move. Then a new surface is exposed and the old one develops an oxide layer. This eventually builds up until you can't get a fresh metal surface to make contact, that's when the resistance jumps up. Gold prevents the oxidation, but gets worn by insertions and vibration cycles. Hence the need for thick gold plating. And you want nickel under the gold.
On another note, if you stuff any connector with silicone grease (not glue!), it will even work under sea water. Forget about sealing the water out, that is very tough. Oh, and water will wick for *any* distance inside a stranded or braided wire to corrode the back side of your connectors. Have a nice day.
Phil's comments are all excellent!
Vibration is indeed an enemy - it can weld the contacts closed, or disrupt the platings we need, or cause fatigue and breakage. Think "ultrasonic welding". That's why I mentioned "shake\not", below.
A microphotograph of the surface of a switch's contact surfaces looks like the result of an ice-breaker ship moving through a frozen ocean. The chunks of oxide get all jumbled up, and inhibit conduction. The resemblance to ice flows is amazing. Low-level signals are blocked but higher voltages can jump the growing barrier.
Pure gold does not oxidize - a "noble" metal. A layer of nickel is supposed to keep the copper from migrating into the gold. [...]
In recent years, switch manufacturers have moved to "selective gold plating", which deposits gold only on the actual contact surfaces. I've always thought this was risky because it inserts variability into an already risky equation. But then I'm not a particularly trusting soul.
The seawater angle is new to me, but makes sense - an excess of conductive ions. I do know that MilSpec salt-spray testing was one of the most corrosive evaluations I ever performed. Our troops need gear that works even in the jungle!
We in the Pacific Northwest are spoiled by our nice, moderate environment. It's depressing (but instructive!) to see your shiny new circuit stop working, grow blue\green crap, and just fall apart overnight with a little heat\cold, moisture, and vibration. It's also enlightening to see a working ckt zap itself to death in a dry Arizona-like environment. You quickly learn words like "Humiseal".
Besides making sure you have gold-to-gold contacts (Whatever happened to rhodium, btw? at one time, it was supposed to save us all...), there is a magic word: "Gas-Tight".
They *used* to rate contacts for this... it depends on the contact design including the geometry and the contact pressure. Patents abound, I'm sure. (Meanwhile, as you've noticed, reliability continues spiraling downward, driven by the convolution of cost and warranty period...)
If it were MY comapany that was going to get sued, I would explicitly not guarantee performance of my POE device except with certain connectors which I'd qualified (or types of connectors; there may also be some sort of type-qualification... mil-spec, aerospace, etc.)
Of course, as someone pointed out, if you fill the connector with silicone grease, that will generally put an end to the corrosion. Some automotive manufacturers do that. Plus, if you're a ham, you've learned to wrap any outdoor connectors in a layer of black sticky stuff that comes in rolls at Radio Shack, prior to taping it with electrical tape and then coat it with ScotchGuard. (Probably not suitable for the decor of an office environment...)
RJ-11 and RJ45 connectors corrode. They work fine in a low dust non-corrosive inside environment.
Why use them?? They are cheap, and are easy to replace when they fail.
My customers like them because:
- They are easy to use.
- They can go to almost any local store to replace them.
As far as current is concerned, I often run 1-2 amps through them. I keep the distance short when using higher currents to minimize voltage drop. I recommend going to 18GA or larger for long distances.