News from ESPEC North America
Will HALT Accelerate Fretting Corrosion?
Friday, December 30 2016
Fretting corrosion is a bit difficult, because it’s really a two stage process.
Recently a ESPEC Qualmark customer emailed us asking if fretting corrosion on connectors could be accelerated using HALT methodologies. Here is our response:
Fretting corrosion is a bit difficult, because it’s really a two stage process. First, you need fretting from vibration to scrape away the gold on the connector, then you need humidity and oxygen to induce the subsequent corrosion of the metallic layers underneath. The vibration in HALT is great at the first part. However, the actual corrosion process isn’t significantly accelerated by the chamber stresses. It is a chemical process that is well described by the Arrhenius Equation, and it takes time. Also, the atmosphere in the chamber is 100% dry nitrogen, so you don’t have the environment to induce corrosion.
If you want to see if a design or connector change will reduce fretting, it’s easy enough to put samples with and without the change in the chamber in a combined environment (the thermal extremes should accelerate connector wear by changing the connector retention and spring forces during vibration), then stop the test every 30 minutes and do a microscopic examination of the connector, looking for fretting damage. You can easily see if a change is resulting in less fretting, or if certain contacts are fretted more than others.
If you really want to see the corrosion happening, do a combined test in the HALT chamber first, then move the specimens to a humidity chamber. The humidity chamber will accelerate the corrosion if the contacts have been fretted enough by the vibration to allow it to happen.
When you’re trying to induce fretting, you want to fixture the product so that the connector can move with respect to the contacts. If you have a board with a connector plugged onto a card edge connector, fixture the board down, then just loosely wire tie the connector on, so it doesn’t fall off, but is free to move and respond to the vibration as it will.
I have seen connector fretting cause failures when doing HALT on devices with card edge connectors, so I know for a fact the vibration will fret the contacts!
Also, don’t go hot enough in temperature to allow the connector to deform, as that will impact your results. Remember that the connector is typically experiencing a retention force of some type, and you don’t have to get too close to the melting point to have that force result in deformation. If it’s a typical PVC connector housing, stay under 80C.
FYI. And remember – If it isn’t broken, you’re not done yet!