Inside the Aluminum Contact Spot

The electrical properties of a power connector - such as the resistance during its service life - depend on the number, size and quality of the tiny contact spots (“a-spots”) where the current crosses the contact interface. Contact degradation or aging are due to processes occurring in and near the contact spots. The current density may here become very high. Careful scanning electron microscopy investigations of contact spots from idealized and heavily stressed aluminum contacts that have carried DC show that the electron flow was accompanied by a mass flow by electromigration. This led to a gradual mass depletion in the cathode, resulting in poorer electrical conductivity and aging. When AC was passed in similar contacts, the high voltage drop caused a local 100 Hz thermal cycling of the contact spots. Subsequent electron microscopy examinations of sectioned contact spots suggest that the associated thermal expansion and contraction caused thermal fatigue and cracking, impairing their current carrying ability.