Long Term Interconnect Creep and Weld Fatigue under Cyclic Thermal Load in Heavy Formats
Thermal expansion and cell swell induce cyclic shear stresses that trigger weld fatigue and interconnect creep, requiring compliance flexure loops.
Kirkendall Voids
Meaning
Diffusion induced cavities form at the interface of two dissimilar metals due to unequal rates of atomic migration. In battery interconnects, kirkendall voids often appear between copper busbars and tin or nickel coatings. These microscopic gaps weaken the bond over time.
Growth Factor
Elevated temperatures accelerate the movement of atoms across the grain boundaries. The density of kirkendall voids increases as the battery pack experiences more thermal cycles during normal operation.
Mechanical Reliability
Structural failure of the joint occurs when the concentration of voids reaches a level that cannot support the mechanical stress of vibration. Preventing kirkendall voids requires careful selection of interlayers and plating thicknesses.
Interface Management
Barrier layers like palladium or specific nickel alloys can slow the migration of copper atoms into the surrounding solder. While kirkendall voids are a known phenomenon in electronics, their impact on high current battery tabs is more severe due to thermal expansion stresses. Testing for these defects involves long term aging studies followed by shear testing or electron microscopy.
A well designed plating system reduces the rate of void formation to a level that exceeds the planned service life of the vehicle. This focus on interface stability ensures the electrical path remains intact throughout the product life.