TA.TEC Consulting
Loop Forming: How Reverse Movement Influences Heavy-Wire Bond Lifetime
How reverse movement during loop forming pre-deforms the heel – and why high reverse values can reduce fatigue lifetime.
The study investigated how different levels of reverse movement during loop forming affect the heel region of 300 µm aluminum heavy-wire bonds.
Reverse movement introduced plastic deformation and visible wrinkling in the heel before lifetime testing. Increasing reverse movement also changed the local microstructure and surface topology.
Under the investigated conditions, moderate reverse movement had only a limited effect. At reverse factors above approximately 30%, lifetime decreased significantly.
Loop forming is not only a geometric process step. Reverse movement can plastically pre-deform the heel before cyclic loading begins.
The study shows that high reverse values can reduce fatigue lifetime. The authors therefore recommend using only slight reverse movement to achieve high lifetimes.
Heel surface topology showed a clearer relationship with lifetime than the reverse-factor setting alone.
The results apply to the investigated 300 µm aluminum wires, loop-forming conditions, and the LCT and modified BAMFIT test methods used.
LCT and modified BAMFIT produced different stress distributions, fracture locations, and material-dependent lifetime behavior. The results therefore do not establish a universal threshold for other wire diameters, materials, loop geometries, machine parameters, or test methods.
Research Source
Florens Felke, Anne Groth, Martin Hempel, Bernhard Czerny, Golta Khatibi, Torsten Döhler, Ute Geissler
Effect of the loop forming process on the lifetime of aluminum heavy wire bonds under accelerated mechanical testing
Microelectronics Reliability 154 (2024) 115337
Engineering Insight PDF herunterladen (Englisch)