Bending fatigue behavior of eutectic Sn-3.5Ag solder bump bonded on FR4-PCB was
characterized by experimental and finite element method (FEM). To investigate an effect of stress
state on bump failure, which had not been weighed in conventional Coffin-Manson model of
Nf=K ⋅εp
-1~-2, ‘fatigue frequency variable’ and ‘bump viscoplasticity’ were included in analysis
procedure. As experimental results, with increasing fatigue cycles from 3,000 to 10,000, bond
strength decreased from 98.9% to 76.5%, and from 97.5% to 67.1% at the fatigue frequencies of
2.5Hz and 5.0Hz, respectively. Stress state could be critical components to determine fatigue life,
which should be combined in Coffin-Manson criteria. FEM calculation showed that higher bending
frequency led to higher normal stress development at the solder and IMC interface, but smaller plastic
strain in bump. However, bending fatigue experiment revealed discrepant results from that of
Coffin-Manson criteria. Higher bending frequency, which was predicted to give rise to smaller εp at
solder, showed dramatic bond deterioration of solder bump on UBM (under bump metallurgy). This
was confirmed experimentally through SEM (scanning electron microscopy) observation as cracks
were found at the solder bump and UBM interfacial IMC, Ni3Sn4, in case of the higher bending
frequency.
Effect of ultrasonic rolling strengthening on the bending fatigue characteristic of 12Cr2Ni4A steel gear
