Anti-buckling vertical microprobes with branch springs

Abstract

This thesis describes anti-buckling vertical probes with branch springs for the wafer-level testing of integrated circuit (IC) chips. Conventional vertical probes require a guide structure to prevent buckling due to the overdrive requirement ($\gt 40 \mum$); however, the guide structure not only increases the cost of fabrication, but it also requires a troublesome assembly procedure. Two problems of previous guide-free vertical probes were the small overdrive due to abrupt stiffness change and the demand for novel prober motion. The proposed vertical probes have branch springs on the left and right sides of the main spring to prevent buckling in the absence of a guide structure. The branch-spring force, generated when the branch spring contacts with the main spring, prevents buckling by means of moment compensation. The requirements of the vertical probes for IC chip testing are pitch ($\lt175 \mum$), overdrive ($\gt40 \mum$), contact resistance $(\lt0.5 \Omega)$ and touchdowns $(\gt 10^6 cycles)$. A sufficient contact force is needed to reduce contact resistance but excessive contact force must be prevented to avoid the plastic deformation caused by overstress. In this paper, the design parameters of the proposed vertical probe are the repeat number of the basic unit ($\It{N}$), the line width of the probe ($\It{W}$), the gap ($\It{G}$) and the longer beam length of the branch spring $\It{L_{bs}}$. The values of these design parameters are determined by parametric study in order to satisfy the design requirements of the vertical probes for IC chip testing. Among the design parameters, $\It{N}$ and $\It{W}$ are related to the initial stiffness. G is related to the stiffness change point, and $\It{L_{bs}}$ is related to the second stiffness. From the parametric study, $\It{N}$ = 9, $\It{W}$ = $10 \microm$, $\It{G}$ = $5 \mum$ and $\It{L_{bs}}$= $80 \mum$ were chosen based on the criteria of low contact resistance, yield strength, and large overdrive. The pro...