The study proposes a new probe design method for reducing the maximum stress of the probe used for vertical probe card in wafer probing, in which a plurality of circular arcs in different radii are provided to drop the stress so as to avoid the probe being overstressed while in operation. Probing force of the vertical probe is determined from the deformation of bending and bucking of the probe body similar to cobra shape. It can be controlled either by the axial offset of the needle tip and that of the insert part or by the thickness of the cobra body. This paper presents a methodology combining the nonlinear finite element method with the quadratic searching optimization to derive the optimal geometry design of the cobra body. In geometry optimization, the curve of the cobra is modeled by a dimensionless 4th order polynomial and the coefficients of the polynomial are set as the design variables, the contact force between needle tip and probing weld pad is constrained to be greater than a special level. The objective function is to define the minimized maximum stress of the probe during probing. A case study under a contact force of 11.0 g is discussed as the optimal cobra probe is close to a curve with two arcs, and it can provide approximately 17.6% reduction in the maximum stress comparing with the conventional single-arc cobra needle. (C) 2008 Elsevier Inc. All rights reserved.
