An Experimental and Numerical Investigation Into Wafer Probing Parameters Based on Thin Wafer Breaking Strength

Mechanical contact caused using excessive probe force produces an oversized scrubbing mark that may result in damage to the die pad and the silicon chip breaking for thin wafer. Therefore, investigating the relationship between the wafer thickness and the limited breaking stress of the wafer, and applying this relationship as a basis for establishing suitable design rules for a multilayer needle layout are crucial. In this paper, two experimental techniques, three-point-bending test and ball-on-ring, were setup and carried out to measure the force- displacement relation of various wafer thicknesses (100-25 μm). The results from the testing then coupled with finite-element analysis to reverse finding the breaking stress/strain as a function of wafer thickness. In addition, experimental setup with a single tungsten needle probe contact with Al pad were employed to investigate the relations between the overdrive, beam length, and scrub mark length. A 3-D computational probing simulation model was developed and verified against the experimental data. The model is then used to examine the effects of the beam length, overdrive distance (OD), and shooting angle on the maximum stress induced within the wafer. Finally, a four-layer probe card needle shape design has been demonstrated as a practical application example; it is shown that for a wafer thickness of 25 μm and an OD of 60 μm, the allowable shooting angles of a four-layer needle probe card are as follows: 1500 μm and 0°-4° (Layer 1); 2100 μm and 0°-9° (Layer 2); 3500 μm and 0°-15° (Layer 3); and 4000 μm and 0°-15° (Layer 4).