A study of the mechanical and fatigue properties of metallic microwires

Abstract

A B S T R A C TThere is an increasing necessity to record the deformation characteristics of microelements containing freestanding bond wires. The data required are either mechanical or thermal such as Young's moduli, stress-strain values, fatigue-and thermal-strain data, but the nominal strength of a structure changes by scaling its size. Due to this size effect, material data cannot be taken from macrospecimens, thus special testing procedures were introduced. Laser optical sensors based on the speckle correlation technique were applied to determine non-contacting strain values with high strain resolution. For the mechanical properties tensile tests were used. A special ultrasonic resonance fatigue system is described for testing freestanding microwires.In this study the stress-strain and fatigue response of microwires of Cu with a purity of 99.99+% with diameters between 10 and 125 µm with a typical bamboo structure have been investigated. A size dependence of the yield strength which increased with decreasing diameter was observed, while the fracture elongation showed contrary behaviour. Fatigue life also decreased with reduced diameters.An explanation is presented based on formed oxide layers, strengthening effects by dislocation pile-ups and a pronounced localization of formed slip steps acting as notches being most dominant in the very thin microwires.Keywords Cu; deformation behaviour; fatigue life; laser speckle strain sensor; microwires; size effect; ultrasonic fatigue testing system. I N T R O D U C T I O NMetallic microwires of Cu, Al and Au with diameters of a few microns have a wide application range as interconnects in microelectronic packages (e.g. microelectronic devices, sensors, microelectromechanical systems = MEMS). During service life bonding wires are subjected to severe cyclic mechanical or thermal loading. 1 One of the major factors determining the reliability of microdevices is the mechanical performance of the bonding wires under service conditions. This strongly depends on the dimensional and microstructural characteristics of the wires described in terms of a mechanical size effect. 2 In order to understand the basic deformation mechanisms tensile tests have proven to be a helpful tool. The effect of diameter on the mechanical properties of microwires has been investigated for many years.

Effects of Thickness and Crystallographic Orientation on Fatigue Life of Single-Crystalline Copper Foils

“…Yang et al 13) showed that the fatigue life of copper wire decreases with a decrease in their diameter from 50 to 30 µm. Weiss et al 14) and Khatibi et al 15) also obtained similar results in the diameter range of 10 to 125 µm. Similarly, Dai et al 16,17) reported that fatigue resistance of copper foils decreases with a decrease in the foil thickness in the range from 20 to 420 µm.…”

Recent Observations on Tin Pest Formation in Solder Alloys

“…Size reduction generally results in a greater surface area:volume ratio and hence favors tin pest formation on free, but not occluded, surfaces. In contrast, smaller samples are usually associated with higher strengths via reduced grain size 18,19 and would be expected to offer higher resistance to tin pest formation. The existence of fewer grains, as for example in fine wires, introduces a further complexity, because of anisotropy effects.…”

High-cycle fatigue behavior of thin-walled CoCr tubes

“…Size effects on mechanical and fatigue properties of small scaled materials have been subject of numerous investigations [30,31]. A majority of these studies have shown a consistent size effect on the tensile ductility of materials [14,30]. It has been observed that decreasing the ratio of thickness (t) to grain size (d) in thin polycrystalline materials results in a strong reduction of plasticity due to the incompatibility of the neighboring grains and strain localization.…”

“…Ultrasonic resonance fatigue testing systems, working at about 20 kHz, are frequently used for determination of lifetime curves and studying the crack growth behavior of metallic materials up to gigacycle fatigue regime [12,13]. In addition to the conventionally used dumbbell shaped bulk samples these systems can also be used for testing of special geometries such as thin wires [14], foils [15] and thin walled tubes [16,17].…”