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Abstract: Copper wires are increasingly used in place of gold wires for making bonded interconnections in microelectronics. In this paper, a microstructural study is reported of cross-sectioned free air balls (FABs) made with 23μm diameter copper bonding wire. It was found that the FAB is comprised of a few columnar grains and a large number of fine subgrains formed within the columnar grains around the periphery of the FAB. It was determined that conduction through the wire was the dominant heat loss mechanism during cooling, and the solidification process started from the wire-ball interface and proceeded across the diameter then outward towards the ball periphery. The microstructure of the Cu ball bond after thermosonic bonding was investigated. The result showed that the subgrain orientations were changed in the bonding process. It is evident that metal flow along the bonding interface was from the central area to the bond periphery during thermosonic bonding. [Copyright &y& Elsevier]

Abstract: Copper wires are increasingly used in place of gold wires for making bonded interconnections in microelectronics. There are many potential benefits for use of copper in these applications, including better electrical and mechanical properties, and lower cost. Usually, wires are bonded to aluminum contact pads. However, the growth of Cu/Al intermetallic compounds (IMC) at the wire/pad interfaces is poorly understood, and if excessive would increase the contact resistance and degrade the bond reliability. To study the Cu/Al IMC growth in Cu ball bonds, high temperature aging at 250°C for up to 196h has been used to accelerate the aging process of the bonds. The Cu/Al IMCs growth behavior was then recorded and the IMC formation rate of 6.2±1.7×10−14 cm2/s was obtained. In addition to the conventional yz-plane cross-section perpendicular to the bonding interface, a xy-plane cross-section parallel through the interfacial layers is reported. Three IMC layers were distinguished at the Cu/Al interfaces by their different colors under optical microscopy on the xy-plane cross-sections of ball bonds. The results of micro-XRD analysis confirmed that Cu9Al4, and CuAl2 were the main IMC products, while a third phase is found which possibly is CuAl. During the aging process, IMC film growth starts from the periphery of the bond and propagates inward towards the centre area. Subsequently, with increased aging time, cavities are observed to develop between the IMC layer and the Cu ball surface, also starting at the bond periphery. The cavitation eventually links up and progresses toward the centre area leading to a nearly complete fracture between the ball and the intermetallic layer, as observed after 81h. [Copyright &y& Elsevier]

Abstract: The Sn3.5Ag0.75Cu (SAC) solder joint reliability under thermal cycling was investigated by experiment and finite element method (FEM) analysis. SAC solder balls were reflowed on three Au metallization thicknesses, which are 0.1, 0.9, and 4.0μm, respectively, by laser soldering. Little Cu–Ni–Au–Sn intermetallic compound (IMC) was formed at the interface of solder joints with 0.1μm Au metallization even after 1000 thermal cycles. The morphology of AuSn4 IMC with a small amount of Ni and Cu changed gradually from needle- to chunky-type for the solder joints with 0.9μm Au metallization during thermal cycling. For solder joints with 4μm Au metallization, the interfacial morphology between AuSn4 and solder bulk became smoother, and AuSn4 grew at the expense of AuSn and AuSn2. The cracks mainly occurred through solder near the interface of solder/IMC on the component side for solder joints with 0.1μm Au metallization after thermal shock, and the failure was characterized by intergranular cracking. The cracks of solder joints with 0.9μm Au metallization were also observed at the same location, but the crack was not so significant. Only micro-cracks were found on the AuSn4 IMC surface for solder joints with 4.0μm Au metallization. The responses of stress and strain were investigated with nonlinear FEM, and the results correlated well with the experimental results. [Copyright &y& Elsevier]

Abstract: Cu bonding wire is more and more used for interconnections to integrated circuits (ICs) to reduce cost and increase performance compared to Au wire. To eliminate underpad damage for Cu wire applications, it is worthwhile to reduce the hardness of the free-air ball (FAB). Short heat affected zone (HAZ) and high HAZ breaking load are often required for advanced microelectronics packaging in order to decrease the loop height and thereby the package thickness. Online measurements of deformability and HAZ breaking force at temperatures close to the bonding temperature of 220°C are new tools used in this study to evaluate the effects of electronic flame off (EFO) current and firing time on the Cu FAB deformability and the HAZ length and tensile strength. FABs with 50μm diameter formed from a 25μm diameter Cu wire with a breaking load of 118.6mN were used. EFO currents and firing times ranged from 40 to 250mA and 0.11 to 0.90ms, respectively. Average FAB deformability factors, HAZ breaking forces, and HAZ lengths were in the rounded ranges of 36.64–44.09% (with a deformation force of 0.60N), 107.7–116.8mN, and 167–215μm, respectively. When produced with 250mA current during 0.11ms, the FABs are 7.01–7.89% more deformable than when produced with 45mA during 0.9ms, the HAZ breaking force is 7.53–9.37% higher, and the HAZ length is 7–90μm shorter. [Copyright &y& Elsevier]