Footprint study of ultrasonic wedge-bonding with aluminum wire on copper substrate

Abstract

The effects of the process parameters of ultrasonic power and normal bonding force on bond formation at ambient temperatures have been investigated with scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) analysis. A model was developed based on classical microslip theory 1 to explain the general phenomena observed in the evolution of bond footprints left on the substrate. Modifications to the model are made due to the inherent differences in geometry between ball-bonding and wedge-bonding. Classical microslip theory describes circular contacts undergoing elastic deformation. It is shown in this work that a similar microslip phenomenon occurs for elliptical wireto-flat contacts with plastically deformed wire. It is shown that relative motion exists at the bonding interface as peripheral microslip at lower powers, transitioning into gross sliding at higher powers. With increased normal bonding forces, the transition point into gross sliding occurs at higher ultrasonic bonding powers. These results indicate that the bonding mechanisms in aluminum wire wedge-bonding are very similar to those of gold ball-bonding, both on copper substrate. In ultrasonic wedge-bonding onto copper substrates, the ultrasonic energy is essential in forming bonding by creating relative interfacial motion, which removes the surface oxides.

Deformation Behavior of Thick Aluminum Wire during Ultrasonic Bonding

“…Therefore, the suppression of vibration amplitude will suppress the probability of formation of the initial adhesion. The condition at which "z U max is achieved will correspond to the boundary of microslip and gross-sliding proposed by Lum et al, 5) since the condition satisfies both the strong frictional force and large frictional motion simultaneously. The bonding force has an optimum value which depends on the ultrasonic power to be applied.…”

“…1) Their results indicate that the frictional sliding occur only at the periphery, instead of the entire contact area. Lum et al have investigated the adhesion behavior between Au 4) or Al 5) wires and Cu substrate in detail and proposed a model which describes the ratio of sliding and stationary area. Their model predicts that the entire interface sliding is achieved by setting the bonding parameters with low bonding force and high ultrasonic power as a function of the static friction coefficient of the interface.…”

Damage Evolution in Al Wire Bonds Subjected to a Junction Temperature Fluctuation of 30 K

“…The lack of clarity is attributable to several factors, including the ambiguous nature of the wire bonding mechanism itself, over which there are still a number of views. [20][21][22][23][24][25][26] This is compounded by the fact that the concept of fatigue damage generally lags behind other wear out phenomena in terms of the degree of understanding and consensus of relevant mechanisms of initiation, accumulation, and how these may be quantified. [27][28][29] In view of all these factors, the additional insight that can be gained from a 'same sample' x-ray computed tomography study is instrumental.…”

“…Additionally, the possibility of three-dimensional scrutiny of degradation as it develops can provide new perspectives and crystallise our understanding of wire bond wear-out. Although wire bond lift-off due to thermal cycling fatigue is well documented, [17][18][19] there are complexities in the degradation mechanisms that are not yet fully elucidated. The lack of clarity is attributable to several factors, including the ambiguous nature of the wire bonding mechanism itself, over which there are still a number of views.…”

Relation between Vibration of Wedge-Tool and Adhesion of Wire to Substrate during Ultrasonic Bonding

“…Typically, the technique has been applied to form electrical interconnections between integrated-circuit chips and outer circuits 1,[5][6][7][8] . In addition, the interconnections formed by ultrasonic bonding are free of low melting-point phases like solders.…”