Thermosonic Wire Bonding Research Articles

Study of Complex Looping With Five Kinks in Thermosonic Wire Bonding by Using Variable-Length Link-Spring Model

Complex looping with five kinks has been commonly used in the stacked die 3-D packaging and simulation models were desired to help control the loop profile in modern thermosonic wire bonding. The traditional finite-element model was time consuming and cost intensive, and the simple link-spring analysis model cannot provide precision simulation of the complex looping as it ignores the stage when the capillary is moving upward from the first bond to the highest point. In this paper, a variable-length link-spring model is proposed to deal with the above problems. In this model, the wire segments and moment equilibrium equations at the opening end of the wire are dynamically added during the capillary upward movement stage, to simulate the wire feeding and loop formation process. The experiment was carried and the complete wire loop profile in the model agree well with the experimental results. Based on this verified model, the effects of various parameters on loop profiles with five kinks were studied. It seems that the first kink height has little effect on the final loop profiles, while the second kink height can change the loop profiles and sweep resistance obviously.

Fatigue testing method for fine bond wires in an LQFP package

A mechanical testing setup was developed to study the fatigue response of fine thermo-sonic wire bond connection in low profile quad flat packages (LQFP). The testing set-up was designed to induce pre-defined multi-axial stresses in the wire bond loops of non-encapsulated packages in order to mimic their deformation behavior during the thermo-mechanical loading. Lifetime curves were obtained up to 1.0E7 loading cycles with fatigue failure occurring in the heat affected zone of the ball bond. The experimental fatigue data in combination with extended FEA provided the basis for a Coffin Manson lifetime model. The proposed fatigue testing procedure can be applied as a highly efficient method for evaluation of various wire bonded packages by using a limited number of test samples and simultaneous testing of several wire bonds.

Investigation of Complex Looping Process for Thermosonic Wire Bonding

Wire looping is a critical part of a modern wire bonder. A major concern in wire bonding is the loop profile and loop height consistency, particularly for complex loops with more than four kinks for 3-D packages. To understand the effects of various parameters on complex loop profiles, a 3-D finite element (FE) model is developed with LSDYNA. Experiments observation of the looping process is carried out using high speed camera to verify the FE model. This FE model is used for investigating the effect of material properties, wire tension force, and clamp action on loop profile. Experimental and simulation results show that a complex capillary trace is used for a complex loop, and six kinks are formed. The loop profile is mainly determined by the number of kinks, kink position and deformation. The upward stage of capillary trace is mainly used for forming kinks and shape the loop profile, the downward stage only slightly changes the loop profile. Larger tension force, open clamp, softer materials, and larger reverse motion parameter can reduce the loop height. A constant wire tension force and precise clamp action can improve the loop consistency.

Effect of Capillary Trace on Dynamic Loop Profile Evolution in Thermosonic Wire Bonding

Thermosonic wire bonding remains the most commonly used interconnection technology in microelectronic packaging, and looping is an important aspect in modern wire bonders. To identify the loop formation mechanism, the effect of capillary trace on the standard wire looping process was studied. Dynamic looping processes with different capillary trace parameters and reverse motions of 4, 8, and 16 mil were recorded by a high-speed camera. The capillary trace and wire profile evolution were obtained from the looping videos using a digital image processing program, and the relationship between capillary trace and loop profiles was analyzed. A finite-element model was established to study the strain distribution on wire during looping. Experimental and simulation results show that the wire profile of the standard loop is mainly affected by capillary position and is not sensitive to capillary velocity. The upward capillary trace mainly affects the loop configuration, including the number, position, and deformation of kinks and the loop length. The downward capillary trace affects the stress states, loop height, kink deformation, and loop profiles. A kink is the wire with the largest local curvature, and it is a plastically deformed wire segment with little elastic core. The kink has two functions: 1) shaping the loop and 2) isolating the pulling force on the first bond and neck caused by capillary movement. This paper can be of great help in loop profile optimization in the industry and in academic research of loop dynamics.

Dynamics of Free Air Ball Formation in Thermosonic Wire Bonding

In thermosonic wire bonding, a free air ball (FAB) created by electronic flame-off (EFO) is crucial to a reliable first bond. To reveal the mechanisms of FAB and ripple formation, the dynamic FAB formation process was monitored and analyzed with a high-speed camera and digital data acquisition system. FAB formation was found to occur in four stages: initiation, preheating, melting, and solidification. The FAB formation speed and model depend only on the EFO current. The FAB formation speed can be as high as 90.4 μm/ms at an EFO current of 60 mA. A minimum EFO time is required for FAB formation, which depends on the EFO current. A semiempirical model FAB=ηln[ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> - 0.5 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> )]- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> , was developed to describe the relationship between the FAB size and the EFO time at different EFO currents. The ripple formation mechanism was investigated by analyzing the solidification stage images. Transient droplet vibration caused by high-speed rolling-up and skin-forming solidification in the molten gold drop caused by a large temperature gradient are two major factors in ripple formation. This analysis is expected to be useful for understanding the FAB formation mechanism and optimizing the EFO parameters.

Gold Ball Wire Bonding with Heated Tool for Automotive Microelectronics

Microelectronics used in automotive applications have grown considerably in the last few years with more high tech electronics controlling more functions in automobiles. In an effort to have more precise control and to reduce vehicle weight manufacturers are integrating more functions into smaller packages. Many of these packages are embedded in molded plastic. This causes challenges when it comes to wirebonding these devices. They often cannot be heated to traditional Gold Ball Thermosonic wirebonding temperatures of 120 – 150C. However, using a heated capillary to bond the parts which remain at room temperature simplifies the process considerably. Alternatives such as pre-heating the parts in an oven and complex hot gas handler systems are not required. With a resistive wire coil heater surrounding a standard (or long capillary for deep access) sufficient heat can be provided to the wire bond site for a strong and reliable interconnect. The bonding surface can be any material used in gold ball bonding: aluminum bond pads on die, plated contacts, ceramic substrates or plated copper traces on PCBs. This paper will show that this heated tool process has been successfully utilized with 1mil Au wire and many of the standard die and substrate materials with little impact on process parameters.

Effect of Conduction Pre-heating in Au-Al Thermosonic Wire Bonding

This paper presents the recent study by investigating the vital responses of wire bonding with the application of conduction pre-heating. It is observed through literature reviews that, the effect of pre-heating has not been completely explored to enable the successful application of pre-heating during wire bonding. The aim of wire bonding is to form quality and reliable solid-state bonds to interconnect metals such as gold wires to metalized pads deposited on silicon integrated circuits. Typically, there are 3 main wire bonding techniques applied in the industry; Thermo-compression, Ultrasonic and Thermosonic. This experiment utilizes the most common and widely used platform which is thermosonic bonding. This technique is explored with the application of conduction pre-heating along with heat on the bonding site, ultrasonic energy and force on an Au-Al system. Sixteen groups of bonding conditions which include eight hundred data points of shear strength at various temperature settings were compared to establish the relationship between bonding strength and the application of conduction pre-heating. The results of this study will clearly indicate the effects of applied conduction pre-heating towards bonding strength which may further produce a robust wire bonding system.

Tail Breaking Force in Thermosonic Wire Bonding with Novel Bonding Wires

Tail breaking forces (TBFs) are measured for various process conditions to understand phenomena such as short tail formation. TBFs obtained with several Cu wires are compared to find the most suitable Cu wire type that improves consistent tail formation. In situ online TBF measurement method is developed. The highest TBF obtained is 61.59 + 9.10mN. The highest Cpk value obtained is 2.97 + 0.33 when lower specification limit of 10 mN is assumed.

Formation and growth of intermetallics in thermosonic wire bonds: Significance of vacancy–solute binding energy

This paper reviews the formation and growth of intermetallics at the interface during thermosonic bonding of fine metal wires in microelectronics packaging. The nucleation of intermetallics relies on deformation of solid, inter diffusion couple of the bonding elements and energies supplied. Inter diffusion of atoms play crucial role on the formation of intermetallics. The physical, thermal and atomic properties of the joining metals, especially the vacancy–solute binding energy and the atomic radii significantly influences the inter diffusion of atoms. Growth of intermetallics on thermal aging also depends on the atomic properties of the bonding elements. The paper also introduces the importance of vacancy solute binding energy on the formation/growth of intermetallics.

Ball Bonding Process Robustification with regard to Space Applications

Space applications require being able to pass 100% pull test, before and after thermal aging, and to conform to visual inspection. Gold thermosonic wirebonding (ball bonding) on Aluminum metallized silicon dies, is usually felt to be a sensitive process, due to the need for effectively controlling intermetallic. As the criteria are very stringent, the process setup of small batches can be a cumbersome and costly operation. In this paper, we propose an original sequence of methods to ensure correlation of success in all these aspects, while using only a pull tester and keeping the batch setup time and cost down. It relies on design of experiments (DoE) and analysis of variance (ANOVA), to estimate the effects of significant factors and determine an optimum. An original statistic is used to better answer the objective. The visual aspects of the bonds are also studied. The purpose is to offer a process map showing safe bonding conditions, where changes made to improve visual aspects will not impact the pull test values. Finally, we have quantified the visual quality, and have related it to the pull test.

On the Microcontact Mechanism of Thermosonic Wire Bonding in Microelectronics: Saturation of Interfacial Phenomena

Thermosonic ball bonding is widely used in interconnections to integrated circuits. This work used a thermosonic bonding machine and a shear tester to study the effects of preload, preheat temperature, ultrasonic power, welding time, and wire diameters on shear force. The results reveal that shear force of thermosonic wire bonding can be elucidated from the viewpoint of interfacial microcontact phenomena such as energy intensity, interfacial temperature, and real contact area. As the energy intensity is increased, the shear force increases, reaches a maximum, and then decreases. The energy intensity at which the shear force reaches maximum is called the saturated energy intensity. After saturation (that is, the establishment of maximum atomic bonds), any additional energy input will damage the bonding, decreasing the shear force. The saturated energy phenomenon can be further confirmed by using wires with different diameters. It is also observed that the saturation phenomena, and thereby the optimal shear force, occurs under a certain range of interfacial temperature.

A Novel Method for Enabling the Thermosonic Wire Bonding of Gold Wire onto Chips with Copper Interconnects

The wire bonding of chips with copper interconnects is a challenging issue. The present study develops a method in which the substrates are stored in a temperature and humidity controlled environment such that copper oxide films of appropriate thickness are formed on the substrate prior to the gold-copper thermosonic bonding process. It is shown that this film can be removed during the bonding process, hence rendering the wire bonding process feasible without the requirement for an inert gas environment or the use of metallic cap layers.

Study of Interfacial Phenomena Affecting Thermosonic Wire Bonding in Microelectronics

Thermosonic ball bonding is the method of choice for many interconnections to integrated circuits. This study investigated the effects of bonding parameters on bonding strength using a thermosonic bonding machine and a shear tester. Theoretical analyses were conducted to relate bonding strength to interfacial contact phenomena. Our results show that bonding strength of thermosonic wire bonds can be explained based on frictional energy intensity and real contact area. When the ultrasonic power is small, the bonding strength increases with increasing real contact area mainly caused by the bonding force. Increasing the bonding force can hardly increase the frictional energy intensity, but it can increase the real contact area, thus increasing the shear force. For larger ultrasonic power, the ultrasonic power plays an important role in increasing the bonding strength at the interface between the wire and the pad. Increasing the ultrasonic power increases both the frictional energy intensity and the real contact area, thus increasing the shear force until before the frictional energy intensity reaches a critical point. Moreover, increasing the welding time increases both the frictional energy intensity and the real contact area, thus increasing the shear force before the critical frictional energy intensity is attained; this is far smaller than the critical frictional energy intensity when the ultrasonic power is varied.

Failure mechanisms of aluminum bondpad peeling during thermosonic bonding

Aluminum bondpad peeling was observed in a newly developed thermosonic wirebonding process for chip-on-board assembly. Through detailed failure analysis and with the help of finite element analysis on stress simulation, the true root cause of the peeling is identified. It is found that the true root cause is the effect of skidding force as a result of the constrained movement of the bonding tool as bonding is done on a chip assembled in a plastic casing. With a change in the bonding tool movement, the peeling phenomenon is completely eliminated.

Study of wire bonding looping formation in the electronic packaging process using the three-dimensional finite element method

This paper presents a three-dimensional finite element model of the thermosonic wirebond looping process. The gold wire heat affected zone load-displacement relationship was investigated using precision micro-force tensile tests. The temperature-dependent elastoplastic behaviors of gold wire were taken into account in the nonlinear finite element model. Both the solid and equivalent shell wire models were used; exhibiting very close agreement, indicating that equivalent model was adequate. The simulation results were verified against the experimental results. The standard triangle loop (STD2-Mode) and trapezoidal loop (LOW2-Mode) were simulated. The finite element model closely predicted the profile of the tested loops. This model was used for an extensive looping factor parameter study. No obvious looping parameter effects were observed for the STD2 Mode. The loop shapes depend primarily on the wire properties, bonding pad distance and tier height. Both the looping and material parameters have significant effects on the loop shape for the LOW2 Mode. Optimizing the reverse angles could reduce the stress on the wire.

A high-quality cross-sectional transmission electron microscope specimen preparation technique for structural and interfacial property studies in microelectronic packaging

A new versatile procedure employing low-angle ion milling and a self-clamping titanium grid to produce high-quality cross-sectional transmission electron microscopy (TEM) specimens for microelectronic packaging structures is proposed. Large electron transparent areas are generated with minimal mechanical damage, ion-induced damage, differential thinning in inhomogeneous materials, and surface roughness. An application example on gold thermosonic wire bonds is demonstrated.

Gold wire weakening in the thermosonic bonding of the first bond

Factors that are likely to weaken gold wire during thermosonic wirebonding of the first bond on the die pad are studied. The studies show that weakening is due mainly to the golf club ball formation, grain growth of wire, neckdown formation, tie-bar severance, and the wire-scratching phenomenon. Golf club ball formation can be eliminated by long tail length and wire grain size after recrystallization, can be reduced by low current supplied and short spark time during the EFO excitation. Neckdown and tie-bar severance can be removed by introducing reverse loop and ensuring no indexing problem during the wirebonding process. Wire scratching and dragging due to inside chamfer can be eliminated when the capillary for bonding is not too worn out.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermosonic Gold Wire Bonding to Copper Conductors

The use of thick film copper conductors in hybrid microcircuitry requires that wire bonding to their surfaces be demonstrated as a feasible and reliable interconnection technique. Previous work has shown that acceptable and reliable ultrasonic aluminum wire bonds can be made to copper. The conditions necessary to make successful thermosonic gold wire bonds to copper are described. Items covered are the types of tool to use, the influence of stage temperature, the maximum dwell time for the parts on the stage, and the bonding window that yields the best results. In addition, information is provided on the reliability of the bonds following aging at temperatures in the range 150° to 350°C, in high temperature-humidity conditions such as 40°C, 90 percent RH, and 85°C, 85 percent RH and after 100 thermal cycles between -55°C and 150°C.