Effects of process parameters on bondability in thermosonic copper ball bonding

Thermosonic copper ball bonding is an absorbing interconnection technology that serves as a viable and cost saving alternative to gold ball bonding. Its excellent mechanical and electrical characteristics make copper ball bonding attractive for high-speed, power devices and fine-pitch applications. However, copper is easily oxidized and harder than gold, which causes some critical process problems in connection with bondability. In this study, a 50 mum copper wire with purity of 99.99% was bonded on aluminum metallization with thickness 3 mum using an ASM angle 60 automatic thermosonic ball/wedge bonder. Experimental studies of copper free air balls (FABs) formation and bonding process were conducted to establish the bonding mechanism and to explain the effects of process parameters on bondability. A micro-slipping model was proposed to account for the effects of the ultrasonic power and bonding force on bondability. It was found that the bondability was determined by a slip area at the bonding interface. The occurrence of bonding only at the periphery of the contact area between FAB and aluminum metallization was attributed to partial slips at the bonding interface. Variation in the ultrasonic power and bonding force that lead to different stick-slip modes, can effect bondability in the ultrasonic bonding process. It is important to set a proper bonding time to achieve interatomic bonding without causing fatigue rupture of microjoints. It was also found that preheating of the chip to a certain temperature can improve bondability

Review of Direct Metal Bonding for Microelectronic Interconnections

Microelectronic interconnections require advanced joining techniques. Direct metal bonding methods, which include thercomsonic and thermocompression bonding, offer remarkable advantages over soldering and adhesives joining. These processes are reviewed in this paper. The progress made in this area is outlined. Some work concerned with the bonding modeling is also presented. This model is based on the joint interface mechanics resulting from compression. Both bump and substrate deformation are taken into account. The improved understanding of the relationship between the deformation and bonding formation may provide more accurate joint evaluation criterion.Singapore-MIT Alliance (SMA

High reliability bond program using small diameter aluminum wire

The program was undertaken to characterize the performance of small diameter aluminum wire ultrasonically bonded to conductors commonly encountered in hybrid assemblies, and to recommend guidelines for improving this performance. Wire, 25.4, 38.1 and 50.8 um (1, 1.5 and 2 mil), was used with bonding metallization consisting of thick film gold, thin film gold and aluminum as well as conventional aluminum pads on semiconductor chips. The chief tool for evaluating the performance was the double bond pull test in conjunction with a 72 hour - 150 C heat soak and -65 C to +150 C thermal cycling. In practice the thermal cycling was found to have relatively little effect compared to the heat soak. Pull strength will decrease after heat soak as a result of annealing of the aluminum wire; when bonded to thick film gold, the pull strength decreased by about 50% (weakening of the bond interface was the major cause of the reduction). Bonds to thin film gold lost about 30 - 40% of their initial pull strenth; weakening of the wire itself at the bond heel was the predominant cause. Bonds to aluminum substrate metallization lost only about 22%. Bonds between thick and thin film gold substrate metallization and semiconductor chips substantiated the previous conclusions but also showed that in about 20 to 25% of the cases, bond interface failure occurred at the semiconductor chip

A Study on the Effect of Bond Stress and Process Temperature on Palladium Coated Silver Wire Bonds on Aluminum Metallization

In the past ten years, the increasing price of gold has motivated the wire bonding industry to look for alternative bonding wire materials in the field of microelectronics packaging. A new candidate wire to replace gold is palladium coated silver wire. In this thesis, the effect of the two specific process parameters “bond stress” and “process temperature” on the ball bonds made with the new candidate wire are investigated. Using 20 μm diameter wire and various level-combinations of these process parameter, ball bonds are produced according to a special accelerated optimization method to result in a target diameter of 46 ± 0.5 μm and target height of 16 ± 0.5 μm. Three different levels are used for each of the specific process parameters. After pre-selecting a few process parameters, the accelerated method determines the levels for the process parameters “impact force” and “electric flame-off current” with a 2×2 design of experiments. Then, the ultrasound parameter is maximized up to a level where a pre-selected ultrasonic deformation occurs to the bonds, maintaining the target bond diameter and height. The bond quality is measured by measuring the shear strength of the bonds. The results show that • the bond geometry is not affected by the bond stress, • the optimized specific process parameters vary by less than ~0.5 % when bond stress values are varied from 60 to 100 MPa, • the variations in optimized parameters are larger than ~3.0 % when the BT is changed from 100 to 200 ºC, • ball bonds achieve acceptable shear strength (> 120 MPa) when the values for both, bond stress and bond temperature, are high, • ultrasound level and shear stress interact, the higher shear stress the lower the ultrasound level required. An average shear strength of ~120 MPa is achieved with 11.4 % ultrasound, 100 MPa bond stress, and 200 ºC bond process temperature. In summary, a robust methodology is presented in this thesis to efficiently optimize the ball bonding process as demonstrated with the new candidate wire has a bondability similar to that of gold wire with only minor adjustment in the bonding process needed

Design of a Smart Ultrasonic Transducer for Interconnecting Machine Applications

A high-frequency ultrasonic transducer for copper or gold wire bonding has been designed, analyzed, prototyped and tested. Modeling techniques were used in the design phase and a practical design procedure was established and used. The transducer was decomposed into its elementary components. For each component, an initial design was obtained with simulations using a finite elements model (FEM). Simulated ultrasonic modules were built and characterized experimentally through the Laser Doppler Vibrometer (LDV) and electrical resonance spectra. Compared with experimental data, the FEM could be iteratively adjusted and updated. Having achieved a remarkably highly-predictive FEM of the whole transducer, the design parameters could be tuned for the desired applications, then the transducer is fixed on the wire bonder with a complete holder clamping was calculated by the FEM. The approach to mount ultrasonic transducers on wire bonding machines also is of major importance for wire bonding in modern electronic packaging. The presented method can lead to obtaining a nearly complete decoupling clamper design of the transducer to the wire bonder

Study of Au Ball Bond Mechanism and Reliability on Pd/Ni/Cu Substrate

Microelectronic wire bonding is a manufacturing process used to electrically connect integrated circuits with circuit boards or other substrates. Conventionally, balls are molten at the end of a Au bonding wire and subsequently bonded on Al metallization of a integrated circuit. However, Pd/Ni metallization has recently been used for its improved mechanical properties. The bondability, bonding mechanism, and reliability of Au ball bonds on Pd are studied in this thesis. The substrates were produced in this project using three different materials. The base material is polished Cu in the shape of a coupon (1.0 cm × 1.0 cm × 0.5 mm). Cu coupons are plated with Ni (1.0 μm) using an electroless process, followed by electrolytic plating of a layer of Pd (0.3 μm), resulting in an arithmetic mean roughness of the surface of 0.08 μm (baseline sample, sample 0). Higher roughness values of 0.2, 0.4, and 0.5 μm are artificially produced by rolling (sample 1), sanding (sample 2), and sandblasting (sample 3), respectively, on the Cu surface before plating Ni and Pd. A 25 μm diameter Au wire is used for bonding on the polished and roughened substrates with a process temperature of T = 220 °C, and it was found that ≈ 4 % to ≈ 18 % less ultrasonic amplitude was required for successful bonding on the roughened substrates compared to the polished substrate. Bondability is measured by shear testing the ball bonds. An average ball bond strength achieved on the polished substrate is 130 MPa. This value is lower on the roughened substrate with the exception of the sandblasted substrate. Long-term thermal aging at 250 °C was performed with ball bonds on samples 0-3 for durations of ≈ 300 h. The reliability of the bonds is characterized by non-destructive contact resistance analysis during aging and destructive cross section analysis after aging. Contact resistance values for the ball bonds range from 1.6 to 3.5 mΩ at 20 °C before aging, and does not correlate with roughness. For the baseline sample, contact resistance of the ball bonds decreases during aging by -6 % (median value), which indicates electrical integrity of the interconnections at high temperature. This decrease possibly is due to interfacial gap filling by Au or Pd diffusion. In contrast, the contact resistance increases for the roughened samples 1-3 and changes are 0.4, 5, and 14 %, respectively (median values). A conclusive explanation for this increase has not yet been found. After 250 h of aging, a TEM analysis showed Au to Pd diffusion in the baseline sample with a diffusion depth of ≈ 0.1 μm Au. No intermetallics, voids, or contamination is found on the interfaces after aging according to nanohardness, SEM, and TEM analyses. No bond lift-offs or electrical opens were found for the aging temperature and durations chosen. No conclusive evidence for the presence of Au-Pd intermetallics or voids is found

Long-wavelength GaInNAs/GaAs Vertical-cavity Surface-emitting Laser for Communication Applications

This paper presents a comprehensive study of optical and electrical properties of vertical-cavity surface-emitting lasers(VCSELS) for long wavelength communication applications. The device consists of GaInNAs/GaAs multi-quantum wells QWs that enclosed between standard top and bottom epitaxially grown on AlGaAs/GaAs distributed Bragg reflectors. The impact of driven currents and injecting optical powers through QWs layers on the output light emission is addressed. Room temperature spectra measurements are performed at various applied currents using 980 nm pump laser and maximum intensity amplitude at around 21 dB was achieved

Transforming data into information to control and improve a ribbon bonding process

Thesis (S.M.)--Massachusetts Institute of Technology, Sloan School of Management; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering; in conjunction with the Leaders for Manufacturing Program at MIT, 2004.Includes bibliographical references (p. 83).In a complex production process attached to every unit are considerable quantities of data that provide many details regarding process performance. On many occasions, although this data is collected in a database or in some cases manually, output is never generated from this data. Often the data is reviewed by operators or engineers but because of its complexity no real conclusions are drawn from it and hence the data is never used to take action or make appropriate decisions. This work explores the theme that the use of information transformed from data is critical in making the necessary decisions and actions in a problem solving process. This methodology is carried out in solving significant yield and rate problems of a ribbon bonding process used to produce state-of-the-art surface radars at Raytheon Company. Transforming data into information particularly through the use of visual tools became essential in determining root causes by bringing forward underlying issues. This led to increased confidence in making the right decisions and ultimately led to implementation of process improvement solutions. Work for this thesis resulted in several process improvement initiatives as well as the implementation of an automated data management tool designed using extensive visual controls to provide real-time process feedback to operators. The process improvement initiatives involved implementation of a new cleaning process prior to ribbon bonding as well as the design, manufacture, and implementation of a work stage that added heat to the bonding process. These process improvement initiatives resulted in the elimination of the yield and rate problems and led to cost avoidance savings of over $2.6M for the first two radars. More importantly, the(cont.) lessons learned from the methodology introduced in this work and used to solve ribbon bond process problems will lead to lower production costs on all future radars. Thesis Supervisor: Daniel E. Whitneyby Brett A. Balazs.S.M

Process Quality Improvement in Thermosonic Wire Bonding

This thesis demonstrates the feasibility of methods developed to increase the quality of the crescent bond together with the tail bond quality. Low pull force of the crescent bond limits the usage of insulated Au wire in microelectronics assembly. Premature break of the tail which results in the stoppage of the bonding machine is one of obstacles to overcome for Cu wire. The primary focus of this thesis is to understand the tail and crescent bonding process and then to propose methodologies to improve thermosonic wire bonding processes when Cu and insulated Au wires are used. Several series of experiments to investigate the crescent and tail bonding processes are performed on auto bonders. Cu and insulated Au wires with diameters of 25mm are bonded on the diepads of Ag leadframes. For simplicity, wire loops are oriented perpendicular to the ultrasonic direction. It was found that the crescent bond breaking force by pulling the wire loop (pull force) with insulated Au wire is about 80 % of that of bare Au wire. A modification of the crescent bonding process is made to increase the pull force with insulated Au wire. In the modified process, an insulation layer removing stage (cleaning stage) is inserted before the bonding stage. The cleaning stage consists of a scratching motion (shift) toward to the ball bond in combination with ultrasound. Bonds are then made on the fresh diepad with the insulation removed from the contact surface of the insulated Au wire. This process increases the pull force of the crescent bond up to 26% which makes it comparable to the results obtained with bare Au wire. An online tail breaking force measurement method is developed with a proximity sensor between wire clamp and horn. Detailed understanding of tail bond formation is achieved by studying tail bond imprints with scanning electron microscopy and energy dispersive x-ray analysis. Descriptions are given of the dependence of the tail breaking force on the bonding parameters, metallization variation, and cleanliness of the bond pad. Simultaneous optimization with pull force and tail breaking force can optimize the Cu wire bonding process both with high quality and robustness. It is recommended to first carry out conventional pull force optimization followed by a minimization of the bonding force parameter to the lowest value still fulfilling the pull force cpk requirement. The tail bond forms not only under the capillary chamfer, but also under the capillary hole. The tail breaking force includes both the interfacial bond breaking strength and the breaking strength of the thinned portion of the wire that will remain at the substrate as residue. Close investigations of the tail bond imprint with scanning electron microscopy indicate the presence of fractures of the substrate indicating substrate material being picked up by Cu wire tail. Pick up is found on Au and Cu wires, but the amount of pick up is much larger on Cu wire. The effect on the hardness of the subsequently formed Cu free air ball (FAB) as investigated with scanning electron microscopy and micro - hardness test shows that Cu FABs containing Au and Ag pick ups are softer than those without pick up. However, the hardness varies significantly more with Au pick up. The amount of Au pick up is estimated higher than 0.03 % of the subsequently formed FAB volume, exceeding typical impurity and dopant concentrations (0.01 %) added during manufacturing of the wire

Design, processing and testing of LSI arrays hybrid microelectronics task

Those factors affecting the cost of electronic subsystems utilizing LSI microcircuits were determined and the most efficient methods for low cost packaging of LSI devices as a function of density and reliability were developed