Microstructural evolution of ultrasonically bonded high purity Al wire during extended range thermal cycling

This paper concerns the reliability of ultrasonically bonded high purity thick aluminium wires at elevated temperature. To date, the evolution of the microstructure of wire bonds during thermomechanical exposure and its influence on reliability have not been fully characterised and understood, particularly as they pertain to thermal cycling regimes which exceed 125 °C. Shear testing, indentation hardness and fine-scale microstructural data are reported here which show that the rate of wear-out can be influenced not only by the thermal cycling range (ΔT), but more importantly by the maximum temperature and duration to which bonds are exposed. There is evidence that significant annealing occurs during thermal cycling regimes with high T_max values, which results in the removal of some of the damage accumulated and a reduction in the rate of crack propagation. The rate of bond degradation is also found to be faster for 99.99% (4 N) than 99.999% (5 N) pure Al wires. Analysis of the two wire compositions after thermal cycling suggests that this difference could be attributable to a difference in their creep resistance. In conclusion, our findings suggest that high purity Al wire bonds may be suitable for operation at temperatures which exceed 125 °C.     

Direct gold and copper wires bonding on copper

The key to bonding to copper die is to ensure bond pad cleanliness and minimum oxidation during wire bonding process. This has been achieved by applying a organic coating layer to protect the copper bond pad from oxidation. During the wire bonding process, the organic coating layer is removed and a metal to metal weld is formed. This organic layer is a self-assembled monolayer. Both gold and copper wires have been wire-bonded successfully to the copper die even without prior plasma cleaning. The ball diameter for both wires are 60 μm on a 100 μm fine pitch bond pad. The effectiveness of the protection of the organic coating layer starts from the wafer dicing process up to the wire bonding process and is able to protect the bond pad for an extended period after the first round of wire bond process. In this study, oxidization of copper bond pad at different packaging processing stages, dicing and die attach curing, have been explored. The ball shear strength for both gold and copper ball bonds achieved are 5 and 6 g/mil² respectively. When subjected to high temperature storage test at 150 °C, the ball bonds formed by both gold and copper wire bond on the organic coated copper bondpad are thermally stable in ball shear strength up to a period of 1440 h. The encapsulated daisy chain test vehicle with both gold and copper wires bonding have passed 1000 cycles of thermal cycling test (−65 to 150 °C). It has been demonstrated that orientation imaging microscopy technique is able to detect early levels of oxidation on the copper bond pad. This is extremely important in characterization of the bondability of the copper bond pad surface.     

Reliability of thick Al wire: A study of the effects of wire bonding parameters on thermal cycling degradation rate using non-destructive methods

The effect of bonding parameters on the reliability of thick Al wire bond is investigated. Samples were prepared with 25 different designs with 5 different bonding parameters such as time, ultrasonic power, begin-force, end-force and touch-down steps (pre-compression) with 5 levels. The bond signals of ultrasonic generator were collected during bonding in order to obtain prior quality information of bonded wires. 3D X-ray tomography was then used to evaluate bond quality during passive thermal cycling between −55 °C and 125 °C. Tomography datasets were obtained from the as-bonded condition and during cycling. The results clearly show ultrasonic power, appropriate levels of begin-force and touch-down steps are all important for achieving a well attached and reliable bond. Analysis of the virtual cross-sections indicates a good correlation between the bond signal (i.e. the initial bond quality) and wire bond damage/degradation rate. An improved understanding of the wire bonding process was achieved by observing the effect of the complex interaction of bonding parameters on the ultrasonic generator signals and degradation rate under thermal cycling.     

Improved thermal management of low voltage power devices with optimized bond wire positions

This paper focuses on optimization of bond wire positions as a method to improve thermal management of power semiconductors. For this purpose, robustness of a new low-voltage MOSFET generation with an optimized multiple bond wire arrangement and device shape is compared to an older device design with lower number of bond wires. 2D electrical simulation is used to evaluate the lateral distribution of power dissipation due to the gate voltage de-biasing effect. 3D thermal finite element simulation and infrared thermography measurements are employed to analyze the corresponding surface temperature distribution. Finally, tests under extreme single pulse short-circuit conditions demonstrate the effectiveness of thermal management for improving robustness in automotive applications.     

Die degradation effect on aging rate in accelerated cycling tests of SiC power MOSFET modules

In order to distinguish the die and bond wire degradations, in this paper both the die and bond wire resistances of SiC MOSFET modules are measured and tested during the accelerated cycling tests. It is proved that, since the die degradation under specific conditions increases the temperature swing, bond wires undergo harsher thermo-mechanical stress than expected. The experimental results confirm the die-related thermal failure mechanism. An improved degradation model is proposed for the bond-wire resistance increase in case of die degradation.     

Reliability and lifetime evaluation of different wire bonding technologies for high power IGBT modules

IGBT modules for power transmission, industrial and traction applications are operated under severe working conditions and in harsh environments. Therefore, a consequent design, focused on quality, performance and reliability is essential in order to satisfy the high customer requirements. One of the main failure mechanisms encountered in high power IGBT modules subjected to thermal cycles is wire bond lift-off, which is due to the large thermal expansion coefficient mismatch between the aluminum wires and the silicon chips. The paper describes various bonding technologies using different wire materials directly bonded onto chip metallisation as well as the ABB solution where the wire is bonded on a thin molybdenum strain buffer soldered onto the chip. We assess in the present paper the potential of these technologies to enhance module reliability and lifetime through a power cycling test. Failure analysis results are presented and the failure mechanisms related to each technology are explained in detail.     

Analytical approach to temperature evaluation in bonding wires andcalculation of allowable current

Wirebonding is still the most common technique being applied to device assembly. Since the entire electrical power for the chip has to be delivered through the wires, considerable current densities may occur. As a result, bond wires are heated up and in case of too excessive current wires or surrounding materials might suffer and subsequently fail. In order to increase reliability of semiconductor devices it is important to know the resultant temperature due to a given current and deduced from this, the allowable loading so that a maximum temperature will not be exceeded. In this paper universally valid formulas for steady state, single pulse and periodic loading are introduced. They are derived from the heat diffusion equation resulting from a mathematical model which is proposed for simplification. In case of ceramic packages radiation and convection effects are considered whereas conduction through the molding compound is taken into account if the wire is encapsulated in plastic. The formulas also enable comparison between the effects of heat conduction through the wire and through the molding compound. Besides, the system of differential equations considers the temperature dependence of the specific resistance and the thermal conductivity of the wire material. Corrections for very thin plastic packages and multiple bonding are suggested. The formulas have been checked by both experimental data and numerical computation by means of Finite Element Analysis     

Chip-on-Board (CoB) technology for low temperature environments. Part I: Wire profile modeling in unencapsulated chips

An analytical model based on elastic strain energy minimization is developed for estimating the wire profile in unencapsulated ball-wedge wire bond configuration for chip-on-board (CoB) technology. The wire profile is applicable to ball-wedge bonds with a height offset, and is modeled using a piece-wise continuous polynomial function (cubic spline) with appropriate boundary conditions at the two bond sites. Plastic deformation is ignored in the current analysis as a first-order approximation, since the interest is in parametric sensitivity studies. The model is useful for estimating the wire profile for different offset heights and wire spans, and serves as a starting point for subsequent thermomechanical stress analysis after encapsulation. Parametric studies are presented to explore the wire profile for different CoB geometries.     

Bond reliability under humid environment for coated copper wire and bare copper wire

There is growing interest in Cu wire bonding for LSI interconnection due to cost savings and better electrical and mechanical properties. Conventional bare Cu bonding wires, in general, are severely limited in their use compared to Au wires. A coated Cu bonding wire (EX1) has been developed for LSI application. EX1 is a Pd-coated Cu wire to enhance the bondability.Bond reliability at a Cu wire bond under a humid environment is a major concern in replacing Au wires. The bond reliability of EX1 and bare Cu was compared in the reliability testing of PCT and UHAST (Unbiased HAST). The lifetimes for EX1 and the bare Cu in PCT testing were over 800 h and 250 h, respectively. Humidity reliability was significantly greater for EX1. Continuous cracking was formed at the bond interface for the bare Cu wire. Corrosion-induced deterioration would be the root cause of failure for bare Cu wires. The corrosion was a chemical reaction of Cu-Al IMC (InterMetallic Compound) and halogens (Cl, Br) from molding resins. EX1 improves the bond reliability by controlling diffusion and IMC formation at the bond interface. The excellent humidity reliability of the coated Cu wire, EX1 is suitable for LSI application.     

Reliability evaluation of a silicon-on-silicon MCM-D package

This paper presents the results of reliability testing on a multichip module technology with active silicon substrates. The modules use flip-chip technology to attach silicon chips to the active substrate and this assembly is then packaged into a plastic ball grid array package. Performance was evaluated using two custom designed test chips incorporating thermal, thermomechanical, electrical and reliability test structures. A rigorous environmental test sequence including temperature, cycling, humidity, highly accelerated stress test and power cycling were carried out on the demonstrators. A full destructive physical analysis was then performed, consisting of die/substrate shear, wire bond pull tests and microsectioning.     

Enhanced power cycling performance of IGBT modules with a reinforced emitter contact

A typical emitter contact of an IGBT consists of a front metallization and bond wires. In this study, the power cycling performance of a special emitter contact design is experimentally verified. The emitter contact includes a metal plate, which is Ag-sintered to the metallization and wire bonded on the top surface. Either Cu or Al bond wires were implemented. Power cycling tests were performed to investigate the performance of such IGBT modules. The results were very promising and a cycling lifetime was achieved, which is about 20 times higher than the lifetime of typical IGBT modules. For a better understanding of the experimental results, the electrical and thermal response of the IGBT modules were simulated by FEM. The results of this study, provide a key for high-reliability designs of the emitter contact of IGBT modules with superior power cycling capability.     

Effect of wire diameter on the thermosonic bond reliability

Thermosonic bonding process is a viable method to make reliable interconnections between die bond pads and leads using thin gold and copper wires. This paper investigates interface morphology and metallurgical behavior of the bond formed between wire and bond pad metallization for different design and process conditions such as varying wire size and thermal aging periods. Under thermal aging, the fine pitch gold wire ball bonds (0.6 mil and 0.8 mil diameter wires) shows formation of voids apart from intermetallic compound growth. While, with 1-mil and 2-mil diameter gold wire bonds the void growth is less significant and reveal fine voids. Studies also showed void formation is absent in the case of thicker 3 mil wire bonds. Similar tests on copper ball bonds shows good diffusional bonding without any intermetallic phase formation (or with considerable slow growth) as well as any voids on the microscopic scale and thus exhibits to be a better design alternative for elevated temperature conditions.     

Microstructural Investigation of Interfacial Features in Al Wire Bonds

In the present study the microstructure of ultrasonically bonded Al wires on AlSiCu and AlSi metallization was investigated by means of scanning electron microscopy, electron back-scattered diffraction, and high-resolution transmission electron microscopy techniques. Detailed microstructural investigations were conducted on samples in the as-bonded condition, subsequent to power cycling tests, and after long-time thermal exposure to reveal the temperature-dependent evolution of the interfaces and the metallization layer. Typical interfacial features were found to be ultrafine and nanoscaled grains of Al and Al₂O₃, amorphous Al oxide particles, voids, and pores, with regions of high density of dislocations and dislocation loops within the larger grains of the wire and metallization. The observed interface features confirm the suggested mechanism of formation of bonding interface by emergence of submicron grains at the thin interfacial boundary between the metallic pair as a result of dynamic recrystallization and interdiffusion. While isothermal and/or thermomechanical cycling lead to strong grain growth in the metallization layer and the Al wire, the nanostructured interfacial regions mainly remain, indicating a high thermal stability and strength of the interface. Furthermore, evaluation of a large number of wire bonds prepared using standard bonding conditions showed the presence of a certain percentage of nonbonded areas and microstructural variations between the interconnects processed under nominally identical conditions. However, it was found that, if a sufficient effective bonding interface is provided, the long-time reliability of Al wire bonds is maintained due to the stability and strength of the nanostructured interface.     

Assembly of Copper Column Interconnect Flip Chip

This paper addresses a new flip chip interconnection technology, flexible flip chip connection (F2C2) technology, for attaching silicon chips to a chip carrier using flexible copper wires. F2C2 is a novel approach to create area array flip chip interconnections using a matrix block of wires encapsulated with a heat-resistant, dissolvable substance. A slice from the wire matrix ingot is first attached to the chip using solder. The other end of the slice is then matched and soldered to the footprint of a substrate. Finally, the encapsulating, dissolvable substance is removed from the body of the slice, leaving the chip attached to the carrier by the inter- disposed, flexible copper wires. The compliant copper wire interconnections can accommodate the coefficient of thermal expansion (CTE) mismatch problem between die and substrate, thus eliminating the need for underfill.     

A SQUID-Based Nondestructive Evaluation System for Testing Wires of Arbitrary Length

The high field sensitivity of superconducting quantum interference devices (SQUIDs), especially at low frequencies, makes them ideally suited for applications in nondestructive evaluation. For testing of conducting wires, such as aluminum bond wire, we developed a special cryostat, which allows for pulling a wire of arbitrary length (which is kept at room temperature) through a niobium flux transformer connected to a niobium dc SQUID. The wire is excited by either passing an alternating current through it, or by exciting eddy-currents in the wire. The cryostat is made from a stainless steel inner vessel; the outer tube is from fiberglass. The gradiometric pickup loops are wound on a German-silver tube. As the wire under test is at room temperature, thermal noise produced by the wire is limiting the sensitivity of the system, rather than thermal noise produced by the stainless steel dewar.     

Thermomechanical response and stress analysis of copper interconnects

This study is devoted to thermomechanical response and modeling of copper thin films and interconnects. The constitutive behavior of encapsulated copper film is first studied by fitting the experimentally measured stress-temperature curves during thermal cycling. Significant strain hardening is found to exist. Within the continuum plasticity framework, the measured stress-temperature response can only be described with a kinematic hardening model. The constitutive model is subsequently used for numerical thermomechanical modeling of Cu interconnect structures using the finite element method. The numerical analysis uses the generalized plane strain model for simulating long metal lines embedded within the dielectric above a silicon substrate. Various combinations of oxide and polymer-based low-k dielectric schemes, with and without thin barrier layers surrounding the Cu line, are considered. Attention is devoted to the thermal stress and strain fields and their dependency on material properties, geometry, and modeling details. Salient features are compared with those in traditional aluminum interconnects. Practical implications in the reliability issues for modern copper/low-k dielectric interconnect systems are discussed.     

Room-Temperature Nanoindentation Creep of Thermally Cycled Ultrasonically Bonded Heavy Aluminum Wires

Recent findings suggest that creep occurs during thermal cycling of ultrasonically bonded wires, the extent of which is influenced by the nature of the temperature cycle, particularly its peak temperature. In this work, this hypothesis is investigated through a study of the power-law creep behavior of bonded 375-μm aluminum wires that have been thermally cycled. Data from a study of two wire purity levels (99.999% and 99.99%) and two different cycling profiles (?55°C to 125°C and ?60°C to 170°C) are presented. Room-temperature creep stress exponents are derived for the wire bonds from constant-load nanoindentation tests and compared with their respective microstructures.     

Effects of Cu and Pd addition on Au bonding wire/Al pad interfacial reactions and bond reliability

Finer pitch wire bonding technology has been needed since chips have more and finer pitch I/Os. However, finer Au wires are more prone to Au-Al bond reliability and wire sweeping problems when molded with epoxy molding compound. One of the solutions for solving these problems is to add special alloying elements to Au bonding wires. In this study, Cu and Pd were added to Au bonding wire as alloying elements. These alloyed Au bonding wires—Au-1 wt.% Cu wire and Au-1 wt.% Pd wire—were bonded on Al pads and then subsequently aged at 175°C and 200°C. Cu and Pd additions to Au bonding wire slowed down interfacial reactions and crack formation due to the formation of a Cu-rich layer and a Pd-rich layer at the interface. Wire pull testing (WPT) after thermal aging showed that Cu and Pd addition enhanced bond reliability, and Cu was more effective for improving bond reliability than Pd. In addition, comparison between the results of observation of interfacial reactions and WPT proved that crack formation was an important factor to evaluate bond reliability.     

环氧树脂模填加剂模压处理后对MOS场效应晶体管引线键合可靠性的影响(英文)

氧化硅填充环氧树脂在作为芯片封装的密封中对微电子器件的可靠性与功能起着一种主要影响。在帮助了解铝引线键合内部类似功率MOS场效应晶体管的一种含铅电子组件的可靠性,在热力学机械特性试验结果表明由于栅离层的焊线接近引线框架,在键合后从-40℃到125℃的功率热循环下,导致了键合样品的一种电气开路。采用有限元模型模拟了这种封装应力,根据经验检验结果本模拟预示那些在引线键合支撑的焊跟处具有较高的平面剪切力。由数字模拟和试验数据获得的结果可作为选定适当模塑添加剂的一种指导路线。     

An analysis of the reliability and design optimization of aluminium ribbon bonds in power electronics modules using computer simulation method

Ribbon bonding technique has recently been used as an alternative to wire bonding in order to improve the reliability, performance and reduce cost of power modules. In this work, the reliability of aluminium and copper ribbon bonds for an Insulated Gate Bipolar Transistors (IGBT) power module under power cycling is compared with that of wire bonds under power and thermal cycling loading conditions. The results show that a single ribbon with a cross section of 2000 μm × 200 μm can be used to replace three wire bonds of 400 μm in diameter to achieve similar module temperature distribution under the same power loading and ribbon bonds have longer lifetime than wire bonds under cyclic power and thermal cycling conditions. In order to find the optimal ribbon bond design for both power cycling and thermal cycling conditions, multi-objective optimization method has been used and the Pareto optimal solutions have been obtained for trade off analysis.