Copper Wire Bond Research Articles

Multiphysics Effect of Temperature and Humidity on Copper Wire Bond Degradation via Enthalpy-Based Water-Vapor Energy

Multiphysics Effect of Temperature and Humidity on Copper Wire Bond Degradation via Enthalpy-Based Water-Vapor Energy

Evaluation of Copper and Gold Wire Bonds in PEM Devices

Abstract The shift from gold to copper wire bonds used in plastic encapsulated microcircuits (PEMs) has been gradual for the last few years. Many agencies and critics question the long-term robustness and reliability of copper wire bonds; however, PEM manufacturers have been thoroughly evaluating and optimizing the copper wire bond process over the last few years to produce highly reliable products. When the manufacturing process is optimized and closely controlled and monitored, copper wire bonds can be a reliable and stronger alternate to gold wire bonds. This paper details a copper wire bond study that specifically compares copper wire against gold wire bonds following combined environmental life stress test conditions. In this study, bond pull testing indicates good performance of both copper and gold wire bonds. The methods used in this analysis (destructive physical analysis, wire pull testing, and environmental testing) were effective techniques for evaluation of bond wire integrity, detection of manufacturing defects, and overall assessment of copper and gold wire bond processes. Based on wire bond testing in this analysis, the reliability and integrity of optimally developed copper wire bonds appear to be superior to or even stronger than gold wires in the same package type. Equivalent reliability was demonstrated for both copper and gold wire bonds in this test.

Copper Wire Bonding: A Review.

This paper provides a comprehensive review on copper (Cu) wire bonding. Firstly, it introduces the common types of Cu wire available in the market, including bare Cu wire, coated Cu wire, insulated Cu wire, and alloyed Cu wire. For each type, their characteristics and application areas are discussed. Additionally, we provide detailed insights into the impact of Free Air Ball (FAB) morphology on bonding reliability, including its effect on bond strength and formation mechanisms. Next, the reliability of Cu wire bonding is analyzed, with a focus on the impact of intermetallic compounds and corrosion on bonding reliability. Specifically, the formation, growth, and stability of intermetallic compounds at bonding interfaces are discussed, and their effects on bonding strength and reliability are evaluated. The detrimental mechanisms of corrosion on Cu wire bonding and corrosion inhibition methods are also analyzed. Subsequently, the applications of simulation in Cu wire bonding are presented, including finite element analysis and molecular dynamics simulations, which provide important tools for a deeper understanding of the bonding process and failure mechanisms. Finally, the current development status of Cu wire bonding is summarized, and future research directions are discussed.

Effects of electrolyte towards copper wire metallurgical interconnection in semiconductor

Copper (Cu) wire has been extensively used in the semiconductors industry to replace gold wire because of its cost-effectiveness and high performance. However, it has a massive challenge because of its high oxidation rate, high hardness, and high susceptibility to corrosion which is time-sensitive under environmental conditions. One of the attractive factors to investigate is the electrolytes used in the assembly process of copper wire. However, these electrolyte chemicals may potentially affect the quality of the copper wire bond’s metallurgical interconnection, manifesting as copper corrosion. Therefore, this paper will investigate the mechanism of the electrolyte reaction and the chemicals. In addition, the metallurgical morphology of the copper wire observes through electron microscopy. The results suggest that corrosion occurs with a specific time rate, electrolyte type-dependent and metallurgical interconnection system. The Copper wire bonded on silver (Ag) plated lead frame (Cu-Ag-Cu) interconnection experiences a significant morphological change in most compared with other electrolyte systems. Furthermore, since it is a bimetallic element (Cu and Ag) thus, the corrosion type is galvanic.

JEDEC's Generation of Wire Bond Pull Test Methods to Address Pulling of Copper Wire Bonds

Abstract More than 50 years ago when the wire pull test method was initially added to Mil-Std 883, in Condition D of Method 2011, Bond Strength (Destructive Bond Pull Test), the test procedure and minimum pull force values were based on pull testing of mostly ultrasonic wedge bonded aluminum and gold wires of just a few different diameters. The minimum pull force values from that original data were extrapolated to cover a much wider range of wire diameters for both gold and aluminum wires. Since the release of this test method the electronics industry has manufactured copper ultrasonic wedge bonds, widely adopted copper thermosonic ball bonding roughly 15 years ago, and even developed a niche market for silver thermosonic ball bonding. The industry also developed specialty bonds such as security bonds, reverse bonds also called "stitch on ball", and even multi-loop wires and ribbons. In all that time neither the test procedure nor the minimum pull force values in Method 2011 were reviewed to determine their appropriateness for these new materials or new types of bonds, even though the industry widely referenced the test method for all of them and thus, by default, accepted its use for all of them. In late 2013, I led a working group within JEDEC's JC14.1 subcommittee, Reliability Test Methods for Packaged Devices, to update JEDEC JESD22-B116, Ball Bond Shear Test Method, to expand its scope to include the shearing of Cu ball bonds. It took the working group three years to address the necessary technical issues to ensure that the revised test method adequately addressed the shearing of copper ball bonds and propose minimum acceptable shear values. The working group produced a greatly improved document with drawings and images depicting the different shear fail modes of both gold and copper bonds and added several informative annexes to aid in the performing of the test method. By 2018 it was apparent that none of the most commonly referenced wire pull test methods in the electronics industry had made any significant progress in updating their documents to include Cu wire bonds. Therefore, the JC14.1 working group agreed to work jointly with the JC-13.7 Subcommittee, New Electronic Device Technology, to create a new, wire pull test method document under JC14.1 that would be a companion to the JESD22-B116. This new document will use Method 2011, Conditions C and D as its basis, but expand on its scope to cover copper wire bonds, both ultrasonic wedge and thermosonic ball bonds. The new test method will describe the process for a ball pull test and a stitch pull test that are referenced for copper bonds by AEC Q006, Qualification Requirement for Component Using Copper (Cu) Wire Interconnection. The test method will also provide guidance on how to perform pull testing on several different bond types used today including reverse bonds, multi-loop bonds, and stacked die. The working group plans to propose minimum pull values for copper wire bonds which JC14.1 will reference in JESD47, Stress-Test-Driven Qualification of Integrated Circuits. After the joint working group completes its work, which is targeted for some time in 2022, JC13.7 would then be able to use the output of this working group to update Method 2011 Conditions C & D. This paper will first briefly discuss the updates made to B116 to cover Cu wire bonds, but mainly focus on the work that has so far been completed by the joint working group, including a general outline of the proposed new document, JESD22-B120, Wire Bond Pull Test Methods.

Experimental parameter identification and validation of a process model for ultrasonic heavy wire bonding

Abstract Ultrasonic heavy wire bonding is a standard process in packaging technologies of power semiconductor modules. Due to increasing demands on reliability of the electrical contacts under high temperature loads, copper wires with significantly better electrical and thermal properties compared to aluminum are used more often nowadays. This results in new challenges in process development due to higher process forces and ultrasonic power; for this purpose, a simulation model has been developed to improve process development. The process model is based on a co-simulation with sub-models for the different physical phenomena. The sub-models are based on parameters, which need to be identified from measurements. This contribution focusses on the identification of the material model. Therefore, a method is presented, which allows for an iterative identification of the stress-strain characteristics from compression tests based on a modified tensile-compression machine. In compression tests under ultrasonic load, the bond wire material behavior under ultrasonic load is investigated to characterize the so-called ultrasonic-softening-effect. The simulation model with identified model parameters is then used to predict main-effects-diagrams for aluminum and copper wire bond processes on DCB. The simulation results are validated by comparison to results from parameter studies of ultrasonic heavy wire bonding experiments.

Effects of alloying elements in high reliability copper wire bond material for high temperature applications

Recently, a shift from Au bonding wire to Cu bonding wire is expected for automotive application. Palladium coated copper (PCC) wire is strong candidate since it is widely used in LSI packages owing to its high performance and low cost. In order to apply PCC wire to automotive devices, long-term reliability specific to automobiles including stable operation under harsh environment with high temperature and humidity is required. However, pitting corrosion at the wedge bond portion was reported with PCC wire under high temperature storage life (HTSL) test. Depending on the surrounding environment such as mold compound, it is difficult for conventional PCC wire to achieve the target of automotive application. A newly developed PCC wire (EX1R) has thus developed. By using additive element in Cu core, halide induced interface corrosion and also pitting corrosion can be prevented successfully. In this paper, effect of additive element on the corrosion resistance will be discussed. The result shows that additive element forms protective passivation layer on the intermetallic compound (IMC) and Cu surface. Thereby, EX1R improves bond reliability even under severe reliability test condition.

Substructural phenomena in Cu wire bond after laser assisted manufacturing in electronic packaging

This paper studies the crystallographic and substructural details in copper wire bonds on integrated circuit chips. Copper is regarded a potential replacement of gold wire bonding technology used in electronic device manufacturing. The innovative manufacturing process adopted in this work involves simultaneous application of laser heat and ultrasonic pressure. The beneficial effects of laser heating are apparent to overcoming the processing challenges associated with high hardness of copper. The application of laser yields columnar gains and grain size gradient. The laser-assisted samples have undergone a different deformation mechanism which is indicated by the non-localized nature of deformation. The dynamic thermal restorations in the laser-assisted sample are obvious.

Life prediction of copper wire bonds in commercial devices using principal component analysis (PCA)

Copper has been widely adopted as a viable alternative to gold for wire bonding, due to its lower cost, better electrical and thermal conductivity, lower tendency for wire sweep due to a higher modulus and slower intermetallic compound (IMC) formation with aluminum bond-pad. However, these IMCs can cause an increase in resistance and separation between bonding interface which causes an open circuit. Wire bond and component reliability must consider all factors in combination with the change of wire bond materials and processes. How can users of off-the-shelf components determine the effects of wire bond material changes on the reliability of the components in their systems? How will these changes affect high reliability applications such as automotive, military and harsh environments?Although physics-of-failure based models can represent the failure mechanisms individually, the complex nature of the mixed failure mechanism has inhibited development of an analytical equation to represent it. In addition, geometrical and material characteristics of the entire package along with bonding process parameters play a crucial role in magnitude of stress experienced by these bonds, portraying difficulty in developing one such equation for use to all copper wire bonded devices. This study has aimed to develop a data-based predictive method based on the principal component analysis technique to provide an estimate of the wire bond time to failure for plastic encapsulate components. Extensive thermal cycling experiments have been performed on multiple commercial devices to obtain training data for the model. This method of wire bond fatigue and failure prediction is an adaptive technique which can achieve greater accuracy as more data is added. This method aims to provide a life-time estimate of copper wire bonded component with inputs such as initial geometrical, material properties, and wire bond related attributes for a specific package.

Challenges and Resolution for Copper Wirebonding on Tapeless Leadframe Chip-on-Lead Technology

This technical paper discusses a methodological and systematic way of resolving key challenges during introduction of Chip-On-Lead package specifically wirebonding issues that leads to production dilemma during production ramp-up of products using copper wire in tapeless leadframe. The project was intended to determine the “Red-X” or the major cause of yield detractors that may lead to quality issue during wirebonding process.
 Problem solving tools were showcased in this paper such as Data Analysis, Cause and Effect, Design-of–Experiment (DOE) and mechanical dimensional analysis which provided substantial impact in determining the real root-cause of the problem. Step-by-step elimination of variables was achieved with the use of statistical engineering tools. Outcome of the project eliminated the occurrence of Non-Stick-On-Pad (NSOP) during wirebonding process without cost involved and just optimizing the available in-house resources. The improvements also enhanced the quality of the product after final test which on the other hand lower the risk of having potential customer complaint in the future.

Chip-On-Lead Semiconductor Package with Copper Wirebonding

Chip-On-Lead Semiconductor Package with Copper Wirebonding

Characterization of Rate-Dependent Constitutive Behavior of Copper Free-Air Ball and Its Use for Wire Bonding Process Simulation

Copper (Cu) wire bond technology is an increasingly popular alternative to gold (Au) wire bond in microelectronic devices due to its superior electrical performance and low cost. Cu wires are also approximately 25% more thermally conductive than gold wires, resulting in better heat dissipation. At present, validated constitutive models for the strain rate and temperature-dependent behavior of Cu free-air ball (FAB) appear to be largely missing in the literature. The lack of reliable constitutive models for the Cu FAB has hampered the modeling of the wire bonding process and the ability to assess risk of fracture in ultralow-k dielectric stacks. The challenge to FAB characterization is primarily due to the difficulty in performing mechanical tests on spherical FAB of micrometer size. To address this challenge, we describe a custom-built microscale tester and carry out compression tests on the FAB in this paper. The Anand viscoplastic constitutive model is developed for Cu FAB through an inverse modeling procedure. In the inverse procedure, constitutive model parameter values are iterated through an automated optimization procedure, until the load–displacement response matches the experimentally observed response. Using the estimated Anand model parameters, a finite-element analysis of the impact and ulatrasonic vibration stages of wire bonding process is carried out, and the resulting dielectric fracture risk is analyzed.

Influence of Initial Shear Strength on Time-to-Failure of Copper (Cu) Wire Bonds in Thermal Aging Condition

Abstract Copper (Cu) wire bond is used in a majority of microelectronic devices but has not been fully accepted by all industries due to potential reliability issues. Good quality bond is believed to provide high reliability. Shear strength and intermetallic (IMC) coverage are being used as indicators, however there is no developed model. High shear strength is a result of large IMC coverage laterally under the ball, but the reliability of the bond is also related to IMC longitudinal growth (thickness). This work involves studying effect of shear strength on reliability of a variety of experimental Cu ball bonds by performing temperature aging experiments on test devices, in QFN packages. The ball bonds are made by altering ultrasonic power and time to obtain different IMC coverage. All test packages are monitored for resistance change at specific intervals by performing four point resistance measurement. Resistance increase is analyzed with initial shear strength to determine whether “good quality” bonds always lead to high reliability.

Effective decapsulation of copper wire-bonded microelectronic devices for reliability assessment

The wire bonding industry has made a major shift in wire materials from gold to copper, primarily due to cost concerns. Copper wire-bonds are now present in many commercial off-the-shelf (COTS) devices but minimally used in automotive, industrial, or military-grade applications due to lack of detailed understanding about reliability concerns. A thorough study of wire bond reliability includes performing bond shear and pull strength measurements before and after stress testing. This in turn requires a special decapsulation procedure for copper wire-bonded devices because, unlike gold, copper is chemically potent. Many techniques for copper wire-bonded device decapsulation exist and can be categorized into laser-, plasma-, and acid-based processes. This paper reviews some of these techniques and discusses the decapsulation mechanism, which involves decomposition of the epoxy resin. By understanding the decapsulation mechanism and available techniques, a unique decapsulation method was developed. The effectiveness of this method is presented along with scanning electron microscopy (SEM) images of the results, which indicate minimal etching of copper wire bonds. The critical parameters of this technique are also identified, a suitable range of input for each parameter is analyzed theoretically, and a design of experiment (DOE) is conducted to find optimal values for each parameter. Several SEM images are provided to show both the good and bad results from the DOE. An image method for measuring effectiveness of decapsulation is also presented.

Development of Model for Identification of Process Parameters for Wet Decapsulation of Copper–Aluminum Wirebond in PEMs

Decapsulation is a failure analysis technique often used to expose the die and first-level interconnects such as wirebonds by dissolving the surrounding epoxy molding compound (EMC). The wet decapsulation technique, which uses fuming acids, works very well for traditional gold (Au) wirebonds. On the other hand, its latest alternative, copper (Cu) wirebond, reacts with the nitric acid vigorously and undergoes severe corrosion. It is important to develop an acid chemistry that can be used to perform decapsulation of Cu–Al incorporated plastic encapsulated microelectronics (PEMs) without damaging the Cu wires. This paper presents the wet decapsulation technique based on different ratios of the red fuming nitric acid and concentrated sulfuric acid. Quality of post-decap part was examined using reduction in the wire diameter and changes in ball shear strength. Reduction in wire diameter was monitored with the scanning electron microscopy (SEM), and shear strength was measured using a DAGE2400 shear tester. These tests were performed on the PEMs molded with different EMCs, wire diameters, and pad thickness to cover process as well as geometric variability. Artificial neural network with Bayesian regularization-based model has been developed correlating the decapsulation process parameters with the post-decap wire diameter reduction and shear strength of the wirebond. Stepwise regression was used to identify the significant variables that affect the decap process. The influential variables were then used to develop a predictive model for prediction of the percent reduction in wire diameter and change in shear strength separately. Models were then validated with the test data set.

Effects of Bond Pad Thickness on Shear Strength of Copper Wire Bonds

Abstract Copper (Cu) wire bonding is now widely accepted as a replacement for gold (Au), however, its use in high reliability applications is limited due to early failures in high temperature and humid conditions. The Au to Cu wire transition is mainly driven by cost savings though there are other advantages to Cu such as better electrical and thermal conductivity, slower intermetallic compound (IMC) formation and reduced wire sweep during transfer molding. Some automotive, industrial and aerospace industries are still reluctant to adopt Cu wire bonded products due to perceived risks of wire and bond pad cracks, the potential for corrosion, and some lack of understanding about its reliability in harsh conditions. A wire bond is considered good if destructive sampling qualification tests and periodic monitors pass for the batch. Tests include wire pull strength, wire bond shear, IMC coverage, and thickness of bond pad aluminum (Al) remaining beneath the bond. Nondestructive inspections also verify acceptable ball diameter and Al “splash”. This paper focuses on the bond shear test and its contribution to Cu ball bond reliability assessment, especially when changing Al bond pad thickness. A new revision of the JEDEC Wire Bond Shear Test Method, JESD22-B116B, has just been released, to include Cu wirebonds for the first time. It helps to clarify shear test failure modes for Cu ball bonds. However, there are still questions to be answered through research and experimentation, especially to learn the extent to which one may predict Cu ball bond reliability based on shear test results. Pad Al thickness is not considered in the current industry standards for shear test. Yet bond pad Al thickness varies widely among semiconductor products. This research is intended to contribute toward improved industry standards. In this study, power and time bonding parameters along with bond pad thickness are studied for bond strength. Several wire bonds are created at different conditions, evaluated by optical microscope and SEM, IMC% coverage and bond shear strength. Similar bonding conditions are repeated for bond pads of 4um, 1um and 0.5um thickness.

Effect of in-situ free air ball laser heating on bonding strength and grain structure for copper wire bond

As the need for integrated circuits increase year after year, researchers around the globe are constantly experimenting for innovative approaches to use copper wire as a medium of interconnectivity. Copper is being used as an alternative to replace gold to produce interconnections in microelectronics. However, the use of copper wire is known to pose threats to low-k devices with thin structure. The impact of hard copper free air ball onto bonding surfaces is undesirable as it can damage the bond pad and the silicon die. In-situ heating of free air ball is expected to reduce its hardness and mitigate columnar grains. This research paper investigates the effects of in-situ free air ball laser heating on bonding strength and grain structure for 99.999% purity (5N) copper wire bonding. Wire bonding was performed on bond pads consisting of NiPdAu metallization and was wire bonded under 16 different temperature conditions. Results of this study showed significantly higher as-bonded ball shear strengths and reduced columnar grains when in-situ laser heating is applied. The results of this study can be helpful for semiconductor plastic packaging with copper wire bonding on thin die structures by further improving copper wire bonding quality without the need to use excessive bonding temperature, higher bonding force and excessive ultrasonic energy.

Consumable and Process Improvement for Large Copper Wire Bonding

Abstract Aluminum (Al) wire has certain limitations and cannot satisfy the increased demands for power electronics interconnects. Copper (Cu) wire has many benefits and is replacing Al wire in high power, high temperature applications. Cu wire is much harder and wears the bond tool much faster than Al wire. Consumable lifetime and process stability have to be improved so that Cu wire interconnect can be accepted for mass production. The bond tool wear mechanism is investigated. Bond process parameter, bond tool tip geometry, and bond tool material affect bond tool lifetime and bond performance. By combining the bond tool tip design and bond process optimization, an ultra-shallow groove bond tool that has a groove depth (GD) of 40% wire diameter achieved producing 260,000 Cu wire bonds. The groove opening angle (GOA) affects wire confine capability of a bond tool and a larger GOA is preferred for bond tools with a shallower GD. Alternative bond tool material shows benefits of improving bond appearance and further extending bond tool lifetime.

Reliability aspects of copper metallization and interconnect technology for power devices

The introduction of thick copper metallization and topside interconnects as well as a superior die attach technology is improving the performance and reliability of IGBT power transistor technologies significantly.The much higher specific heat capacity and higher thermal conductivity increases the short circuit capability of IGBTs, which is especially important for inverters for drives applications. This opens the potential to further optimize the electrical performance of IGBTs for higher energy efficiency.The change in metallization requires the introduction of a reliable barrier against copper diffusion and copper silicide formation. This requires the development of an efficient test method and reliability assessment according to a robustness validation approach.In addition, the new metallization enables interconnects with copper bond wires, which yield, together with an improved die attach technology, a major improvement in the power cycling capability.

Effect of Pedestal Temperature on Bonding Strength and Deformation Characteristics for 5N Copper Wire Bonding

In recent years, copper has increasingly been used to replace gold to create wire-bonded interconnections in microelectronics. While engineers and researchers in the semiconductor packaging field are continuously working on this transition from gold to copper wires to reduce costs, the challenge remains in producing robust and reliable joints for semiconductor devices. This research paper investigates the effect of pedestal temperature on bonding strength and deformation for 99.999% purity (5N) copper wire bonding on nickel-palladium-gold (NiPdAu) bond pads. With increasing pedestal temperature, significant thinning of the copper ball bond can be achieved, resulting in higher as-bonded ball shear strengths while producing no pad damage. This can be helpful for low-k devices with thin structures, so as to prevent the use of excessive bond force and ultrasonic energy during copper wire bonding.