Microelectronic Packaging Research Articles

Thermally Conductive and Electrically Insulating Epoxy Composites Filled with Network-like Alumina In Situ Coated Graphene.

With the rapid development of the electronics industry, there is a growing demand for packaging materials that possess both high thermal conductivity (TC) and low electrical conductivity (EC). However, traditional insulating fillers such as boron nitride, aluminum nitride, and alumina (Al2O3) have relatively low intrinsic TC. When graphene, which exhibits both superhigh TC and EC, is used as a filler to fill epoxy resin, the TC of blends can be much higher than that of blends containing more traditional fillers. However, the high EC of graphene limits its application in cases where electrical insulation is required. To address this challenge, a method for coating graphene sheets with an in situ grown Al2O3 layer has been proposed for the fabrication of epoxy-based composites with both high TC and low EC. In the presence of a cationic surfactant, a dense Al2O3 layer with a network structure can be formed on the surface of graphene sheets. When the total content of Al2O3 and graphene mixed filler reached 30 wt%, the TC of the epoxy composite reached 0.97 W m-1 K-1, while the EC remained above 1011 Ω·cm. Finite element simulations accurately predicted TC and EC values in accordance with experimental results. This material, with its combination of high TC and good insulation properties, exhibits excellent potential for microelectronic packaging applications.

Investigation on Rheological Characterization of Isotropic Conductive Adhesives

In the field of microelectronic packaging technology, isotropic conductive adhesives (ICAs) have been used widely as interconnection materials between chips and substrates. ICAs are composed of silver powder particles and adhesive matrix. The paper analysed the rheological properties of ICAs at different temperatures in order to better apply ICAs in dispensing and jet printing processes. The viscosity, viscoelasticity, and thixotropic behaviours of ICAs were studied using various experimental methods such as oscillatory stress shear, and rotational rheology. The rheological behaviours of ICAs conforms to the Cross Model. In SAOS flow field, the ICAs has G”>G’ in LVR region, which shows the viscous component is dominant and ICAs represent viscoelastic fluid characteristics.

Research Progress of Palladium-Plated Copper Bonding Wire in Microelectronic Packaging.

Wire-bonding technology is the most commonly used chip interconnection technology in microelectronic packaging. Metal bonding wire is the key material for wire bonding and plays an important role in the reliability of electronic devices. In recent years, palladium-plated copper (PdCu) bonding wire has been widely used because of its low cost, good electrical and thermal conductivity, the fact that it is not easy to oxidize, and its high reliability. Therefore, it is necessary to review its research progress. In this paper, the preparation and application of palladium-plated copper bonding wire are reviewed. Firstly, the preparation methods of electroplating, electroless plating, and direct plating are introduced. Secondly, the factors affecting the distribution of Pd in free air balls and bonding interfaces, the effect of Pd on the formation and growth of intermetallic compounds in PdCu wire, stitch bond, and reliability of PdCu wire are summarized and analyzed in the application process. Finally, its development prospect is prospected. Hopefully, this review can help readers to have a comprehensive understanding of the preparation and application of palladium-plated copper bonding wires, and can accelerate the promotion of its application in more fields in the future.

A Hybrid Pressureless Silver Sintering Technology for High-Power Density Electronics

Aerospace and defense applications present unique challenges for material suppliers. As increasing adoption of advanced semiconductor materials and Diverse Accessible Heterogeneous Integration technologies, power density of defense and aerospace devices increases rapidly. Traditional die-attaching technology is becoming an increasingly limiting factor in microelectronics packaging for the next generation aerospace and defense systems. Metal particle sintering creates a porous metal foam, which can significantly enhance heat transfer within the sintered material. However, the traditional silver sintering requires very high sintering temperatures that cannot be tolerated by typical microelectronics devices. In addition, the sintered metal foam contains open-cell pores that can absorb/entrap moisture and dusts, which poses a reliability risk. This article introduces an advanced hybrid silver sintering material, which incorporates highperformance silver sintering with high reliability and process-friendly of epoxy-based die-attaching technology. This hybrid sintering paste can be processed without applying any pressures in temperature ranges that are normal in microelectronics packaging processes. Preliminary experimental studies, including Scanning Electron Microscopic study, volume resistivity tests, die shear strength tests, and thermal resistance tests, were performed for developing a low temperature sintering schedule that is compatible with normal assembly processes of high-power density electronics devices. The results indicated that the hybrid material could achieve silver sintering at 150 degrees C and offer 36 enhancement on thermal performance comparing with a widely used die-attach material.

Research Progress of Electroplated Nanotwinned Copper in Microelectronic Packaging.

Copper is the most common interconnecting material in the field of microelectronic packaging, which is widely used in advanced electronic packaging technologies. However, with the trend of the miniaturization of electronic devices, the dimensions of interconnectors have decreased from hundreds of microns to tens of or even several microns, which has brought serious reliability issues. As a result, nanotwinned copper (nt-Cu) has been proposed as a potential candidate material and is being certified progressively. Firstly, the physical properties of nt-Cu have been widely studied. Notably, the higher thermal stability and oxidation resistance of the (111) texture causes nt-Cu to maintain excellent physical properties under high-temperature serving conditions. Secondly, recent works on the electrolyte and electroplating processes of nt-Cu on wafer substrates are summarized, focusing on how to reduce the thickness of the transition layer, improve the twin density, and achieve complicated pattern filling. Thirdly, nt-Cu can effectively eliminate Kirkendall voids when it serves as UBM or a CuP. Additionally, the high (111) texture can control the preferred orientation of interfacial intermetallic compounds (IMCs) at the Cu-Sn interface, which should be helpful to improve the reliability of solder joints. nt-Cu has superior electromigration resistance and antithermal cycling ability compared to ordinary copper RDLs and TSVs. Above all, nt-Cu has attracted much attention in the field of microelectronic packaging in recent years. The preparation-performance-reliability interrelationship of nt-Cu is summarized and displayed in this paper, which provides a solid theoretical basis for its practical applications.

Study on preparation and sintering properties of nano-silver-coated tin slurry

A nano-sized silver-coated tin (Sn@Ag) slurry was prepared by heterogeneous flocculation method by adjusting the pH value of solution and selecting different dispersants. The slurry improved the oxidation resistance of tin and its dispersibility in silver matrix. The sintering strength of nanometre Sn@Ag slurry increases with the increase of Sn content. When the Sn content reaches 5%, the shear strength of the joint reaches the highest 50 MPa, which is more than 10 MPa higher than that of the pure nanometre silver slurry sintered joint. The increase of shear strength is due to the fact that the equilibrium phase formed after sintering is Ag–Sn replacement solid solution and intermetallic compound Ag 3 Sn, which have the effect of replacement solution strengthening and dispersion strengthening, respectively. It is proved by experiments and analysis that the application of nano-silver paste in chip interconnection is feasible. The research of this subject provides experimental reference and theoretical basis for the application of new generation interconnect materials in power devices and promotes the development of microelectronics packaging technology.

Reliability and failure modelling of microelectronic packages based on ultrasonic nondestructive evaluation data

Reliability testing and failure modelling is crucial and very challenging for modern electronics, especially safety critical electronics-based systems working under harsh environmental conditions. In this paper, an approach based on ultrasonic Non-Destructive Evaluation (NDE) is proposed to establish a failure model for solder joints which is urgently needed for prognostic and health management of electronics. Printed Circuit Boards (PCB) containing flip-chip and Area Array packages were designed and aged using Accelerated Thermal Cycling (ATC) testing. During ATC testing solder joint degradation was regularly monitored by taking out the test boards from the temperature chamber at four thermal cycle intervals for ultrasonic micro-imaging. Data pre-processing techniques including dynamic range-based intensity normalization, gain compensation, and calibration for random transducer defocusing errors were developed to enhance the data consistency, integrity and accuracy. Furthermore, solder joint image labelling and segmentation methods were developed by exploiting the geometrical features of the solder joints to extract individual solder joints and Region of Interests (ROIs) from the sequential ultrasound images collected throughout the testing. The mean intensity of the ROIs was used as the precursor to degradation to build the solder joint failure model, from which cycles to failure for individual solder joints are finally derived for life prediction.

Enhancement of the electrical and thermal conductivity of epoxy-based composite films through the construction of the multi-scale conductive bridge structure

Electrically conductive adhesive films (ECAFs), a novel electrically and thermally adhesive material, have been widely used in the large-area bonding of microelectronics packaging. However, it is still a challenge to optimize the silver micro-flakes (Ag-MFs) dispersion, and build a more effective electrically and thermally conductive network. In this study, we improved the dispersion of Ag-MFs in epoxy matrix through the intermolecular hydrogen bonding, and proposed a new multi-scale conductive bridge structure. Specifically, poly (3,4-ethylenedioxythiophene)/poly (styrene sulfonate) (PEDOT/PSS or P/P for short) can form the intermolecular hydrogen bonding with Ag-MFs based on the sulfonic groups of PSS chain and the carboxyl group located on the surface of Ag-MFs, resulting in improving dispersion of Ag-MFs. Besides, PEDOT can occur conformation change from benzoid conformation to quinone conformation and form PEDOT conductive crystalline nanofibril, and then multi-scale conductive bridge structure with Ag-MFs were constructed. Moreover, the initial viscosity of epoxy-based composite is maintained within 106 mPa s, which means Ag-MFs is uniformly dispersed after the addition of P/P. Scanning electron microscopy (SEM) and oscillating viscoelastic measurement were used to demonstrate the improved dispersion of Ag-MFs after the addition of P/P. In addition, atomic force microscope (AFM) and Raman spectroscopy were used to verify the conformation change of PEDOT. Compared with ECAF without P/P, the electrical and thermal conductivity of ECAF with 6 wt% P/P was enhanced by 73.1% and 166.3%, respectively. Meanwhile, scattering parameter (S-parameter) measurements demonstrated that ECAF with 6 wt% P/P had excellent radio-frequency behavior at 2–4 GHz, indicating high-performance in the microelectronics packaging industry.

Understanding Criticality of Thermal Performance in Thermal Interface Material Applications

Thermal interface material (TIM) is used in between a heat generating component (e.g., microelectronic packaging) and a heat spreading component (e.g., heatsink or cooling plate) to create an effective path for the heat (Phonon) to travel. Standard heatsink and heat generating component surfaces are generally uneven and rough. Actual metal to metal contact is no more than 10%. These surface imperfections allow air to get trapped in between the two surfaces. Air, being a thermal insulator, prevents the heat from dissipating and thus the device/system fails to maintain the required operating temperature to meet the reliability and functionality needed. Replacing the air with the “right” thermal interface material is the focus of this research. TIM is a composite of thermally conductive fillers dispersed in a polymer matrix. A higher filler loading causes a higher bulk conductivity. Common practice among design engineers is to utilize output from thermal modelling & simulation to specify a TIM with a certain thermal conductivity to meet the system’s thermal needs. What many engineers miss is the impact of thermal boundary resistance that could have significant effect on the overall thermal management of the design. This paper discusses how to characterize thermal performance of TIM beyond the bulk thermal conductivity. Boundary/interfacial thermal resistance and impedance will be explored as a function of TIM wetting ability, bond line thickness and surface conditions. This paper will discuss the importance of thermal conductivity, thermal resistance/impedance in a real application scenario.

Electrically Insulative Film Adhesive with Enhanced Thermal Performance to Assemble High Power Density Electronic Devices

Assembly adhesives play a critical role in microelectronic packaging and contribute significant value for circuit assembly interconnection, protection, and thermal management. Of particular interest with today’s more powerful systems is thermal management of high-power density electronic systems, including power modules, power amplifiers, communication systems, and directed energy systems. As requirements for performance and functionality increase, heat generated from these devices increases exponentially. For example, local heat flux can easily exceed 50 W/cm2 for IGBT power modules while it may reach several hundred Watts/cm2 at active areas for Gallium Nitride (GaN) power amplifiers. Simultaneously, many of these systems demand a higher level of electrical insulation to eliminate or minimize current leakage to ensure high reliability over a long service life. Effectively and safely dissipating the high heat and ensuring excellent electrical insulation has become an increasingly critical and challenging task in the microelectronics packaging technology for aerospace and defense systems. This paper presents integration of thermally conductive adhesives with an electrically insulative polyimide film into a carefully engineered laminate, which enables high thermal transfer and robust electrical insulation. This new film adhesive provides outstanding compliance and augmented tolerance to large topographical features and warpage (5-7 mils). This film adhesive is one of the potential solutions to addresses the evolving needs of the electronics assembly industry in the areas of management of high-power systems

A Hybrid Pressure-less Silver Sintering Technology for High-power Density Electronics

Integration of smaller, higher-functioning devices and use of advanced high thermal chip structures present thermal management challenges in the aerospace sector where reliability is the top priority and fail-safe processes/materials are the standard. As power density increases rapidly, traditional die attaching technology is becoming an increasingly limiting factor in microelectronics packaging for the next generation aerospace and defense systems. This paper introduces an advanced hybrid silver sintering technology, which incorporates ultra-high thermal and electrical performance of silver sintering with high reliability and process friendly of epoxy-based die attach technology. Unlike traditional silver sintering that requires high temperature and pressure, this hybrid sintering paste can be processed without applying any pressures in temperature ranges that are normal in microelectronics packaging processes. This paper presents results of an application study aimed at developing this unique technology in the field of high-power density devices for aerospace applications.

A novel hybrid method to integrate delicate MEMS components into a FOWLP

A steep increase in IoT- and smart devices fuels the need for ever more miniaturized and highly integrated microelectronic packages. Fan-Out technology is well suited for this task, as it allows for heterogeneous (co-)integration of sensor elements and corresponding circuitry. Following a Chip-First approach in Fan-Out Wafer-Level Packaging (FOWLP), protection of delicate sensor areas during compression molding and formation of redistribution layers (RDL) becomes a key part of the manufacturing process chain. Such areas include thin membranes, antireflective coatings, air bridges or media access for gas sensors. We demonstrate a novel packaging method for a MEMS pressure sensor with corresponding ASIC (application-specific integrated circuit). We integrate vertical interconnect elements (VIE), instead of employing laser drilling for via formation and thereby avoid debris and reduce the overall package thickness. The hybrid approach, at heart, relies on a structured adhesive layer, onto which components are placed prior to compression molding. Due to its thermal and mechanical stability, the patterned thin film adhesive serves as a (first) dielectric layer. Process development includes warpage management to allow for laser debonding from temporary glass carriers and RDL processing compatible with delicate MEMS surfaces. Summarizing, we present a novel process variant to avoid common issues related to sensor surface integrity using Fan Out technology and thus allow for highly miniaturized System in Package (SiP) concepts with sensor integration by mold embedding.

The Influence of CaF2 Doping on the Sintering Behavior and Microwave Dielectric Properties of CaO-B2O3-SiO2 Glass-Ceramics for LTCC Applications

With the rapid development of microelectronic information technology, microelectronic packaging has higher requirements in terms of integration density, signal transmission speed, and passive component integration. Low temperature co-fired ceramics (LTCC) exhibit excellent dielectric properties and low temperature sintering properties, which meets the above-mentioned requirements. This work investigates the effects of CaF2 doping (0–16 mol%) on the glass structure, sintering behavior, crystallization, microstructure, and microwave dielectric properties of the CaO-B2O3-SiO2 (CBS) glass-ceramic system. Glass-ceramics were prepared using the conventional melting and quenching method. The physical and chemical properties of the glass-ceramics were analyzed using various techniques including TMA, SDT, FTIR, XRD, SEM, and a network analyzer. The results indicate that CaF2 doping can effectively reduce the sintering temperature and softening temperature of CBS ceramics. It also substantially improves the densification, dielectric, and mechanical properties. The appropriate amount of CaF2-doped CBS glass-ceramics can be sintered below 800 °C with a low dielectric constant and loss at high frequency (εr < 6, tanδ < 0.02 @ 10~13 GHz). Specifically, 8 mol% CaF2 doped CBS glass-ceramics sintered at 790 °C exhibit excellent microwave dielectric and thermal properties, with εr ~ 5.92 @ 11.4 GHz, tanδ ~ 1.59 × 10−3, CTE ~ 7.76 × 10−6/°C, λ ~ 2.17 W/(m·k), which are attractive for LTCC applications.

Atomistic insights into silver-indium solid solution softening mechanism for microelectronics packaging

Silver-indium solid solution is the key material in advancing low-temperature/pressure solid-state technology for electronics packaging, due to its rare solid solution softening (SSS) phenomenon. In this work, the critical resolved shear stress (CRSS) for slip, ideal shear strength and critical twinning stress of silver-indium disordered solid solutions were systematically evaluated by the generalized planar fault energy (GPFE) analysis, using first-principles modelling. Compared with others, Ag-based metals have much lower ideal shear strength and critical twinning stress, whereas the CRSS for slip of silver-indium solid solutions decreases with the increases of indium content, therefore exhibiting a general trend of softening behaviors. For the first time, the electronic origin of silver-indium SSS mechanism has been unveiled, where the modification of its GPFE structure was the fundamental cause for the reduction of CRSS for slip, critical twinning stress and the corresponding SSS phenomenon. As confirmed by transmission electron microscopy, this work presents a new design criterion for the next-generation solid-state bonding materials development.

A Review of Low-Temperature Solders in Microelectronics Packaging

A Review of Low-Temperature Solders in Microelectronics Packaging

Controlling diffusion in gold bonding materials for high reliability via microalloying of trace rare earth metals

Diffusion plays a vital role in regulating microstructure evolution and properties of metallic materials. However, understandings of how microalloying affects diffusion kinetics in gold bonding materials remain far from comprehensive, which severely threatens the reliability of microelectronic devices in the big-data era. This problem persists challenging due to enormous difficulty in accurate and cost-effective experimental measurements. Here, first-principle based theoretical computations are performed to explore the control of diffusion at elevated service temperatures by trace rare earth (RE) microalloying in gold bonding materials. We identify the light lanthanides can stably bind excess vacancies and the efficacy to modulate vacancy/solute-vacancy evolution depends on the RE concentrations and ambient temperatures. Above critical doping concentrations, self- and impurity-diffusion can be simultaneously hindered. These findings successfully point to a rational microalloying strategy to withstand heat and a novel perspective on improving the service life-time of microelectronics packaging materials.

Optimal design of thermo-compression bonder via ANN-based surrogate modeling under uncertainty

The miniaturisation of electronic devices demands advanced microelectronics packaging systems dominated by interconnection reliability. Thermo-compression bonding (TCB) is a promising approach to improve bonding quality by applying heat and pressure onto the chip. However, TCB has prompted severe mechanical warpage and electrical shortage when the diminutive interconnection is randomly shaped, which yields non-uniform stress and thermal distribution in bonding. Moreover, the mechanism is interpreted by excessive multi-physics simulations or experiments, which impedes computational cost-efficiency. Therefore, this research develops an optimised TCB design via a machine learning-based surrogate model to predict complex TCB behaviour, by certifying its best performance. First, a framework is proposed to extract and propagate the multivariate geometric uncertainties of the interconnection. The framework realistically represents the existing packaging system and predicts TCB behaviour through ANN, which enriches the accuracy and efficiency of the prediction model. Second, an optimal TCB is designed to minimise the warpage and electrical short. With the surrogate model, the optimisation attempts to find the best location and magnitude of a structural pressure, which derives uniform stress concentration or restricted interconnection deformation. The efficacy of the proposed framework is demonstrated with a practical example of an advanced packaging system utilised in actual commercial products.

Arrhenius Fatigue Life Modeling for Lead-Free Solder Joints in Accelerated Combined Fatigue and Creep Tests at Different Operating Temperatures

Frequent failures in interconnected materials in microelectronic packages play a vital role in determining the reliability of electronic devices. Differences in the thermal expansion coefficients of the solder joint, printed circuit board (PCB), and electronic package are a major source of applied stress on solder joints. Due to health concerns and governmental regulations associated with using leaded solder alloys, SAC solder alloys are commonly used as an alternative to Pb-alloys because of their outstanding solderability and reliability. The shear fatigue creep accelerated test at different load levels (16 MPa, 20 MPa, and 24 MPa) was implemented in this study by using a universal micro-testing machine to investigate solder joint reliability. The creep effects were demonstrated by employing 60 seconds of dwell time for each alternating stress value and comparing its impact on solder joint reliability with the fatigue test results. The accelerated test was applied to SAC305 (96.5% Sn, 3% Ag, 0.5% Cu) solder joints with OSP surface finish at different testing temperatures (25°C, 60°C, and 100°C). Seven replicates were considered as a sample size for the reliability analysis. A two-parameter Weibull distribution was employed as the dominant distribution to describe the fatigue behavior at each condition. The characteristic life and shape parameters at each condition were extracted from the obtained Weibull models. The stress–life equation was used to estimate the reliability performance of the solder joints against fluctuations in stress amplitude at differing testing temperatures. The inelastic work per cycle and plastic strain were calculated from the acquired hysteresis loop under each condition. Coffin–Manson and Morrow energy models illustrate the relationships between fatigue life versus plastic strain and inelastic work, respectively. The effects of the oscillated testing temperature on the stress–life model, Morrow energy model, and Coffin–Manson model were elaborated using the Arrhenius equation. The results indicated a severe degradation of fatigue life with increased testing temperature. Raising the stress amplitude had a less significant impact on fatigue life reduction than increasing the testing temperature. The reliability prediction models, as a function of the fatigue properties and testing temperature, were derived from the Arrhenius, Coffin–Manson, and Morrow energy models. The acquired prediction equations produced acceptable goodness-of-fit values. Finally, a general reliability model with a 99% adequacy value (R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) was constructed as a function of testing temperature and stress amplitude.

Crowbar Loading-A New Test Technique to Characterize Interfacial Delamination

This paper presents a new test method called Crowbar Loading (CBL) to characterize interfaces between dissimilar materials. In this test, a cylindrical roller is sandwiched in the delaminating interfaces and acts as a fulcrum to facilitate crack propagation. Although the externally applied load is compressive on one side of the fulcrum, the fulcrum translates the applied external load into an opening-mode load at the crack tip. By varying the location of the roller, this work shows that a wide range of loads can be altered at the loading point. Numerical models were developed to determine the critical crack length from the experimentally measured compliance of the test sample and, thus to determine the critical strain energy release rate (Gc). The crack length, determined using compliance, is also validated through optical measurements. Copper-epoxy mold compound (Cu/EMC) bimaterial samples, commonly seen in microelectronic packages, were successfully characterized using the developed CBL test method. The results from the CBL test have been compared against the double-cantilever beam (DCB) test results.

Low-Temperature Curable Negative-Tone Photosensitive Polyimides: Structure and Properties.

Low-temperature curable negative-tone photosensitive polyimide (n-LTPI) viscous solutions were prepared by dissolving photo-crosslinkable poly (amic ester) (pc-PAE) resin, photophotocrosslinker, photoinitiator, and the heteroaromatic base as curing catalysts, and other additives in organic solvents. Among them, the pc-PAE resin was synthesized by polycondensation of aromatic diacid chloride and diester of 2-ethoxymathacrylate, aromatic diamines in aprotic solvents. After being spun-coated on a silicon wafer surface, soft-baked, exposed to UV light, and developed, the n-LTPI with 2% of imidazole (IMZ) as a curing catalyst produced high-quality photo-patterns with line via resolution of 5 μm at 5 μm film thickness. The photo-patterned polymer films thermally cured at 230 °C/2 h in nitrogen showed 100% of the imidization degree (ID) determined by in situ FT-IR spectroscopy. The thermally cured polymer films exhibited great combined mechanical and thermal properties, including mechanical properties with tensile strength of as high as 189.0 MPa, tensile modulus of 3.7 GP, and elongation at breakage of 59.2%, as well as glass transition temperature of 282.0 °C, showing great potential in advanced microelectronic packaging applications.