Thermo-mechanical Reliability Research Articles

Thermal Cycling Performance of Heterogeneous Solder Ball Joints with Sn-3.0Ag-0.5Cu and Sn-0.75Cu(-Ni,Bi) Solder Ball Composition for BGA Package

Sn-3.0Ag-0.5Cu (SAC305) is one of the most commonly used Pb-free solder composition nowadays. A minor alloying element addition can improve mechanical properties and thermal reliability of the solder joint. Minor amount of Bithmuth (Bi) or Nickel (Ni) addition can improve mechanical properties of solder by solid solution strengthening, or ensure the structural stability of the intermetalllic compounds. Also, low melting temperature solder with Bithmuth can reduce the cost of soldering process and keep components from deterioration. In this study, reliability of heterogeneous and homogeneous solder joint in the ball-grid-array (BGA) package solder joint were investigated. Using Sn-3.0Ag-0.5Cu (SAC305) solder paste and two kinds of solder balls, we have investigated which combination of solder joint improves the thermo-mechanical reliability. SAC305 and SCN (Sn-0.75Cu-Ni,Bi) balls were used for homogeneous and hetrogeneous joints. Solder balls were attached in BGA package. To examine the solder joint reliability, thermal cycling test was conducted after reflow soldering. Shear strength measurement and cross-sectional analysis of the BGA package solder joints were carried out before and after 2000 cycles of the test. The shear strength of homogeneous solder joint decreased to 68.2% and heterogeneous decreased to 43.5%. The cross-sectional analysis using scanning electorn microscopy (SEM) and electorn back-scattered diffraction (EBSD) showed the (Ni,Cu)<sub>6</sub>Sn<sub>5</sub> and Cu<sub>6</sub>Sn<sub>5</sub> IMCs which were generated in the solder joint, and the crack propagated longer in the homogeneous solder joint of SAC305 after the thermal cycling test. IPFM showed the grain refinement effect of the SCN solder joint after thermal cycling. The minor addition of Bi had played a role as dislocation pinning site and became to equiaxied grain, so that the lifetime of heterogeneous SCN solder joint was superior than that of the SAC305.

Effect of Matte-Sn Electroplating Parameters on the Thermo-mechanical Reliability of Lead-free Solder Joints

Most of the Cu/Cu alloy lead-frames of electronic components used for automotive applications contain electroplated matte-Sn terminal finish to improve the wettability of Sn-based Pb-free solders during reflow soldering process. When the solder joints are subjected to combined thermal and mechanical cyclic loading, the influence of matte-Sn electroplating parameters can lead to early and brittle failure of the solder joint. To test this hypothesis, a factorial design of experiments (DOE) has been conducted with LFPAK-MOSFET (hereafter referred to as LFPAK) components plated with different matte-Sn electroplating parameters and reflow soldered with two solder alloys (SAC 305 and Innolot). The LFPAK solder joints were then subjected to thermo-mechanical in-phase cyclic loading under different strain amplitudes. No electrical measurement is done to eradicate the effect of electrical current on the solder joint. The response to the DOE is the crack percentage obtained in the LFPAK solder joints after 1000 and 2000 cycles. The Innolot solder joints exhibited lower crack percentages than the SAC 305. The level of organic additives in the electroplating process of matte-Sn influences the failure mode of the solder joint. Microstructural investigation attributes the nature of failure to the morphology of the (Cu,Ni)6Sn5 IMC phase that forms on the component side of the solder joint.

(Invited, Digital Presentation) 2-D Van Der Waal Materials for Advanced Interconnect Applications

Advanced interconnects require new materials to minimize high frequency signal loss and manage high temperature excursions due Joule heating. In this talk, we will review our on-going material selection modeling, and the thermo-mechanical reliability testing of interconnect prototypes that incorporate 2-D van der Waal thin films. Specifically, we will discuss our use of broadband microwave spectroscopy, that enables the accurate in-operando analysis of the electrical and magnetic properties without compromising the kinetic conditions of the experiment, to investigate high speed signal loss in 3-D integrated test circuits. The BDS method is sensitive to the actual electronic structure of the constituent materials and interfacial processes in the test structures. Using BDS, we have studied and characterized the 2-D van Der Wall, and other chalcogenide thin films in purposely built prototypical 3-D integrated circuit (3D-IC) during cycled high-temperature storage. We show that the microwave signal loss in these devices is attributable to the energy dissipation through the signal’s interactions with the copper barrier / cladding materials.

Thermomechanical Reliabilities of Pb-Free Solder Joints According to Ag Content in Harsh Environment

Pb-free solders based on Sn-Ag-Cu are currently applied to electronic devices. This article investigates the thermomechanical reliability of three different compositions of Pb-free solder joints, Sn-1.0Ag-0.5Cu, Sn-1.2Ag-0.5Cu, and Sn-3.0Ag-0.5Cu in a harsh environment. First, we perform a temperature cycling test (TCT) and a thermal shock test (TST) at temperatures from –55 to 125 °C. In addition, we conduct a combined thermal shock and vibration test (TSVT) using a HALT chamber, at –65 to 150 °C and an acceleration of up to (15 ± 1) Grms, considering the automotive underhood environment. We then analyze the mean lifetimes for each test of Pb-free solder joints dependent on the Ag contents. Finally, we present the lifetime ratios of three different Pb-free solders between TCT and TST, and show that SAC1205 has a similar mean lifetime to SAC305 within about 0.34% in combined TSVT. The results of this article could provide a reference for selecting suitable Pb-free solders in industrial use over harsh environments.

Unveiling High Performance Solder Alloys for Automotive Applications

Vehicle electrification and advances in autonomous driving technologies are creating unique opportunities and challenges in the automotive electronics assembly industry. The ever-increasing demand of reliability at higher temperatures and longer service life, especially for automotive electronics, is leading to the evolution of high reliability solder alloy design in the electronics assembly industry. The combination of harsh operating conditions, increasing power densities, and miniaturization has added to the complexity of assembly designs in the automotive electronics market for which traditional surface mount solders are no longer applicable. Thermo-mechanical reliability of solder alloys under harsh operating conditions has been a primary factor for defining suitability and selection of solder alloy for next generation designs. A next generation high reliability alloy for automotive electronics is introduced herein. The microstructure of this novel alloy consists of a well-distributed eutectic phase and finely distributed precipitates of intermetallic compounds which contributes towards solid solution strengthening as well as precipitation strengthening. It is shown that a high-temperature superior creep strength improvement of more than 100% over existing high reliability alloys is achieved at temperatures of 150°C. This and other mechanical properties of the bulk alloy are then correlated to performance with thermal cycling testing under extreme conditions. The novel alloy showed significantly higher thermal cycling performance, exhibiting more than 40% increase in the characteristic life over other existing high-reliability alloys while maintaining traditional surface mount target reflow temperatures. Considering these results, this new class of alloys has the capability to meet the increasingly challenging reliability requirements of automotive electronics.

Double-Side Cooled SiC MOSFET Power Modules With Sintered-Silver Interposers for a 100-kW/L Traction Inverter

Low-profile double-side cooled power modules are emerging in electric-drive inverters to achieve higher power density and efficiency. However, the rigid interconnection between the devices and two substrates raises the thermo-mechanical reliability issue of double-side cooled modules. In the prior work, we proposed the use of porous sintered-silver (Ag) interposers in the double-side cooled modules, resulting in reduced thermo-mechanical stresses, better heat extraction, and lower package parasitic inductance. To further improve the power density and demonstrate the benefits of using the sintered-Ag interposers, we proposed a revised half-bridge module layout that further reduced the power-loop parasitic inductance by 23%, improved the power handling capability by 44%, and retained a similar thermo-mechanical stress reduction. After carefully selecting high-temperature packaging materials, we fabricated the improved module layout with a reasonably high yield due to the deformable feature of sintered-Ag interposers. The thermal testing showed that the module had excellent power handling capability with a junction-to-fluid thermal resistance of 0.76°C/W due to the double-sided cooling. The electrical characterization at elevated temperatures up to 250°C and the switching characterization validated the functionalities of the power modules with SiC devices aimed for high-temperature and fast switching. Six of the double-side cooled power modules with sintered-Ag interposers were assembled into a segmented traction inverter, demonstrating a high power density of over 100 kW/L.

Development and thermo-mechanical reliability assessment of fiber reinforced polymers in lightweight PV modules towards vehicle-integrated photovoltaics

Weight reduction by omitting the use of bulky glass in c-Si photovoltaic (PV) modules is an important consideration of module development for vehicle-integrated photovoltaics (VIPV). Various approaches to achieve lightweight modules are proposed, yet there are many concerns regarding the reliability of such modules compared to standard glass-glass or glass-backsheet configurations. In this work, we investigate the thermo-mechanical behavior of LW modules with a multiwire design specifically aiming to VIPV applications. The developed modules consist of a commercially available carbon-fiber reinforced polypropylene backsheet and are compared to glass-fiber reinforced polypropylene backsheet modules. To enhance the thermo-mechanical reliability, polymer encapsulant and interconnection foil are substituted by glass-fiber reinforced composite encapsulant with the carbon-fiber reinforced polypropylene backsheet, thereby leading to ∼2.9% fill factor (FF) decrease with a limited degradation after 200 thermal cycles. A failure mechanism analysis using electroluminescence and X-ray-based micro-tomography is carried out after thermal cycling tests, clearly demonstrating that thermal stresses introduce deformation of wire interconnects. A modified high-temperature thermal cycling test (- 40 to 110 °C, 3.5 h) is implemented to observe the fast degradation of interconnects in compliance with VIPV conditions. The resulting fatigue stresses account for wire breakage in-between cells in the glass-fiber reinforced polypropylene module, while this effect is less pronounced in the carbon-fiber reinforced polypropylene backsheet module, indicating better thermo-mechanical reliability of the carbon-fiber reinforced polypropylene backsheet module. Herein, the current results could provide guidelines for lightweight PV module design (with a weight of 4.8 kg/m2) in the thermo-mechanical aspect. This research sheds light on the potential of lightweight modules specifically for VIPV applications.

Copper-Wire Stress Buffers for Extending Lifetime of Double-Sided Bidirectional SiC Modules

The mismatch of thermal expansion coefficients causes tremendous thermo-mechanical stress in die attachment of double-sided bi-directional modules (BMs), thus reducing the reliability of the module. The thermo-mechanical reliability of a double-sided power module is the most critical issue, which needs to be addressed. People proposed ways to improve the reliability by reducing the CTE mismatches using such as Moly spacer instead of copper or junction temperature rise using high conductive spacer. This work first proposed a composite buffer, namely copper-wire-spacer (CWS), with low elastic modulus, capable of reducing thermo-mechanical stress by >50%. The lifetime of the double-sided BM with the CWS buffer is 40.0% and 42.9% longer than that with the conventional solid copper buffer, and is only 6.67% and 6.25% shorter than that with the moly buffer under the thermal shocking tests and the power cycling tests, respectively. The novel method of reducing the elastic modulus of the spacer could be a novel and good concept to reduce the thermo-mechanical stresses as well as improving the reliability of double-sided power modules. It can provide new guidance to design and packaging reliable double-sided power modules.

Design Optimization by Virtual Prototyping Using Numerical Simulation to Ensure Thermomechanical Reliability in the Assembly and Interconnection of Electronic Assemblies

Abstract A methodology is presented that allows the evaluation of the thermomechanical reliability of electronic packages using “virtual prototyping.” Here, a virtual flip chip ball grid array (FC-BGA) is examined in comparison to a reference chip scale package (CSP). The comparison is performed using finite element simulation. A combined measurement-simulation technique is used to calibrate the finite element simulations on a reference object. The adjustment is based on the in-plane deformation field obtained by both simulation and optical measurement. For the latter, an optical sensor is used for in-plane deformation and strain field analysis based on the gray-scale correlation method. The findings obtained can be extrapolated to alternative package types with different but similar design to evaluate their suitability for the desired application before physical fabrication.

Thermo-Mechanical Reliability Study of Through Glass Vias in 3D Interconnection.

Three-dimensional (3D) interconnection technology based on glass through vias (TGVs) has been used to integrate passive devices, and optoelectronic devices due to its superior electrical qualities, outstanding mechanical stability, and lower cost. Nevertheless, the performance and reliability of the device will be impacted by the thermal stress brought on by the mismatch of the coefficient of thermal expansion among multi-material structures and the complicated structure of TGV. This paper focuses on thermal stress evolution in different geometric and material parameters and the development of a controlled method for filling polymers in TGV interconnected structures. In addition, a numerical study based on the finite element (FE) model has been conducted to analyze the stress distribution of the different thicknesses of TGV-Cu. Additionally, a TGV interconnected structure model with a polymer buffer layer is given to solve the crack problem appearing at the edge of RDL. Meanwhile, after practical verification, in comparison to the experimental results, the FE model was shown to be highly effective and accurate for predicting the evolution of stress, and several recommendations were made to alleviate stress-related reliability concerns. An improved manufacturing process flow for the TGV interconnected structure was proposed and verified as feasible to address the RDL crack issue based on the aforementioned research. It provides helpful information for the creation of highly reliable TGV connection structures.

Reliability Behavior of A Resin-Free Nanosilver Paste at Ultra-Low Temperature of 180°C

• Large-area (20 × 20 mm 2 ) bonding successfully achieved by a trimodal particle system • The resin-free trimodal paste could sintered under ultra-low temperature of 180°C • Excellent thermo-mechanical reliability was achieved by proposed resin-free paste In this paper, excellent thermo-mechanical reliability of resin-free silver sintering for large-area (20 × 20 mm 2 ) bonding was successfully achieved by using a trimodal particle system composed of nano-, submicron-, and micron-sized Ag particles. After 1000 cycles of the thermal shock (TS) test, the transient thermal impedance ( Z th ) increase of the proposed resin-free silver paste sintered at 180°C under 2 MPa and 5 MPa is only 5.8% and 6.1%, respectively. Due to the avoidance of resin degradation, the obtained resin-free silver paste as-sintered under 5 MPa exhibt outstanding shear strength (>12.5 MPa) even after 1000 cycles TS test. The obtained cross-sectional microstructure can confirms the excellent thermo-mechanical reliability of the sintered resin-free silver paste, which exhibits a denser and more homogeneous bonding layer with a porosity as low as 12.5%. Furthermore, distinct indications of plastic flow can be observed on the fracture surfaces of corresponding joints before and after aging, which further confirm the superiority of proposed resin-free sintering method. The development of novel resin-free silver paste successfully promotes the thermo-mechanical reliability of silicon carbide (SiC) power devices at a low processing temperature of 180°C, and greatly benefits the practical application of SiC power devices.

Investigation of Vehicle-Oriented Double-Sided Cooling Power Module With BGA Technology

With the increasing power density and integration of power electronics for motor drive applications, lower parasitic parameters, higher heat dissipation, thermomechanical reliability, and manufacturability have become the focused requirements of automotive power modules. New structures and materials have been investigated for the packaging of power modules, and wire-bondless packaging technology has become a research focus. In this article, a wire-bondless power module packaging technology based on ball-grid array (BGA) was developed, and a double-sided cooling (DSC) power module was then fabricated and thoroughly investigated. Thermal, electrical, and mechanical properties of the BGA packaged module were analyzed from the aspects of electrothermal coupling, shear performance, and electrical test. Specifically, by using the BGA interconnection, the thermal resistance was decreased by 27.4% compared to the wire-bond module. Parasitic inductance of BGA-based module was measured to be 10.13 nH by the dynamic test under the conditions of 310 V/120 A, which decreased by 33.3% compared to the wire-bond module counterpart. The findings of this work can help advance the design and fabrication of power modules, specifically DSC power modules.

Reliability and Lifetime Prediction Model of Sintered Silver Under High-Temperature Cycling

Although excellent reliability has been reported for sintered silver as a die-attach material under both thermal and power cycling loads in power electronics applications, the promise of this material as a large-area attachment at temperatures beyond 200 °C needs to be investigated. This article presents insights into the thermomechanical behavior and reliability of sintered silver under extreme thermal cycling conditions. In this study, we bonded sintered silver samples and subjected it to a thermal cycling profile of −40 °C to 200 °C with high ramp rates. We periodically monitored samples under thermal cycling to detect the presence of any failure mechanisms using a scanning acoustic microscope. We also included 95Pb5Sn solder in the study to obtain reference data. Results show the occurrence of cracks in sintered silver followed by a rapid rate of crack growth that exceeded the failure criterion in just 50 cycles. The predominant failure mechanism we observed was adhesive failure. As a large-area attachment, solder exhibited a higher reliability than sintered silver but failed within 100 cycles. Finally, we performed thermomechanical modeling to compute strain energy density values and correlated these with the experimentally observed crack growth rates to formulate a lifetime prediction model for sintered silver.

Reliable Aluminum Wire-Bonded SiC/Si Diodes With Laminated Al/Cu Stress Buffers

Operation temperature of typical wire-bonded power modules, especially for harder silicon carbide (SiC) power chips, keeps increasing. The thermo-mechanical reliability of aluminum (Al) wire-bonds which suffer from the mismatch of coefficient of thermal expansion (CTE) between chip and bonding wires reduces significantly. In this letter, a novel laminated Al/copper (Cu) soft stress buffer was proposed between SiC chip and bonding wires, capable of reducing thermo-mechanical stress of the wires and using soft Al wires with low ultrasonic bonding energy instead sophisticated Cu wires. Lifetime of Al wire bonding on the proposed 10- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m Al/40- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m Cu buffer was 5.14 times longer than that on a SiC chip directly under power cycling with constant Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> = 110 °C. In the end, a lifetime prediction model was established to guide the laminated buffer design for enhancing the reliability of Al wire-bonded Si diodes.

Viscoelastic Influence on the Board Level Assessment of Wafer Level Packages Under Drop Impact and Under Thermal Cycling

Abstract Structural components such as printed circuit boards (PCBs) are critical in the thermomechanical reliability assessment of electronic packages. Previous studies have shown that geometric parameters such as thickness and mechanical properties like elastic modulus of PCBs have a direct influence on the reliability of electronic packages. Elastic material properties of PCBs are commonly characterized using equipment such as tensile testers and used in computational studies. However, in certain applications viscoelastic material properties are important. Viscoelastic influence on materials is evident when one exceeds the glass transition temperature of materials. Operating conditions or manufacturing conditions such as lamination and soldering may expose components to temperatures that exceed the glass transition temperatures. Knowing the viscoelastic behavior of the different components of electronic packages is important in order to perform accurate reliability assessment and design components such as PCBs that will remain dimensionally stable after the manufacturing process. Previous researchers have used creep and stress relaxation test data to obtain the Prony series terms that represent the viscoelastic behavior and perform analysis. Others have used dynamic mechanical analysis in order to obtain frequency-domain master curves that were converted to time domain before obtaining the Prony series terms. In this paper, nonlinear solvers were used on frequency domain master curve results from dynamic mechanical analysis to obtain Prony series terms and perform finite element analysis on the impact of adding viscoelastic properties when performing reliability assessment. The computational study results were used to perform a comparative assessment to understand the impact of including viscoelastic behavior in reliability analysis under thermal cycling and drop testing for Wafer Level Chip Scale Packages.

Thermomechanical Reliability Investigation of Carbon Nanotube Off-Chip Interconnects for Electronic Packages

This article investigates the mechanical reliability benefits of carbon nanotube (CNT) off-chip interconnects for future electronic packages. Finite-element analysis (FEA) of a silicon chip attached to an organic substrate showed that CNT off-chip interconnects with an isotropic modulus of 1 MPa reduced the principal stresses in the silicon chip ~71%–93% compared with lead-free solder balls without underfill. In addition to reducing chip stresses, chip warpage was essentially eliminated by utilizing CNT interconnects compared with solder balls. Parameters of importance for the implementation of CNTs as interconnects such as CNT modulus and height were investigated with finite-element analysis. It was found that chip stresses increase with increasing CNT modulus, but for the modulus range reported in the literature and examined here (0.1–500 MPa), chip stresses were much less than packages utilizing solder balls. Due to the structure and morphology of CNT forests, the different material model types of fully isotropic versus transversely isotropic that can be used to model CNT interconnects were investigated. While the models yielded similar mechanical behavior in the finite-element analyses, the isotropic CNT model was mechanically conservative and simpler to implement and, therefore, sufficient for future thermomechanical reliability studies. To further demonstrate the thermomechanical reliability benefits, silicon chips with CNT off-chip interconnects were fabricated and assembled to organic substrates and subjected to cyclic thermal shock testing. During testing, the daisy chain circuit containing CNT interconnects survived 1660 cycles, demonstrating that CNT interconnects mechanically decouple the chip from the substrate and, therefore, reduce chip stresses caused by coefficient of thermal expansion (CTE) mismatch.

Investigation of Cu-Sn-Cu transient liquid phase bonding for microsystems packaging

ABSTRACT Fabrication, characterization, and thermomechanical reliability of electroplated Cu-Sn-Cu-based transient liquid phase (TLP) bonding required in microsystems packaging are reported. Die-level bonding was achieved using 8-μm-thick electroplated copper pillars covered with 2-μm tin. A mechanical test specimen having rectangular ring-shaped structures was fabricated to measure the strength of Cu-Sn joints. An electrical test vehicle having 250 copper pillars connected in a daisy chain series was designed. The effect of bonding force, temperature, and durations on the shear strength and electrical resistance of the Cu-Sn-Cu joints were investigated. The mechanism behind the improved bonding at higher force, temperature, and duration is reported. Samples bonded at a lower bonding force (5 kg) failed, while those bonded at a higher force showed good strength. A complete transformation of Sn into stable Cu3Sn was essential to achieve good bonding. Due to the presence of Cu3Sn IMC, the bonding strength of samples bonded at higher temperatures and longer duration was higher. Thermal shock and humidity tests were conducted to check the long-term reliability of Cu–Sn–Cu bonded joints. The percentage change in the electrical resistance of a complete daisy chain remained <5% even after 1000 thermal shock cycles.

Thermal Induced Interface Mechanical Response Analysis of SMT Lead-Free Solder Joint and Its Adaptive Optimization.

Surface mount technology (SMT) plays an important role in integrated circuits, but due to thermal stress alternation caused by temperature cycling, it tends to have thermo-mechanical reliability problems. At the same time, considering the environmental and health problems of lead (Pb)-based solders, the electronics industry has turned to lead-free solders, such as ternary alloy Sn-3Ag-0.5Cu (SAC305). As lead-free solders exhibit visco-plastic mechanical properties significantly affected by temperature, their thermo-mechanical reliability has received considerable attention. In this study, the interface delamination of an SMT solder joint using a SAC305 alloy under temperature cycling has been analyzed by the nonlinear finite element method. The results indicate that the highest contact pressure at the four corners of the termination/solder horizontal interface means that delamination is most likely to occur, followed by the y-direction side region of the solder/land interface and the top arc region of the termination/solder vertical interface. It should be noted that in order to keep the shape of the solder joint in the finite element model consistent with the actual situation after the reflow process, a minimum energy-based morphology evolution method has been incorporated into the established finite element model. Eventually, an Improved Efficient Global Optimization (IEGO) method was used to optimize the geometry of the SMT solder joint in order to reduce the contact pressure at critical points and critical regions. The optimization result shows that the contact pressure at the critical points and at the critical regions decreases significantly, which also means that the probability of thermal-induced delamination decreases.

Investigations on High-Power LEDs and Solder Interconnects in Automotive Application: Part I—Initial Characterization

Thermo-mechanical reliability is one major issue in solid-state lighting. Mismatches in the coefficients of thermal expansion (CTE) between high-power LED packages and substrates paired with temperature changes induce mechanical stress. This leads to a thermal degradation of LED modules by crack formation in the solder interconnect and/or delamination in the substrate, which in turn increases junction temperature and thus decreases light output and reduces lifetime. To investigate degradation and understand influence of LED package design and solder material, a reliability study with a total of 1800 samples − segmented in nine LED types and five solder pastes − is performed. First of all, in this paper a state-of-the-art review of high-power LED packages is performed by analyzing and categorizing the packaging technologies. Second, the quality inspection after assembly is realized by transient thermal analysis (TTA), scanning acoustic microscopy (SAM) and X-ray. For TTA, a new method is introduced to separate the thermal resistance of the LED package from solder interconnect and substrate by applying the transient dual interface method (TDI) on samples with different solder interconnect void ratios. Further measurement effort is not required. The datasheet values for thermal resistance are verified and the different LED package types are benchmarked. The void ratio of the solder interconnects is determined by X-ray inspection combined with an algorithm to suppress disruptive internal LED package structures. TTA and TDI revealed that initial thermal performance is independent of solder paste type and that voiding is more critical to smaller LED packages. In addition, lower silver proportion in the paste is found to increase voiding. SAM is less sensitive for initial void detection than X-ray, but it’s applied to monitor crack propagation while aging in combination with TTA. The results of the reliability study, i.e., the crack growth under temperature shock test for the different SAC solders, will be presented in a second independent paper.

A Reliable Way to Improve Electrochemical Migration (ECM) Resistance of Nanosilver Paste as a Bonding Material

Electrochemical migration (ECM) of sintered nano-Ag could be a serious reliability concern for power devices with high-density packaging. An anti-ECM nano-Ag-SiOx paste was proposed by doping 0.1wt% SiOx nanoparticles rather than previously used expensive noble metals, e.g., palladium. The ECM lifetime of the sintered nano-Ag-SiOx was 1.5 to 3 times longer than that of the sintered nano-Ag, due to the fact that the SiOx could protect the Ag from oxidation. The thermo-mechanical reliability of the sintered nano-Ag-SiOx was also improved by sintering under 5 MPa assisted pressure. The lesser porosity and smaller grain boundaries of the sintered nano-Ag-SiOx could also be beneficial to retard the silver ECM. In the end, a double-sided semiconductor device was demonstrated to validate the better resistance to the ECM using the sintered nano-Ag-SiOx.