Accelerated Temperature Cycling Research Articles

Reliability Investigations in Printed Electronic Assemblies

Aerosol-Jet Printing (AJP) technology is a popular additive manufacturing method for achieving complex net geometry with micron-level accuracy. This opens up new possibilities in fabrication of electronic assemblies in 3D curvilinear form factors. Therefore, it is vital to understand not only the performance of AJP printed electronics, but also their reliability under different kinds of life-cycle operational and environmental stresses. Electrochemical migration (ECM) under temperature-humidity-bias (THB) conditions can lead to dendritic growth, which can cause dielectric breakdown, leakage current, and unexpected short circuit. ECM propensity and life prediction models for THB conditions were studied for AJP printed coupons based on silver-nanoparticle (AgNP) inks. Cyclic thermo-mechanical fatigue was studied under accelerated temperature cycling testing and modeling. Failures were found to occur mostly at interconnect transition regions due to stress concentrations caused by sudden transitions in local architecture. Drop/Shock failures for extreme accelerations were studied using a high-G drop tower. Deformations were characterized using high-speed photography and transient finite element modeling was used to study high strain-rate failures. Acknowledgments: Jason Fleischer from the Laboratory for Physical Sciences, Harvey Tseng and Jian Yu from the Army Research Lab

Reliability testing of solder joints under combined cyclic thermal and bending load for automotive applications

The cost and time efficient reliability qualification process of solder joints in automotive electronic assemblies requires accelerated temperature cycling (ATC) testing. This work introduces a load-dependent reliability assessment method on board level for solder joints of multilayer ceramic capacitors (MLCC) under superimposed cyclic thermal and 3-point bending load. For this purpose, a testing setup is developed to enable statistical testing of solder connections under accelerated load scenarios. The condition of solder joints is monitored online by means of impedance measurement until failure.In this work, solder joint failure of a large amount of MLCC 1206 components is experimentally investigated, where the following lifetime influencing factors are studied: solder volume variation and MLCC 1206 components from different suppliers.The resulting lifetime of solder joints with volume variation reveals, that under the same loading conditions smaller solder joints show a significantly lower survival probability compared to solder joints with larger volumes. Furthermore, significant differences in solder joint lifetime between components from two different suppliers are observed.In order to derive a load-dependent failure criterion using impedance measurement data, finite element (FE) simulation is performed under consideration of test boundary conditions. In this way, printed circuit board (PCB) deformation is determined, taking into account both the thermally and mechanically induced strains. It is shown, that the relationship between calculated load-dependent PCB strain amplitudes and experimentally detected solder joint failures are described by a power law leading to component Woehler curves for different failure probabilities.

Multi-stress Cyclic Testing of Roebel Stator Bars for Hydropower

A multi-stress test rig was built to investigate the effect of many start-stops (load cycling) on hydropower generator insulation. To emulate real-life start-stops, stator bars were subjected to accelerated temperature cycling by circulating a 3.5 kA current and then cooling down with fans while high voltage (service voltage) was applied simultaneously to the insulation. A total of 250 load cycles were applied. Partial discharges (PD) were recorded on-line during load cycling, and after every 50 cycles, off-line dielectric loss and PD measurements were conducted. There was an apparent increase in dielectric losses after they had been subjected to a total of 250 load cycles. During load cycling, the PD level generally decreased with temperature, but there was a temporary increase in the PD activity during temperature rise and fall. This trend can be explained by different thermal expansion in copper and insulating material (epoxymica), suggesting that a stator bar that is load-cycled will be exposed to substantially higher PD levels than a stator bar under a uniform load and temperature, leading to further deterioration of the insulation quality.

Developed non-destructive verification methods for accelerated temperature cycling of power MOSFETs

This study deals with the non-destructive test methods applied in the industry for SiC power metal-oxide-semiconductor field-effect transistors (MOSFETs). Scanning acoustic tomography (SAT) is used for testing SiC power MOSFETs, which are widely used in high-power electronic devices and electric vehicles. In this paper, we propose an automatic calculation method through SAT image conversion. Based on the automatically calculated delamination area, the device failure was determined, and the exact failure mechanism was identified. Existing verification methods require a lot of skills. In addition to measuring the electrical characteristics of SiC MOSFETs, internal structure analysis using X-ray and pass/fail judgment through SAM were performed. However, the auto-counting program SAT method reduces unnecessary verification methods and simultaneously detects the process of degradation of the SiC MOSFETs and the cause of internal failure. An SAT image first goes through a program that optimizes image recognition by adjusting the contrast and threshold. These preparation steps play an important role in accurately recognizing the delamination area. These steps and the verification method proposed to evaluate the reliability of SiC power MOSFETs are detailed in this article. The degradation process of the device is presented and discussed by applying an accelerated temperature cycling test and high voltage at the device's gate terminal.

Statistical analysis of accelerated temperature cycling test based on Coffin-Manson model

This paper investigates the statistical analysis of grouped accelerated temperature cycling test data when the product lifetime follows a Weibull distribution. A log-linear acceleration equation is derived from the Coffin-Manson model. The problem is transformed to a constant-stress accelerated life test with grouped data and multiple acceleration variables. The Jeffreys prior and reference priors are derived. Maximum likelihood estimation and Bayesian estimation with objective priors are obtained by applying the technique of data augmentation. A simulation study shows that both of these two methods perform well when sample size is large, and the Bayesian method gives better performance under small sample sizes.

Improved Reliability and Mechanical Performance of Ag Microalloyed Sn58Bi Solder Alloys

Ag microalloyed Sn58Bi has been investigated in this study as a Pb-free solder candidate to be used in modern electronics industry in order to cope with the increasing demands for low temperature soldering. Microstructural and mechanical properties of the eutectic Sn58Bi and microalloyed Sn57.6Bi0.4Ag solder alloys were compared. With the addition of Ag microalloy, the tensile strength was improved, and this was attributed to a combination of microstructure refinement and an Ag3Sn precipitation hardening mechanism. However, ductility was slightly deteriorated due to the brittle nature of the Ag3Sn intermetallic compounds (IMCs). Additionally, a board level reliability study of Ag microalloyed Sn58Bi solder joints produced utilizing a surface-mount technology (SMT) process, were assessed under accelerated temperature cycling (ATC) conditions. Results revealed that microalloyed Sn57.6Bi0.4Ag had a higher characteristic lifetime with a narrower failure distribution. This enhanced reliability corresponds with improved bulk mechanical properties. It is postulated that Ag3Sn IMCs are located at the Sn–Bi phase boundaries and suppress the solder microstructure from coarsening during the temperature cycling, hereby extending the time to failure.

New Electronic Packaging Method for Potted Guidance Electronics to Sustain Temperature Cycling and Survive High-G Applications

Potted Guidance Electronics have been widely used in precision guided munitions. In the current generation of projectiles, soft potting materials have been sufficient to protect the electronics from the G-forces of gun launch at approximately 15 kG while sustaining uncontrolled extreme low temperature storage environments at different locations around the world. With on-going development of long-range precision guided munitions, stronger and hardened potting materials will be needed to survive gun-launch accelerations of 30 kG and higher. In the case of uncontrolled storage environments, the daily temperature fluctuations can act to dislodge/fail electronic components due to the coefficient of thermal expansion (CTE) mismatches between the potting materials and the electronic components. In this paper, a new protective layer method is presented, which consists of two tightly fitted preformed polymer layers, acting to mitigate the CTE mismatches, while only producing insignificant degradation of the supporting structure during extreme high-G projectile launch. The effectiveness of this new method is demonstrated by using finite element based modeling and simulation methods to examine a simplified potted electronics example. In the first step, the example was simulated with and without protective layers during an accelerated temperature cycling (−67 °F to 185 °F), and found that the protective layers were able to mitigate the CTE mismatch problem. During second step, the example compared dynamic responses between potted electronics with and without protective layers during a high-G, ∼15 kG, gun-launch simulations; results also showed that the degradation of the supporting structure introduced by protective layers during gun launch was insignificant.

Probabilistic methodology for reliability assessment of electronic packages

In the mechatronic devices, the finite element analyses are the most used method to determine time-dependent solder joint fatigue response under accelerated temperature cycling conditions, the deterministic analyses are the most used methods. However, the design variables show variability and randomness which will affect the lifetime prediction quality. This paper focuses on solder joint reliability in tape-based chip-scale packages(CSP) with the consideration of uncertainties in material parameters.

Accelerated temperature cycling induced strain and failure behaviour for BGA assemblies of third generation high Ag content Pb-free solder alloys

In severe operation environments where extreme thermal cycles and long dwells are common, such as the engine compartment of motor vehicles, solder alloys resistant to fatigue under thermomechanical stimuli are required. With this in mind, new third generation lead free alloys are being designed to enhance the toughness of interfacial intermetallic layer (IML), and solid solution strengthening is introduced to compensate for the strength loss due to Ag3Sn particle coarsening. In this paper, the reliability of ceramic Ball Grid Array (BGA) assemblies using third generation lead-free alloys under various strain levels and Accelerated Temperature Cycling (ATC) profiles, 0/100 °C and −40/125 °C, are presented. Failure analysis was conducted and cracking along the IML at the solder/package interface was determined to be the root failure mechanism. The ATC test results indicate that the induced fatigue strain on each ring of the BGAs is a function of maximum dwell temperature, alloy choice and temperature change (ΔT) within the thermal cycle, and this is noted to have a significant influence on characteristic lifetimes. Results correlate well with the Coffin-Manson model, and activation energies (EA) were determined to predict the reliability of the electronic on the premise of the same predominant failure mechanism.

Deposition of silver films on copper nanopowders by three-times electroless plating

In order to enhance the oxidation resistance of a copper substrate, silver films were deposited by electroless plating onto pure copper nanopowder. However, the efficiency of electroless plating for nanopowders is very low. Silver-coated copper nanopowders can be successfully achieved by three-times electroless silver plating on copper nanopowder. The microstructure and the chemical composition of the silver-coated copper nanopowders were systematically analyzed. The silver films have shown excellent oxidation resistance, according to the results of accelerated temperature cycling tests. The silver-coated copper nanopowders were characterized using scanning electron microscopy and X-ray diffraction. Following conclusions were obtained under the optimum condition that the reaction temperature was 50 °C, the dosage of EDTA was 4 g/L, hypophosphite (NaH2PO2) was 20.6 g/L. When the silver content reached 25 %, silver was homogeneously distributed around copper nanopowders.

Manufacturability and Reliability Screening of Lower Melting Point, Pb-Free Alloys Containing Bismuth

This paper explores the manufacturability and reliability of three Pb-free Bi-containing alloys in comparison with conventional SAC305 and SnPb assemblies. The first alloy included in the study is a Sn-based alloy with 3.4%Ag and 4.8%Bi, which showed promising results in the National Center for Manufacturing Sciences and German Joint projects. The other two alloy variations have reduced Ag content, with and without Cu. BGA and leaded components were assembled on medium-complexity test vehicles using these alloys, as well as SAC305 and SnPb as baseline alloys, for comparison. Test vehicles were manufactured using two board materials, 170°C glass transition temperature (Tg) and 155°C Tg, with three surface finishes: ENIG, ENEPIG, and OSP. The accelerated temperature cycling (ATC) testing was done at −55°C to 125°C with 30-min dwells and 10°C/min ramps, for 3,000 cycles. Detailed microstructure examination before and after ATC testing is described, as is failure analysis. All three experimental alloys showed excellent performance in harsh-environment thermal cycling. Vibration testing at two G-force test conditions with resistance failure monitoring was performed on the daisy-chained components. A detailed description of the technique for the vibration testing using 2 G and 5 G harmonic dwells is provided. The lowest failure rate found at both the 2 G and 5 G levels was for the Cu-containing alloy known as Violet. These results provide data for further statistical analysis leading to the choice of proper combinations of the solder alloys, board materials, and surface finishes for high-reliability applications.

BGA Board Level Reliability Analysis & Optimization

The solder joint reliability of semiconductor package interconnects to printed circuit boards is critical for product durability. A dominant failure mode is solder fatigue due to the CTE mismatch between the BGA component and PCB at thermal cycling. However, it is well known that other factors can impact fatigue behavior and time to failure such as solder joint geometry, die geometry, solder system, etc. Finite element modeling (FEM) and simulation can play an integral role in providing deeper insight into the impact of these package parameters on the overall assembly. However, a major challenge of accurately modeling these systems includes simulation of multiple length scales from the package, substrate, and solder joints. The FEM approach addressing these can lead to reduced cycle time, accurate simulation, and improved package performance. In this work, the finite element modeling and simulation procedure is demonstrated for a BGA package at accelerated temperature cycling conditions. At the component level, key details regarding the properties and constituents of the BGA package mold compound and substrate are established by coupling measured experimental warpage data and finite element modeling. Comparison of simulated & Thermoire measurements shows excellent agreement at the package level, with warpage correlation achieved over the entire temperature range. At the assembly level, the truncated sphere model is used to arrive at precise solder joint profiles for accurate representation to tie the package to the board. The combined validated package-level results and solder joint profiles are employed towards a subsequent thermo-mechanical analysis of the full BGA assembly. The entire simulation procedure is demonstrated for a BGA design, where inelastic creep and reliability test data are compared. High strain regions in the solder joint array are shown to compare closely with regions of failure from experimental reliability test data. The validated FEM model allows for extrapolating to similar package conditions allowing faster design cycle time and less time consuming experimental work.

Surface finish effect on reliability of SAC 305 soldered chip resistors

PurposeThe purpose of this paper is to assess the long‐term reliability of lead‐free Sn96.5Ag3.0Cu0.5 (SAC305) under accelerated temperature cycling (ATC) conditions. Test vehicles consisted of commercial 2512 ceramic chip resistors soldered to printed circuit boards (PCBs) using three different Pb‐free surface finishes: organic solderability preservative (OSP), immersion silver (IAg) and electroless nickel immersion gold (ENIG).Design/methodology/approachTwo populations of solder joints were monitored continuously during a thermal cycle of 0°C to 100°C with a ramp rate of 10°C/min and a 30 min dwell at the temperature extremes. One population was cycled to 2,500 cycles, the other population was cycled to 8,250 cycles. Failures were defined in accordance with the IPC‐9701A industry test guidelines and failure data are reported as characteristic life, η. Microstructural evolution was characterised using metallographic techniques and back‐scattered scanning electron microscopy (SEM). Fractography was performed on post‐cycled resistors to determine failure mechanisms.FindingsThe results showed that the lifetime of the chip resistors could be ranked as follows: OSP>ENIG>IAg after 8,250 thermal cycles, when>73 per cent of the population had failed. It was found that the relative performance of OSP finishes improved with longer periods of cycling. This was attributed to diffusion of copper into the bulk solder and which subsequently, over time, acted as a dispersion strengthening mechanism. The relatively poor performance of IAg finishes was attributed to void formation.Originality/valueThe paper provides new and valuable information to end‐users about the long‐term reliability of lead‐free solder on three commonly used lead‐free surface finishes. The performance of each surface finish is explained in terms of microstructural evolution and void formation.

Improvement of oxidation resistance of copper by atomic layer deposition

Al2O3 films were deposited by the atomic layer deposition (ALD) technique onto pure copper at temperatures in the range 100–200°C. The chemical composition, microstructure, and mechanic properties of the ALD-deposited Al2O3 films were systematically analyzed. The variations in the film characteristics with substrate temperature were observed. Oxidation trials revealed that 20-nm-thick Al2O3 films deposited at a substrate temperature as low as 100°C suppress oxidative attack on pure copper. The Al2O3 films also showed excellent durability of adhesion strength, according to predictions using the Coffin–Manson model based on the results of accelerated temperature cycling tests. These features indicate that ALD-deposited Al2O3 film is a very promising candidate to be a protective coating for pure copper.

Thermal Cycling Analysis of BGA Solder Joint Reliability

The solder joint reliability of semiconductor package interconnects is critical to product durability. A dominant failure mode is solder fatigue due to the CTE mismatch between BGA component and PCB at thermal cycling. It is well known that besides thermal expansion mismatch of component and board, the solder joint geometry has a great impact on fatigue behavior and time to failure. In this study, a combination of Surface Evolver and finite element analysis are use to predict the solder joint shapes for the assembly of medium pin count BGA's and to estimate the reliability at accelerated temperature cycling conditions. Results of Surface Evolver are compared with the assumption of a truncated sphere. The solder shape predictions are applied for a subsequent thermo-mechanical analysis of the BGA assembly. Inelastic creep deformation is evaluated for critical solder balls, and the Coffin-Manson relation is used to estimate the solder joint lifetime. The entire simulation procedure will be demonstrated for a product design study for high reliability automotive BGA's. A fractional factorial design is defined that considers solder sphere diameter and solder pad sizes on BGA substrate and on PCB side. Resulting creep values and lifetime estimates will be compared.

Effects of package geometry on thermal fatigue life and microstructure of Sn–Ag–Cu solder joint

Soldering experiments of chip scale package devices were carried out by means of diode laser soldering system with Sn–Ag–Cu solders. In addition, pull tests and a scanning electron microscope were used to analyse the effect of processing parameters on mechanical strength of solder joints. Viscoplastic finite element simulation was utilised to predict solder joint reliability for different package geometry under accelerated temperature cycling conditions. The results indicate that under the conditions of laser continuous scanning mode as well as the fixed soldering time, an optimal power and package geometry exists, while the optimal mechanical properties of microjoints are gained.

Thermal Fatigue and Failure Analysis of SnAgCu Solder Alloys With Minor Pb Additions

The thermal fatigue performances of Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98.5</sub> Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.0</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> (SAC105), Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">97.5</sub> Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.0</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> (SAC205), Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">96.5</sub> Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3.0</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> (SAC305) and Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">95.5</sub> Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4.0</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> (SAC405) solder alloys with Pb terminations were investigated by accelerated temperature cycling with and without thermal preconditioning. The performance of the SAC alloys was compared to eutectic SnPb and aged SAC alloys. The test vehicle consists of commercial 2512 ceramic chip resistors soldered to printed wiring boards using the different solder alloy compositions. The solder joints were monitored continuously during a thermal cycle of 0 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C -100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C with a ramp rate of 9 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C/min and a 30 min dwell between temperature extremes. Failures were defined in accordance with the IPC-9701 A industry test guidelines and failure data are reported as characteristic life η (number of cycles to 63.2% failure) from a two-parameter Weibull distribution. The microstructural evolution was characterized using metallographic techniques and back-scattered scanning electron microscopy. The findings show that the lifetime of the alloys can be ranked as follows: SAC 305 ~ SAC 405 >; SAC 205 >; SAC 105 >; SAC305 aged >; SAC 105 aged >; SnPb and to determine mechanisms of failure, electron microscopy analysis and fractography were performed on post-cycled components.

Phenomenological Study of the Effect of Microstructural Evolution on the Thermal Fatigue Resistance of Pb-Free Solder Joints

Unlike SnPb solders, the thermal fatigue reliability of the Sn-Ag-Cu (SAC) solders is believed to be influenced significantly by both the initial and evolving microstructures. This paper presents a phenomenological study of the relationship between the initial SAC solder joint microstructure, the evolving microstructure, and the thermal fatigue performance measured by accelerated temperature cycling (ATC). To reflect the board assemblies that are in field use, commercial surface mount components with multiple geometries and materials and from different package assemblers were joined to the board with different lead free SAC alloys. The initial microstructures of the board level solder joints were altered in a variety of ways including: 1) varying the solder joint cooling rate; 2) varying the number of solder reflow exposures; and 3) exposure to different isothermal temperature exposures. In all cases the solder joint microstructure was exposed to one or more of these treatments prior to exposure to temperature cycling. In addition, some of the test boards were exposed to different cycling dwell times to determine if the microstructural evolution that occurred during ATC testing effected the respective characteristic lifetimes of the joints. The microstructural evolution was tracked and characterized with optical metallography and scanning electron microscopy. These results could have practical implications in terms of limiting the ability to develop acceleration factors and effective strain-based models for predicting Pb-free solder joint life.

Application of Finite Element Method in Optimal Design of Flip-Chip Package

More and more solder joints in circuit boards and electronic products are changing to lead free solder, placing an emphasis on lead free solder joint reliability. Solder joint fatigue failure is a serious reliability concern in area array technologies. In this study, the effects of substrate materials on the solder joint thermal fatigue life were investigated by finite element model. Accelerated temperature cycling loading was imposed to evaluate the reliability of solder joints. The thermal strain/stress in solder joints of flip chip assemblies with different substrates was compared, and the fatigue life of solder joints were evaluated by Darveaux’s crack initiation and growth model. The results show the mechanisms of substrate flexibility on improving solder joint thermal fatigue.

Apparatus for accelerated temperature cycling.

Abstract An automatic control unit, activating electrically operated solenoid valves, is used to regulate the flow of “hot” and “cold” water from two thermostated baths to the jacket of a reaction vessel so that the temperature of a liquid sample within the vessel follows a highly reproducible programmed cycle. A comparison is made of the temperature hysteresis between 23–33° during a 16 min cycle, when the flow rates to the vessel are changed and when the valves are operated manually. The adjustment of temperature hysteresis to effect the accelerated storage testing of liquid preparations is described.