Interface reliability and lifetime prediction of heavy aluminum wire bonds

In this study a high frequency mechanical fatigue testing procedure for evaluation of interfacial reliability of heavy wire bonds in power semiconductors is presented. A displacement controlled mechanical shear testing set-up working at a variable frequency of a few Hertz up to 10 kHz is used to assess the interfacial fatigue resistance of heavy Al wire bond in IGBT devices. In addition, power cyclic tests were conducted on IGBT modules for in-situ measurement of the temperature distribution in the devices and determination of the thermally induced displacements in the wire bond loops. Finite Element Analysis was conducted to calculate the correlation between the thermally and mechanically induced interfacial stresses in the wire bonds. These stress values were converted into equivalent junction temperature swings (ΔT_j) in the devices based on which lifetime curves at different testing frequencies were obtained. Comparison of the fatigue life curves obtained at mechanical testing frequencies of up to 200 Hz with the power cycling data related to the wire bond lift-off failure revealed a very good conformity in the ranges of 50 to 160 K. A lifetime prediction model for Al wire bonds in IGBT modules is suggested by which the loading cycles to failure can be obtained as a function of ΔT_j and the mechanical testing frequency. The proposed accelerated shear fatigue testing procedure can be applied for rapid assessment of a variety of interconnects with different geometries and material combinations. Decoupling of the concurrent failure mechanisms and separation of the thermal, mechanical and environmental stress factors allows a more focused and efficient investigation of the interfaces in the devices.     

Fatigue testing method for fine bond wires in an LQFP package

A mechanical testing setup was developed to study the fatigue response of fine thermo-sonic wire bond connection in low profile quad flat packages (LQFP). The testing set-up was designed to induce pre-defined multi-axial stresses in the wire bond loops of non-encapsulated packages in order to mimic their deformation behavior during the thermo-mechanical loading. Lifetime curves were obtained up to 1.0E7 loading cycles with fatigue failure occurring in the heat affected zone of the ball bond. The experimental fatigue data in combination with extended FEA provided the basis for a Coffin Manson lifetime model. The proposed fatigue testing procedure can be applied as a highly efficient method for evaluation of various wire bonded packages by using a limited number of test samples and simultaneous testing of several wire bonds.     

Fatigue life evaluation of anisotropic conductive adhesive film joints under mechanical and hygrothermal loads

The shear fatigue lives of Anisotropic Conductive Adhesive Film (ACF) joints were evaluated experimentally and theoretically under different testing conditions. The shear fatigue tests of ACF joints were performed with different loading amplitudes. It is found that the fatigue lives of ACF joints decrease with increasing loading amplitudes and Basquin’s equation is fit to predict the fatigue lives of ACF joints. Hygrothermal aging and thermal cycling tests were conducted to investigate the shear strength and lives of ACF joints. The results show that the shear strength and lives of ACF joints decrease with increasing hygrothermal aging time, however increase firstly and then decrease with increasing thermal cycling time. The fatigue life model considering aging damage is proposed and the predictions of the fatigue life agree with the experimental results at different aging time for ACF joints.     

Reliability evaluations for board-level chip-scale packages under coupled power and thermal cycling test conditions

To evaluate conjointly the effects of ambient temperature fluctuation and operation bias on the reliability of board-level electronic packages, a coupled power and thermal cycling test has been proposed. In this study, the sequential thermal–mechanical coupling analysis, which solves in turn the transient temperature field and subsequent thermomechanical deformations, is performed to investigate thermal characteristics along with fatigue reliability of board-level thin-profile fine-pitch ball grid array chip-scale packages under coupled power and thermal cycling test conditions. Effects of different power cycling durations are studied. A pure thermal cycling condition is also examined and compared. Numerical results indicate that, for the coupled power and thermal cycling test, a shorter power cycling duration in general leads to a shorter fatigue life. However, the temperature compensation effect elongates the fatigue life under certain power cycling durations.     

Thermal cycling analysis of flip-chip solder joint reliability

The reliability concern in flip-chip-on-board (FCOB) technology is the high thermal mismatch deformation between the silicon die and the printed circuit board that results in large solder joint stresses and strains causing fatigue failure. Accelerated thermal cycling (ATC) test is one of the reliability tests performed to evaluate the fatigue strength of the solder interconnects. Finite element analysis (FEA) was employed to simulate thermal cycling loading for solder joint reliability in electronic assemblies. This study investigates different methods of implementing thermal cycling analysis, namely using the "dwell creep" and "full creep" methods based on a phenomenological approach to modeling time independent plastic and time dependent creep deformations. There are significant differences between the "dwell creep" and "full creep" analysis results for the flip chip solder joint strain responses and the predicted fatigue life. Comparison was made with a rate dependent viscoplastic analysis approach. Investigations on thermal cycling analysis of the temperature range, (ΔT) effects on the predicted fatigue lives of solder joints are reported     

Simulating package behavior under power dissipation using uniformthermal loading

The thermomechanical behavior of electronic packages under power dissipation is simulated using uniform thermal loading. Two packages are studied, a large periphery leaded plastic quad flat pack (PQFP) package and a more compact plastic ball grid array (PBCA) package, both mounted on a printed circuit board (PCB). Experimentally verified linear elastic finite element models are used to find the displacements at the predicted failure location during power dissipation, and then during uniform thermal loading. The results for the two cases are then analyzed to find correlations between power dissipation levels and equivalent heating temperatures. One use of the results could be to replace power cycling fatigue tests with thermal cycling tests, For the packages studied, the results revealed that very little uniform heating is required to simulate the thermomechanical effects at the failure location resulting from power dissipation. Due to the prestrained state of the packages at room temperature, power dissipation decreases the expansion mismatch while increasing the thermal mismatch between package and PCB     

A Study on the Thermal Fatigue Behavior of Solder Joints Under Power Cycling Conditions

Failure mechanisms exposed by environmental accelerating testing methods such as thermal cycling or thermal shock test, may differ from those at service operating conditions. While the device is heated up or cooled down evenly on its external surface during environmental testing, real operating powered devices experience temperature gradients caused by internal local heating, components' different heat dissipation capability, and ambient temperature variation, etc. In this study, a power cycling technique is introduced to better approximate the field operating conditions so as to activate the field failure modes. Power cycling thermal fatigue test is performed with different ball grid array solder joints, that is, lead contained [Sn/37 Pb (SP)] and lead free [Sn/4.0Ag/0.5 Cu (SAC)], and the result is compared. In order to account for the thermal fatigue life behavior discrepancy for different solder joint composition, real time Moire interferometry is applied to measure the global/local thermo-mechanical behavior during power cycling excursion. Effective damage parameter, the total average shear strain, is extracted from the experiment and applied to account for the difference in fatigue life result of two different solders. In addition, amount of experimentally measured total average shear strain is mutually verified with finite element method analysis. It is clear that total average shear strain of a solder joint can be an effective damage parameter to predict thermo-mechanical fatigue life. A physical mechanism in terms of thermal material property of solder joints' is proposed to offer some thoughts to abnormal shear strain behavior that leads to discrepancies in fatigue life of two solders. An importance of power cycling testing method is emphasized for certain package designs.     

Impact of load pulse duration on power cycling lifetime of chip interconnection solder joints

Thermo-mechanical stress limits the useful life of power modules in the application. Active power cycling tests have been applied for more than three decades to investigate the degradation generated by thermo-mechanical stress in accelerated testing. Different lifetime models were proposed to extrapolate the lifetime from accelerated test results to application conditions. However, these lifetime models did not differentiate between the prominent failure mechanisms of Al wire bond degradation and solder fatigue in classical modules. By combining new, highly reliable interconnection technologies with classical technologies, those failure mechanisms can be investigated separately. In previous publications this concept of separation of failure modes was applied to study the impact of temperature swing and medium temperature on each failure mode in power cycling. In the present study, the impact of power pulse duration on the lifetime of chip solder and Al wire bonds is investigated. The results are another jigsaw piece for the goal of proposing a lifetime model for chip solder interconnections. The empirical data base is furthermore indispensable for the scaling and validation of physic-of-failure approaches in the process of lifetime modelling.     

Reliability of Cu wire bonds in microelectronic packages

In this study the thermo-mechanical response of 25 μm Cu wire bonds in an LQFP-EPad (Low Profile Quad Flat-Exposed Pad) package was investigated by numerical and experimental means. The aim was to develop a methodology for fast evaluation of the packages, with focus on wire bond fatigue, by combining finite element analysis (FEA) and mechanical fatigue testing. The investigations included the following steps: (i) simulation of the warpage induced displacements in the encapsulated LQFP-176-Epad package due to temperature changes, (ii) reproducing the thermally induced stresses in the wire bond loops in an unmolded (non-encapsulated) LQFP package using an accelerated multiaxial mechanical fatigue testing set-up under the displacement amplitudes determined in case (i) and determination of the loading cycles to failure (N_f), (iii) FEA of the experiments performed in (ii) based on the boundary conditions determined in (i) to calculate the states of stress and strain in the wire bonds subjected to multiaxial mechanical cyclic loading. Our investigations confirm that thermal and mechanical cyclic loading results in occurrence of high plastic strains at the heat affected zone (HAZ) above the nail-head, which may lead to fatigue failure of the wire bonds in the packages. The lifetime of wire bonds show a proportional relation between the location and angle of the wire bond to the direction of loading. The calculated accumulated plastic strain in the HAZ was correlated to the experimentally determined N_f values based on the volume weighted averaging (VWA) approached and presented in a lifetime diagram (∆ d - N_f) for reliability assessment of Cu wire bonds. The described accelerated test method could be used as a rapid qualification test for the determination of the lifetimes of wire bonds at different positions on the chip as well as for related improvements of package design.     

Isothermal bending fatigue response of solder joints in high power semiconductor test structures

Fatigue and cyclic delamination behavior of PbSnAg solders which are typically used as die attach material in power semiconductors was investigated. Isothermal bending fatigue tests were performed by using multilayered model test structures consisting of Si chips soldered on ceramic substrates and failure probability curves were obtained up to 1e8 loading cycles. The fatigue experiments were conducted by using an ultrasonic fatigue testing machine equipped with a three point bending set-up at a constant testing temperature of 80 °C. Detailed failure analysis of the fatigued samples revealed a dependency of the failure mode on the chemical composition of the high-Pb soft solders. The main failure modes included interfacial delamination of the Si-chip from the die attach, degradation due to crack propagation in the solder layer and in some cases partial fracture of the chip. Finally the feasibility of high frequency mechanical fatigue testing for screening and evaluation of solder joints in multilayered electronic systems is discussed.     

Design and Control of a Piezoelectric Driven Fatigue Testing System for Electronic Packaging Applications

Failure of solder joints for electronic packaging is an important issue for controlling the reliability of semiconductor devices. However, the complicated coupling between mechanical stressing and temperature and time dependent material properties makes it difficult to explore the fundamental failure control mechanisms using the existing accelerated thermal cycling methods. In addition, the testing speed is also severely restricted by the thermal time constant of the characterization system. In order to decouple the mechanical stress effect from other factors as the first step toward exploring the control mechanisms of failure, a piezoelectric-based fatigue characterization system is developed to replace the thermal cycling and provide fast and purely mechanical stressing cycles. A self-tuning based (STR) adaptive controller is also developed to provide accurate process control during experiments for compensating stiffness variation due to fatigue crack growth. It is found that this STR regulator is more robust than the traditional PID controller. The bandwidth of the system is approximately 70 Hz and is currently restricted by the equivalent time constant of the piezoelectric material. Nevertheless, this speed is sufficient for conducting a successful fatigue testing of solder joints. Finally, preliminary fatigue experiments have been performed on Sn₆₃Pb ₃₇ solders and the reduction of stiffness due to crack growth is clearly visible while the actuation performance is consistent and stable during the entire testing period. In the future, it is possible to operate in conjunction with a temperature control unit and a creep testing scheme to explore both the temperature and time dependent nature of solders in order to fully understand the failure control mechanisms of packaging     

Toward comprehensive reliability assessment of electronics by a combined loading approach

Many electronic applications, such as portable handheld devices or automotive electronics, experience various loadings during their common operation. Recent investigations have shown, however, that the interactions of the different load components can be highly significant. The commonly employed standardized single load tests neglect these interactions and, therefore, do not represent well enough the use environment loading conditions of many electronic devices. Thus, it has become clear that modifications to the reliability evaluation procedures are necessary. But before loading conditions can be combined in a meaningful manner, the failure mechanisms under single load environments and their possible interactions must be clarified. This paper makes a brief review to the reliability of electronic assemblies under different loading conditions from the perspective of failure modes and mechanisms. The failure modes and mechanisms under pure thermal cycling, power cycling, mechanical shock impact, or vibration conditions are discussed first. Thereafter, the interactions of the loading conditions, when they are combined consecutively or concurrently, are discussed.     

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

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

Reliability of Lead-Free Solder Interconnections in Thermal and Power Cycling Tests

Lead-free solder interconnection reliability of thin fine-pitch ball grid array (BGA) lead-free packages has been studied experimentally as well as with finite-element (FE) simulations. The reliability tests were composed of the thermal shock test, the local thermal cycling test (resistors embedded in the board around the package), and the power cycling test (heat generation in the die). A 3-D board-level finite-element analysis (FEA) with local models was carried out to estimate the reliability of the solder interconnections under various test conditions. Due to the transient nature of the local thermal cycling test and the power cycling test, a sequential thermal-structural coupling analysis was employed to simulate the transient temperature distribution as well as the mechanical responses. Darveaux's approach was used to predict the life time of the solder interconnections. Furthermore, the numerical results validated by the experimental results indicated that the diagonal solder interconnections beneath the die edge were the most critical ones of all the tests studied here. It has been found that the fatigue life in the power cycling test was much longer than that in the other two tests. Detailed discussions about the failure mechanism of solder interconnections as well as the microstructural observations of the primary cracks are reported in this paper.      

A fast test technique for life time estimation of ultrasonically welded Cu–Cu interconnects

In this research the quality of the interconnects of the ultrasonically welded Cu terminals to the Cu substrate in the IGBT-module has been investigated. An ultrasonic resonance fatigue system in combination with a laser Doppler vibrometer and a special specimen design was used for shear fatigue testing of these large ultrasonic Cu–Cu welds (about 0.5 cm²). Fatigue life curves up to 10⁹ loading cycles were obtained in a very short period of time. Using this technique it was possible to evaluate the fatigue strength of these interconnects for the first time. The microstructural features of the interconnects were characterized and their crack growth behaviour was studied. Fracture analysis of the fatigued specimen shows that failure occur due to the propagation of the crack beneath the welding interface into the copper substrate. Additionally performed finite element simulations offer an insight into the stress and strain concentrations during the mechanical fatigue tests. As this method is not restricted to the welding geometry, material joints with larger interconnects can be tested likewise. Thus this new technique can be used as a practical and valid fatigue testing method for evaluation of various interconnects.     

A Rapid Life-Prediction Approach for PBGA Solder Joints Under Combined Thermal Cycling and Vibration Loading Conditions

While some electronic products are routinely subjected to concurrent vibration and temperature cycle loading, the ability to accurately model and estimate life expectancy of hardware under such conditions still presents a unique challenge. For combined vibration and temperature cycling, one of the most likely causes of failure is the fatigue of solder interconnects. This paper presents an approach to predict solder joint life under combined thermal cycling and vibration loading conditions, by taking into account temperature effects and loading interactions. Combined loading experiment on a test vehicle populated with PBGA packages was used to demonstrate this approach.      

Chip-on board technology for low temperature environment. Part II: Thermomechanical stresses in encapsulated ball-wedge bond wires

Wire-bonded chip-on-board (CoB) multi chip modules consist of die and bond wires that are encapsulated to protect them from mechanical and chemical damage. This paper describes a rapid-assessment model for the prediction of thermomechanical strains developed in the encapsulated ball-wedge bond wires due to thermal expansions experienced during curing or subsequent environmental changes. The wire profile is modeled using a piece-wise continuous polynomial function (cubic spline) with appropriate boundary conditions at the two bond sites. Plastic deformation is ignored in the current analysis as a first-order approximation. Then a 2D Raleigh-Ritz (RR) model is developed to estimate the thermomechanical stresses in the bond wire due to temperature cycling in the presence of an encapsulant. The purpose of the model is to provide a rapid ranking of the thermomechanical robustness of different wire-bond design options. Results are validated by detailed 2D finite element analysis (FEA) and are compared to fatigue failure data available from thermal cycling tests.     

Application of coupled electro-thermal and physics-of-failure-based analysis to the design of accelerated life tests for power modules

In the reliability theme a central activity is to investigate, characterize and understand the contributory wear-out and overstress mechanisms to meet through-life reliability targets. For power modules, it is critical to understand the response of typical wear-out mechanisms, for example wire-bond lifting and solder degradation, to in-service environmental and load-induced thermal cycling. This paper presents the use of a reduced-order thermal model coupled with physics-of-failure-based life models to quantify the wear-out rates and life consumption for the dominant failure mechanisms under prospective in-service and qualification test conditions. When applied in the design of accelerated life and qualification tests it can be used to design tests that separate the failure mechanisms (e.g. wire-bond and substrate-solder) and provide predictions of conditions that yield a minimum elapsed test time. The combined approach provides a useful tool for reliability assessment and estimation of remaining useful life which can be used at the design stage or in-service. An example case study shows that it is possible to determine the actual power cycling frequency for which failure occurs in the shortest elapsed time. The results demonstrate that bond-wire degradation is the dominant failure mechanism for all power cycling conditions whereas substrate-solder failure dominates for externally applied (ambient or passive) thermal cycling.     

Endurance behavior of conductive yarns

Various companies are industrializing ‘photonic’ textiles for medical and architectural applications. Here we report reliability testing of photonic textiles based on woven textiles with integrated copper-based conductive yarns used to drive attached LEDs. These textiles were subjected to cyclic mechanical stress tests and the cycle life was analyzed in terms of fatigue. Results show that failure is due to wire fractures at the transition from the rigid component to the compliant textile. The results are in good agreement with Cu-fatigue data from literature. This shows that it is possible to estimate the lifetime of electronic textiles under use conditions by the mechanical fatigue of the conducting yarn material properties.     

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

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