Modelling of the reliability of flip chip lead-free solder joints at high-temperature excursions

At high-temperature operations of electronic control devices, Tin-Silver-Copper (SnAgCu) alloy solder joints used to assemble the component of the devices are functioning at homologous temperature above 0.8. In such ambient temperatures, solder alloys have limited mechanical strength and will be sensitive to strain rate. The sensitivity of solder properties to creep/visco-plastic deformation increases the rate of accumulation of plastic damage in the alloy and decreases the number of cycles to failure (Nf) of the joints. Most untimely rupture of solder joints in high-temperature electronics (HTE) system usually culminates in colossal loss of resources and lives. Typical incidences are reported in recent automotive and aircraft crashes as well as the collapse of oil-well logging equipment. To increase the mean time to failure (MTTF) of solder joints in HTE, an in-depth understanding and accurate prediction of the response of solder joints to thermally induced plastic strain damage is crucial. This study concerns the prediction of the reliability of lead-free solder joints in a flip chip (FC) model FC48D6.3C457 which is mounted on a substrate and the assembly subjected to high-temperature excursions. The research investigates the effect of the high-temperature operations on reliability of the joints. In addition, the investigation examines the impact of control factors (component stand-off height (CSH), inter-metallic compound (IMC) thickness, number of thermal cycle and solder volume) on Nf of the joints. A model developed in the course of this investigation was employed to create the assembly solder joints architecture. The development of the model and creation of the bump profile involve a combination of both analytical and construction methods. The assembled package on a printed circuit board (PCB) was subjected to accelerated temperature cycle (ATC) employing IEC standard 60749-25 in parts. The cycled temperature range is between -38 oC and 157 oC. Deformation behaviour of SnAgCu alloy solder in the joints is captured using Anand’s visco-plasticity model and the response of other materials in the assembly were simulated with appropriate model. The results demonstrate that the reliability of solder joints operating at elevated temperatures is dependent on CSH, thickness of IMC and solder volume. It also shows that incorporating the IMC layer in the geometric models significantly improves the level of accuracy of fatigue life prediction to ± 22.5% (from the ± 25% which is currently generally accepted). The findings also illustrate that the magnitude of the predicted damage and fatigue life are functions of the number of ATC employed. The extensive set of results from the modelling study has demonstrated the need for incorporating the IMC layer in the geometric model to ensure greater accuracy in the prediction of solder joint service life. The technique developed for incorporating the IMC layer will be of value to R&D engineers and scientists engaged in high-temperature applications in the automotive, aerospace and oil-well logging sectors. The results have been disseminated through peer reviewed journals and also presentations at international conferences

End-of-Life and Constant Rate Reliability Modeling for Semiconductor Packages Using Knowledge-Based Test Approaches

End-of-life and constant rate reliability modeling for semiconductor packages are the focuses of this dissertation. Knowledge-based testing approaches are applied and the test-to-failure approach is approved to be a reliable approach. First of all, the end-of-life AF models for solder joint reliability are studied. The research results show using one universal AF model for all packages is flawed approach. An assessment matrix is generated to guide the application of AF models. The AF models chosen should be either assessed based on available data or validated through accelerated stress tests. A common model can be applied if the packages have similar structures and materials. The studies show that different AF models will be required for SnPb solder joints and SAC lead-free solder joints. Second, solder bumps under power cycling conditions are found to follow constant rate reliability models due to variations of the operating conditions. Case studies demonstrate that a constant rate reliability model is appropriate to describe non solder joint related semiconductor package failures as well. Third, the dissertation describes the rate models using Chi-square approach cannot correlate well with the expected failure mechanisms in field applications. The estimation of the upper bound using a Chi-square value from zero failure is flawed. The dissertation emphasizes that the failure data is required for the failure rate estimation. A simple but tighter approach is proposed and provides much tighter bounds in comparison of other approaches available. Last, the reliability of solder bumps in flip chip packages under power cycling conditions is studied. The bump materials and underfill materials will significantly influence the reliability of the solder bumps. A set of comparable bump materials and the underfill materials will dramatically improve the end-of-life solder bumps under power cycling loads, and bump materials are one of the most significant factors. Comparing to the field failure data obtained, the end-of-life model does not predict the failures in the field, which is more close to an approximately constant failure rate. In addition, the studies find an improper underfill material could change the failure location from solder bump cracking to ILD cracking or BGA solder joint failures

Thermomechanical fatigue failure of interfaces in lead-free solders

The European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) banned lead from electronic systems from July 1, 2006 onwards, which has led to much interest in leadfree solders in the past years. Among several lead-free solder alternatives, SnAgCu is a widely accepted replacement due to its better creep-fatigue resistance and microstructural stability. SnAgCu has been extensively studied in the past decade, however, there are still issues to be resolved concerning solder reliability, the underlying mechanisms of thermo-mechanical fatigue failure, fatigue life predictions and the overall effect of decreasing component size, driven by the ongoing miniaturization trend. This thesis aims to scientifically contribute to this subject by a coupled experimental-numerical approach. In solder joint reliability, the bump/pad interface has a crucial role, the quality of which is determined by the metallization and interfacial defects. Solder balls, solder paste and cast eutectic SnAgCu are reflowed on Cu, Ni/Au and Cu/Ni(V)/Au metallization layers and the substrate influence on the bulk and interfacial metallurgy is examined. The damage propagation at SnAgCu soldered joints on Cu and Ni/Au substrates are investigated and microstructure related damage localization is identified as the dominant failure mechanism. Therefore, continuum damage approaches are believed to be inadequate for solder joint reliability predictions. Nano-indentation and tensile testing is used for the mechanical characterization of SnAgCu. An assessment on indentation parameters for solders is conducted and the influence of the Ag content on material properties of SnAgCu is presented. One of the main causes of ball grid array (BGA) failure is thermo-mechanical fatigue crack propagation in the solder, which is almost always observed at the bump/pad junction. Motivated by this fact, a combined experimental-numerical study on the cyclic mechanical response of SnAgCu/Ni-Au interface is conducted. In this study, damage evolution at the bond/pad interface is characterized by dedicated fatigue tests. Local deformations leading to crack propagation are simulated by separation of interfaces through a cohesive zone approach. Solder joints are tested under cyclic shear and cyclic tension for different specimen sizes and strain amplitudes. Two different damagemechanisms are observed: local deformations in the bulk and at the bonding interface. The interfacial failure mode is typically favored at a high initial stress, and a small solder volume. Crack propagation is simulated by an irreversible linear traction-separation cohesive zone law accompanied by a non-linear interfacial damage parameter. Later, tensile and shear experiments are used to characterize the cohesive zone parameters for the normal and the tangential opening, respectively. Interfacial fatigue damage in BGA solders is caused by the difference in coefficient of thermal expansion (CTE) of the materials in the package. Apart from this thermal incompatibility in the package, Sn based solders are themselves prone to thermal fatigue damage due to the intrinsic thermal anisotropy of the ß-Sn phase. Thermal fatigue causes local deformations especially at the grain boundaries. Hence, the thermal fatigue response of bulk SnAgCu is investigated as well. Bulk SnAgCu specimens are thermally cycled between -40 and 125¿C and mechanically tested afterwards in order to quantify the thermal fatigue damage. A size dependent cyclic softening behavior is observed. Test specimens are individually modeled including the microstructure and local crystallographic orientations, on the basis of orientation imaging scans (OIM). Both thermal cycling and tensile testing are imposed as boundary conditions. Reproducing the experimental results in the simulations, parameters of a cohesive zone based intergranular fatigue damagemodel are identified. Finally, the intergranular damage law characterized in this study is combined with the bump/pad interfacial damage law, and a 2Dmicrostructure-incorporated fatigue life prediction tool is established. Using this tool, it is shown that the failure mode of a soldered joint depends extensively on its geometry. The model presented above is extended to 3D for a more complete description of the problem. To provide the microstructural input, a database containing OIM scans of several SnAgCu solder balls is constructed. A missing constituent in the model so far, interfacial defects, i.e. voids, are examined statistically using newly manufactured BGA packages, revealing information on their size, position and frequency. Combining all the data collected, i.e. material properties, microstructure, defects, local damage laws, a 3D slice model from a BGA package is constructed. The slice model contains a single solder ball connecting the board and the chip. A series of case studies is created using experimental input such as different microstructures and initial defects allowing a statistical analysis. Fatigue life of these models are predicted and the results are validated by failure distribution analyses of BGA packages provided by the industry. Here the critical solder ball assumption is made: if a solder ball fails, the electrical circuit of the BGA package is open, thus the package fails. Setting a critical damage value for the interfaces accumulating fatigue damage, a good agreement with the experiments and simulations is obtained. It is seen that microstructural modeling allows to predict and understand the scatter in the solder ball fatigue life observed in reality. Finally, the effect of solder ball size and geometry on interconnect reliability is dis cussed on the basis of numerical analyses. For this purpose, a geometry factor and a microstructure factor is defined, and their influence on damage evolution is discusse

Thermomechanical fatigue failure of interfaces in lead-free solders

The European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) banned lead from electronic systems from July 1, 2006 onwards, which has led to much interest in leadfree solders in the past years. Among several lead-free solder alternatives, SnAgCu is a widely accepted replacement due to its better creep-fatigue resistance and microstructural stability. SnAgCu has been extensively studied in the past decade, however, there are still issues to be resolved concerning solder reliability, the underlying mechanisms of thermo-mechanical fatigue failure, fatigue life predictions and the overall effect of decreasing component size, driven by the ongoing miniaturization trend. This thesis aims to scientifically contribute to this subject by a coupled experimental-numerical approach. In solder joint reliability, the bump/pad interface has a crucial role, the quality of which is determined by the metallization and interfacial defects. Solder balls, solder paste and cast eutectic SnAgCu are reflowed on Cu, Ni/Au and Cu/Ni(V)/Au metallization layers and the substrate influence on the bulk and interfacial metallurgy is examined. The damage propagation at SnAgCu soldered joints on Cu and Ni/Au substrates are investigated and microstructure related damage localization is identified as the dominant failure mechanism. Therefore, continuum damage approaches are believed to be inadequate for solder joint reliability predictions. Nano-indentation and tensile testing is used for the mechanical characterization of SnAgCu. An assessment on indentation parameters for solders is conducted and the influence of the Ag content on material properties of SnAgCu is presented. One of the main causes of ball grid array (BGA) failure is thermo-mechanical fatigue crack propagation in the solder, which is almost always observed at the bump/pad junction. Motivated by this fact, a combined experimental-numerical study on the cyclic mechanical response of SnAgCu/Ni-Au interface is conducted. In this study, damage evolution at the bond/pad interface is characterized by dedicated fatigue tests. Local deformations leading to crack propagation are simulated by separation of interfaces through a cohesive zone approach. Solder joints are tested under cyclic shear and cyclic tension for different specimen sizes and strain amplitudes. Two different damagemechanisms are observed: local deformations in the bulk and at the bonding interface. The interfacial failure mode is typically favored at a high initial stress, and a small solder volume. Crack propagation is simulated by an irreversible linear traction-separation cohesive zone law accompanied by a non-linear interfacial damage parameter. Later, tensile and shear experiments are used to characterize the cohesive zone parameters for the normal and the tangential opening, respectively. Interfacial fatigue damage in BGA solders is caused by the difference in coefficient of thermal expansion (CTE) of the materials in the package. Apart from this thermal incompatibility in the package, Sn based solders are themselves prone to thermal fatigue damage due to the intrinsic thermal anisotropy of the ß-Sn phase. Thermal fatigue causes local deformations especially at the grain boundaries. Hence, the thermal fatigue response of bulk SnAgCu is investigated as well. Bulk SnAgCu specimens are thermally cycled between -40 and 125¿C and mechanically tested afterwards in order to quantify the thermal fatigue damage. A size dependent cyclic softening behavior is observed. Test specimens are individually modeled including the microstructure and local crystallographic orientations, on the basis of orientation imaging scans (OIM). Both thermal cycling and tensile testing are imposed as boundary conditions. Reproducing the experimental results in the simulations, parameters of a cohesive zone based intergranular fatigue damagemodel are identified. Finally, the intergranular damage law characterized in this study is combined with the bump/pad interfacial damage law, and a 2Dmicrostructure-incorporated fatigue life prediction tool is established. Using this tool, it is shown that the failure mode of a soldered joint depends extensively on its geometry. The model presented above is extended to 3D for a more complete description of the problem. To provide the microstructural input, a database containing OIM scans of several SnAgCu solder balls is constructed. A missing constituent in the model so far, interfacial defects, i.e. voids, are examined statistically using newly manufactured BGA packages, revealing information on their size, position and frequency. Combining all the data collected, i.e. material properties, microstructure, defects, local damage laws, a 3D slice model from a BGA package is constructed. The slice model contains a single solder ball connecting the board and the chip. A series of case studies is created using experimental input such as different microstructures and initial defects allowing a statistical analysis. Fatigue life of these models are predicted and the results are validated by failure distribution analyses of BGA packages provided by the industry. Here the critical solder ball assumption is made: if a solder ball fails, the electrical circuit of the BGA package is open, thus the package fails. Setting a critical damage value for the interfaces accumulating fatigue damage, a good agreement with the experiments and simulations is obtained. It is seen that microstructural modeling allows to predict and understand the scatter in the solder ball fatigue life observed in reality. Finally, the effect of solder ball size and geometry on interconnect reliability is dis cussed on the basis of numerical analyses. For this purpose, a geometry factor and a microstructure factor is defined, and their influence on damage evolution is discusse

Peripheral soldering of flip chip joints on passive RFID tags

Flip chip is the main component of a RFID tag. It is used in billions each year in electronic packaging industries because of its small size, high performance and reliability as well as low cost. They are used in microprocessors, cell phones, watches and automobiles. RFID tags are applied to or incorporated into a product, animal, or person for identification and tracking using radio waves. Some tags can be read from several meters away or even beyond the line of sight of the reader. Passive RFID tags are the most common type in use that employ external power source to transmit signals. Joining chips by laser beam welding have wide advantages over other methods of joining, but they are seen limited to transparent substrates. However, connecting solder bumps with anisotropic conductive adhesives (ACA) produces majority of the joints. A high percentage of them fail in couple of months, particularly when exposed to vibration. In the present work, failure of RFID tags under dynamic loading or vibration was studied; as it was identified as one of the key issue to explore. Earlier investigators focused more on joining chip to the bump, but less on its assembly, i.e., attaching to the substrate. Either of the joints, between chip and bump or between antenna and bump can fail. However, the latter is more vulnerable to failure. Antenna is attached to substrate, relatively fixed when subjected to oscillation. It is the flip chip not the antenna moves during vibration. So, the joint with antenna suffers higher stresses. In addition to this, the strength of the bonding agent i.e., ACA also much smaller compared to the metallic bond at the other end of the bump. Natural frequency of RFID tags was calculated both analytically and numerically, found to be in kilohertz range, high enough to cause resonance. Experimental investigations were also carried out to determine the same. However, the test results for frequency were seen to be in hundred hertz range, common to some applications. It was recognized that the adhesive material, commonly used for joining chips, was primarily accountable for their failures. Since components to which the RFID tags are attached to experience low frequency vibration, chip joints fail as they face resonance during oscillation. Adhesives having much lower modulus than metals are used for attaching bumps to the substrate antennas, and thus mostly responsible for this reduction in natural frequency. Poor adhesive bonding strength at the interface and possible rise in temperature were attributed to failures under vibration. In order to overcome the early failure of RFID tag joints, Peripheral Soldering, an alternative chip joining method was devised. Peripheral Soldering would replace the traditional adhesive joining by bonding the peripheral surface of the bump to the substrate antenna. Instead of joining solder bump directly to the antenna, holes are to be drilled through antenna and substrate. S-bond material, a less familiar but more compatible with aluminum and copper, would be poured in liquid form through the holes on the chip pad. However, substrates compatible to high temperature are to be used; otherwise temperature control would be necessary to avoid damage to substrate. This S-bond would form metallic joints between chip and antenna. Having higher strength and better adhesion property, S-bond material provides better bonding capability. The strength of a chip joined by Peripheral Soldering was determined by analytical, numerical and experimental studies. Strength results were then compared to those of ACA. For a pad size of 60 micron on a 0.5 mm square chip, the new chip joints with Sbond provide an average strength of 0.233N analytically. Numerical results using finite element analysis in ANSYS 11.0 were about 1% less than the closed form solutions. Whereas, ACA connected joints show the maximum strength of 0.113N analytically and 0.1N numerically. Both the estimates indicate Peripheral Soldering is more than twice stronger than adhesive joints. Experimental investigation was carried out to find the strength attained with S-bond by joining similar surfaces as those of chip pad and antenna, but in larger scale due to limitation in facilities. Results obtained were moderated to incorporate the effect of size. Findings authenticate earlier predictions of superior strengths with S-bond. A comparison with ACA strength, extracted from previous investigations, further indicates that S-bond joints are more than 10 times stronger. Having higher bonding strength than in ACA joints, Peripheral Soldering would provide better reliability of the chip connections, i.e., RFID tags. The benefits attained would pay off complexities involved in tweaking

MODELING THE PHYSICS OF FAILURE FOR ELECTRONIC PACKAGING COMPONENTS SUBJECTED TO THERMAL AND MECHANICAL LOADING

This dissertation presents three separate studies that examined electronic components using numerical modeling approaches. The use of modeling techniques provided a deeper understanding of the physical phenomena that contribute to the formation of cracks inside ceramic capacitors, damage inside plated through holes, and to dynamic fracture of MEMS structures. The modeling yielded numerical substantiations for previously proposed theoretical explanations. Multi-Layer Ceramic Capacitors (MLCCs) mounted with stiffer lead-free solder have shown greater tolerance than tin-lead solder for single cycle board bending loads with low strain rates. In contrast, flexible terminations have greater tolerance than stiffer standard terminations under the same conditions. It has been proposed that residual stresses in the capacitor account for this disparity. These stresses have been attributed to the higher solidification temperature of lead free solders coupled with the CTE mismatch between the board and the capacitor ceramic. This research indicated that the higher solidification temperatures affected the residual stresses. Inaccuracies in predicting barrel failures of plated through holes are suspected to arise from neglecting the effects of the reflow process on the copper material. This research used thermo mechanical analysis (TMA) results to model the damage in the copper above the glass transition temperature (Tg) during reflow. Damage estimates from the hysteresis plots were used to improve failure predictions. Modeling was performed to examine the theory that brittle fracture in MEMS structures is not affected by strain rates. Numerical modeling was conducted to predict the probability of dynamic failure caused by shock loads. The models used a quasi-static global gravitational load to predict the probability of brittle fracture. The research presented in this dissertation explored drivers for failure mechanisms in flex cracking of capacitors, barrel failures in plated through holes, and dynamic fracture of MEMS. The studies used numerical modeling to provide new insights into underlying physical phenomena. In each case, theoretical explanations were examined where difficult geometries and complex material properties made it difficult or impossible to obtain direct measurements

Deposition and application of electroless Ni–W–P under bump metallisation for high temperature lead-free solder interconnects

A reliable and robust diffusion barrier, commonly known as under bump metallisation (UBM), is indispensable in solder interconnects in order to retard the interfacial reaction rate, hence the growth of intermetallic compounds (IMCs). However, electroless Ni-P coatings are not adequate to inhibit interfacial reactions effectively since the formation of columnar structure and voids in the crystalline Ni3P layer in hybrid automotive devices (operating temperature above 300ºC) can significantly deteriorate the mechanical integrity of solder joints. In this thesis, electroless Ni-W-P coatings, as an effective UBM capable to serving under high temperature (up to 450ºC), are developed, characterised and subsequently applied onto the high temperature lead-free solder interconnects. [Continues.

A Finite Element approach to understanding constitutive elasto-plastic, visco-plastic behaviour in lead free micro-electronic BGA structures

This work investigates the non-linear elasto-plastic and visco-plastic behaviour of lead free solder material and soldered joints. Specifically, Finite Element (FE) tools were used to better understand the deformations within Ball Grid Array solder joints (BGA), and numerical and analytical methods were developed to quantify the identified constituent deformations. FE material models were based on the same empirical constitutive models (elastic, plastic and creep) used in analytical calculations. The current work recognises the large number of factors influencing material behaviour which has led to a wide range of published material properties for near eutectic SnAgCu alloys. The work discovered that the deformation within the BGA was more complex than is generally assumed in the literature. It was shown that shear deformation of the solder ball could account for less than 5% of total measured displacement in BGA samples. Shear displacement and rotation of the solder balls relative to the substrate are sensitive to the substrate orthotropic properties and substrate geometry (relative to solder volume and array pattern). The FE modelling was used to derive orthotropic FR4 properties independently using published data. An elastic modulus for Sn3.8Ag0.7Cu was measured using homologous temperatures below 0.3. Suggested values of Abaqus-specific creep parameters m and f (not found in literature) for Sn3.8Ag0.7Cu have been validated with published data. Basic verification against simple analytical calculations has given a better understanding of the components of overall specimen displacement that is normally missing from empirical validation alone. A combined approach of numerical and analytical modelling of BGAs, and mechanical tests, is recommended to harmonise published work, exploit new material data and for more informed analysis of new configurationsEPSRC-funded PhD studentshi

Electrical termination techniques

A technical review of high reliability electrical terminations for electronic equipment was made. Seven techniques were selected from this review for further investigation, experimental work, and preliminary testing. From the preliminary test results, four techniques were selected for final testing and evaluation. These four were: (1) induction soldering, (2) wire wrap, (3) percussive arc welding, and (4) resistance welding. Of these four, induction soldering was selected as the best technique in terms of minimizing operator errors, controlling temperature and time, minimizing joint contamination, and ultimately producing a reliable, uniform, and reusable electrical termination