Search

This paper reports experimental observations showing that a current flow produces an effect of strengthening a solder joint against a shear load. This conclusion is found from a single-joint shear test conducted on Sn–1Ag–0.5Cu wt% (SAC 105) joints with in situ current stressing varied from 700 to 1400 A/cm2 at room temperature. To isolate the current effect from the Joule heat effect, the same type of tests were conducted at room temperature, 40, 50, and 80 °C, without current applied to the joint, thus mimicking the condition of steady-state temperature resulting from Joule heating. Comparative testing was also conducted after aging the samples at 150 °C for 200 h. These tests produced indications suggesting that the current flow causes the maximum shear load to increase, while the rise in temperature by the Joule heat effect results in the opposite effect. Experimentally, as much as a 9.5% increase in the maximum shear load was observed from the isothermally aged sample tested under a current density of 700 A/cm2, while an ~ 25% reduction was estimated to result from a temperature increase by Joule heating. The potential mechanism for these observations is discussed.

In this paper, a combination of in-situ chemical synthesis and electrolysis methods was applied for the preparation of zinc oxide/graphene oxide quasi core-shell nanoparticles. Pure zinc oxide was also prepared using the same method at different temperatures. 100 °C was selected for synthesizing the ZnO/GO quasi core-shell (zinc oxide as a core and few layers of graphene oxide as a shell). The particle size was 15 ± 2 nm in all samples. In both processes, oxygen was the main reason for the connection between the core and the shell. XRD patterns also confirmed the formation of ZnO (with regular wurtzite structure) and ZnO/GO quasi core-shell structures. Raman spectroscopy results confirmed ZnO formation at various temperatures, but little shift was observed for ZnO/GO quasi core-shell after conjugating. HRTEM, BET, PL and UV-Vis were able to define and confirm the formation of the quasi core-shell structure and the mechanism of connection at the interface. Photoluminescence spectroscopy showed a few numbers of PL peaks that were not related to the presence of graphene. These sub-peaks depend on the kind of synthesis and creation of defects (based on defects chemistry). The results indicated that graphene just changed the intensity of the PL peaks. Our data indicated that the rate of MB degradation for ZnO/GO quasi core-shell structures was less than that of the pure zinc oxide. After conjugating between ZnO and GO, two mechanisms are suggested. The first is the band structure modification in the boundary of the core and shell. The second is the diffusion of carbon atoms or functions as dopant in ZnO surface. This report addresses how band structure modification, defect chemistry; microstructural features can affect the photocatalytic and photoluminescence behavior in a specific core-shell structure.

ZnO/graphene oxide core-shell nanoparticles (ZGNPs) were prepared by via in-situ chemical synthesis and electrolysis. This report addresses the disagreement in the photocatalytic and photoluminescence behavior in this system. XRD, Raman, HRTEM, PL and UV–Vis were able to confirm ZnO nanoparticles and core-shell structures formation at various temperatures. A little shift was observed in Raman for ZGNPs after conjugating. PL showed a few peaks that were not related to the graphene, which depends on the synthesis route and defects. After conjugating between ZnO and Graphene, two mechanisms are suggested. The first is band structure modification in the boundary of core and shell. The second one is diffusion of functional group in GO as a dopant on ZnO surface.

Wirebonding with bare Cu and palladium-coated copper (PCC) wires have reemerged as mainstream packaging technologies. This paper investigates the failure mechanisms in the PCC wirebond packages under electromigration (EM) tests. First, the directionality of failures at high current density and temperature conditions is discussed. Results illustrated that the intermetallic compound (IMC) growth rate is different when the EM flux direction is in the opposite direction of the chemical flux. Energy dispersive X-Ray spectroscopy (EDX) results paved the way for a fundamental understanding of wirebonding failure. Intermetallic compounds of CuAl 2 and Cu 9 Al 4 are observed after interfacial focused ion beam (FIB) cuts are made to obtain clear images of the failure site through scanning electron microscopy (SEM) and EDX. The Pd coating relocates from the Cu wire surface towards the ball area during the initial bonding process. Therefore, a small concentration of Pd alloys with the bulk Cu within the wire, which is small enough to go unnoticed during the manufacturing process. However, the Pd concentration makes a large enough difference to reduce the contact area between the ball bond and the Al pad. This is due to the solid-solution hardening that causes the ball to undergo a greater amount of work hardening during the bonding process. The smaller contact area yields a greater current density and resistance, enhancing the EM effects in PCC wirebonds. Throughout the EM test, Pd atoms migrate toward the Al pad, creating a higher concentration(∼6at%) of Pd near the interface between the ball bond and the Al pad. Interestingly, Pd has a minor effect on the failure mechanism and crack propagation owing to its concentration and resultant increased hardness at the interface. These results can lead to future studies that may lead to improvements in PCC wirebonding reliability.

The concern about the reliability and need for the miniaturization has stimulated intense development efforts to yield new viable joint structures. One such development is to achieve the joint without a Cu pillar by instead integrating an embedding ball of Cu within the solder joint. With the ball occupying the joint volume, the use of solder is considerably reduced, preventing an IMC phase formation with the complete consumption of Sn in the solder. While this new joint structure has its benefits, there are also unknowns in terms of the failure mechanism induced by electromigration (EM). This paper addresses the concerns about the reliability and need for the miniaturization of microelectronic packages. We have carried out extensive studies of the solder joint with an embedded Cu ball in order to provide answers to a few key questions, and this paper reports a few outstanding findings. The samples used in our study are a microjoint with a ∼50um diameter Cu ball. The microstructure, specifically IMC growth at Cu-ball/solder interfaces, was characterized with its exposure to various thermal and EM loads. To our surprise, the resultant findings suggest that such a structure does not necessarily offer better resistance against EM-induced failure, nor does it offer much management against IMC growth. One notable finding is that EM failure occurs without the expected impedance because the EM flux is in fact intensified with the Cu ball in a limited amount of Sn. Also found is the possibility of an accelerated growth of IMC at the surface of the solder joint when the diffusion barrier coated on Cu ball is prevented IMC formation in center of the joint. The mechanism by which EM and the reaction-induced failure is accelerated by the ball, will be presented along with in-depth characterization results.

Electromigration (EM) induced failure is inevitable in wafer-level chip scale package microelectronic packages (WCSP), especially with the implementation of lead-free solders. Many factors contribute to EM failure such as joule heating and current crowding. EM can induce void formation, which can eventually lead to open-circuit failure. Due to its nature, EM is a critical failure concern to the microelectronic industry and can be influenced by current conditions. This study examines the failure mechanisms in solder joints implemented in WCSP packages in Direct Current (DC) and DC-pulse current conditions with varying Duty Factors (DF). DF represents the on-off time for DC to flow through the device under test (DUT). Further, a transient simulative study using finite element method (FEM) explores the failure mechanism and investigates the stress development with DC and DF conditions. Findings suggested that a lower duty factor yielded longer time to failure (TTF). Meanwhile, higher pulsed DC DF yielded a lower TTF than DC. This study aims to explain the failure mechanism with each DF. This study aims to explain this phenomenon and suggests the need for further exploration.

With increasing thermal loads in electronic packages, liquid cooling is a preferred option for superior cooling capability. Closed loop liquid cooling systems with microchannel cold plates have demonstrated superior thermal performance. The widespread adaptation of this technology, however, is subject to long term reliability concerns, especially caused by corrosion with a water-based coolant. Galvanic corrosion, which is caused by the difference in electrode potentials is observed to be the dominant failure mechanism.In this paper, the design and development of flow loops suitable for capturing the corrosion behavior within the loop is discussed. Careful consideration of all components within the loop and the rationales behind the choices are highlighted. The experimental methodology details various sensors and measurements required to assess corrosion development in the loop. Experimental results from the flow loop tests are presented and engineering recommendations are made for flow loop design, choice of working fluid and measurements to be made in-situ. Although there is a clear indication of corrosion progression based on the experimental data of fluid chemistry degradation, no significant impact is observed on thermal performance of the current microchannel cold plate design for the duration of the current test conditions.

This paper reports the corrosion mechanism active in microchannel cold plates used in a liquid cooling system and proposes a kinetic model describing the rate of corrosion-induced failure as a function of testing conditions. The corrosion failure mechanism investigated in this paper is galvanic corrosion because the cold plate is typically made of Cu and is assembled using brazing alloys and there exists a galvanic potential between the Cu and the brazed area. A series of experimental characterizations indicates that the brazed joint is subjected to galvanic attack when exposed to a coolant, a mixture of water and propylene glycol (PG), with a galvanic potential sufficient to dissolve the braze component with an accelerated rate. Various testing on the galvanic corrosion finds that the braze (a ternary alloy of Cu, Ag, and P) becomes an anode in the galvanic pair and loses the component element by the process of dissolution. This type of galvanic corrosion is found to exist even with corrosion inhibitors present in the coolant, necessitating the corrosion assessment methodology that can predict the rate of cold plate failure with the use of the "accelerated testing" and the prediction model. Our research leads to the development of the micro-galvanic cell testing as well as the zero resistance ammeter (ZRA) methodologies. Our investigation with these testing methodologies presents clear evidence showing that the galvanic corrosion is the most active and serious form of corrosion in the cold-plate with the galvanic pair exerting as high as ~0.3V anodic potential on the brazed joint. It is also found that the rate of corrosion can be further accelerated with temperature and the external potential purposely applied across the Cu and the braze. The resulting galvanic corrosion kinetics collected in the form of current may be used to predict the corrosion rate at use conditions as they are found to follow the form of an Arrhenius-type kinetics model with a consideration of the corrosion acceleration factor.

This paper concerns the electromigration (EM) failure mechanisms in solder interconnects under low frequency pulsed direct current (DC) conditions. In our study, the accelerated EM tests of Wafer-level Chip Scale Package (WCSP or WLCSP) samples are conducted under 4 different pulsed DC conditions: 0.1 Hz pulsed DC with duty factors (DFs) of 33%, 50%, 75%, and 100% (DC). The result of our testing suggests that there are at least two competing factors affecting the failure rate in an opposite manner under pulsed DC EM conditions. Specifically, when compared with the cumulative damage model (estimates the damage only during the on period), the failure kinetics is found to be more accelerated at high DF and decelerated at a lower DF. This conclusion is drawn from the observation that the EM failure rate shows an extremely nonlinear relationship with the DF and also that the failure occurs faster under a high DF than under solely DC conditions, which is not possible without a mechanism assisting the EM failure. Furthermore, it is found that there is a 2- stage resistance change before EM failure, an indication of the change in the dominant failure mechanism. The results may indicate that the pulsed DC effect on the EM failure mechanism is far more complex than anticipated with the possible involvement of a damage mechanism other than EM such as thermal fatigue.

This paper investigates the effects of the under bump metallization (UBM) on Electromigration (EM) reliability of SAC solder joints and presents evidence suggestive of hidden mechanisms controlling the EM failure. This conclusion is based on the observation that the EM resistance does not show a monotonic increase with UBM thickness, but rather decreases above a certain critical limit. Since a thicker UBM would provide a greater supply of Cu, our observation contradicts the conventional view on the role of UBM on prolonging EM failure. We initially ascribed it to the increased of EM-prone microstructures or weak- links at the SAC/UBM interface active in thicker UBM layers. This is considered to be due to a more even distribution of EM flux across the joint by reducing current crowding at the corners of the joint. However, switching the test configuration, reducing current crowding, yielded results that disagreed with our proposition. It is found that the impact of the current configuration, and thereby the level of current crowding, on EM resistance does not vary much with the UBM thickness. This defies initial prediction that there would be a smaller impact in samples with thicker UBMs when the current configuration is switched. These results suggest that the EM failure mechanism is affected by UBM in a more complicated manner. Considering hidden factors such as the change in thermal stress with UBM thickness that act against the growth of EM void is necessary in order to better understand the mechanism.

We report high-temperature magnetic properties of hybrid nanocomposite magnets composed of Sm2 Co17 and Nd2Fe14B hard magnetic phases prepared via high-energy ball milling and warm compaction. The magnetic and structural properties of the Sm2Co17/Nd2Fe 14B nanocomposite magnets were investigated for different phase fractions of the two hard phases. The effective interphase exchange coupling resulting from the small grain size (∼14 nm) of the hard phases leads to a single-phase-like demagnetization behavior with improved energy product ${(BH)}_{{{\max}}}$ . The Sm2Co17/Nd2Fe14B isotropic nanocomposite magnets exhibit a ${(BH)}_{{{\max}}}$ value up to 15.5 MG·Oe at room temperature. High-temperature magnetic characterizations show that the Sm2Co17/Nd2Fe14B hybrid nanocomposite magnets have remarkably better thermal stability than single-phase Nd2Fe14B magnets and also have enhanced ${(BH)}_{{{\max}}}$ compared to single-phase Sm 2Co17 magnets.

This paper reports experimental observations evidencing that the intermetallic compound phase interfaced with Cu in the Al-Cu diffusion couple is most likely α2-Cu3Al phase, not γ-Cu9Al4 phase as previously assumed, and that its growth to a critical thickness may result in interface failure by stress-driven fracture. These conclusions are made based on an interdiffusion study of a diffusion couple made of a thick Cu plate coated with ∼ 2-μm-thick Al thin film. The interface microstructure and lattice parameter were characterized using scanning electron microscopy and x-ray diffraction analysis. Specimens aged at temperature between 623 K (350°C) and 723 K (450°C) for various hours produced consistent results supporting the main conclusions. It is found that disordered α2-Cu3Al phase grows in a similar manner to solid-state epitaxy, probably owing to its structural similarity to the Cu lattice. The increase in the interface strain that accompanies the α2-Cu3Al phase growth ultimately leads to interface fracture proceeding from crack initiation and growth along the interface. This mechanism provides the most consistent explanation for interface failures observed in other studies.

This paper describes the kinetic and microstructural mechanism of electromigration (EM) failure found in low-profile solder joints where EM and intermetallic phase formation compete for the same volume of Sn. The low-profile solder joint used in our study was made of 20-25um thick solder situated in between a Cu pillar and a Ni coated Cu lead frame (LF). The samples were EM tested in a temperature range of 140-170oC with the current densities varying between 35-45 KA/cm2 in an oil bath to induce failure without Joule Heat induced artifacts. Our studies on EM failure kinetics and microstructural mechanism have produced two key findings. The first finding suggests that the EM diffusivity (Z*D) of diffusing species (Sn, Ni, Cu) in the solder matrix can be uniquely ranked from microstructural analysis, and it is estimated to be (Z*D) Cu> (Z*D) Sn>(Z*D) Ni. This difference in EM diffusivity causes Cu-Sn and Ni-Sn intermetallic compounds (IMC) to develop in distinctively different manners under EM, leading to different EM failure mechanisms. The second finding is that EM in low-profile solder joints consists of multiple failure stages: a) with EM-related voiding in Sn dominating at lower temperatures; while b) thermally-induced IMC growth and invasion competes with EM-induced Sn voiding at high temperatures leading to the complete failure of each joint.

This paper reports the extreme sensitivity of the microstructure evolution in Sn-Ag-Cu (SAC) micro-solder joint, consisting of 15–20 µm thick solder alloy and electrolytic Ni plated Cu electrodes, to electromigration (EM) conditions resulting from a kinetic competition between the growth of Ni3Sn4 intermetallic compound (IMC) and EM of Sn in solder matrix. Microstructural analysis of samples tested at various conditions reveals that the voiding in the solder joint develops to the full extent only when the EM rate of Sn in the solder alloy is sufficiently greater than the growth rate of Ni3Sn4 IMC because the IMC phase is EM-inactive. The decisive evidence for such behavior is found from the samples tested at 160 and 170 °C under 35 kA/cm2. The samples tested at 170 °C show the classic solder joint microstructure that typically develops under the influence of EM, that is the exaggerated growth of Ni3Sn4 at the anode as a result of Ni-EM and voids at the cathode interface of the joint as a result of Sn-EM. Contrarily, such characteristic features are not found in samples tested at 160 °C, instead, the joint is fully converted to Ni3Sn4 without sign of its biased growth and voiding activities. The near absence of biased EM microstructure at 160 °C is believed to result from the stress-gradient developed by Sn EM counteracting the EM force on Ni. No such effect is at work at higher temperatures due to the significantly higher initial Sn EM rate which leads to voids and reduces the stress gradient.

This paper reports the mechanism of the power conversion efficiency (PCE) improvement in the ZnO nanowires (NW) based solar cells by using CdS/CdSe nanocomposite sensitizers instead of a single CdS quantum-dot (QD) sensitization layer. Two cells with the different type of the sensitization layers were essentially consists of the high-density ZnO nanowire (NW) and a sensitization layer of either CdS-QD or CdS/CdSe nanocomposite, which were produced by an in-situ sequential assembly process of both ionic layer absorption and reaction (SILAR) and chemical bath deposition (CBD). Measurement on the PCE revealed that the cell with CdS/CdSe nanocomposite showed a three-fold increase in PCE compared to the one with a CdS-QD layer. While such improvement in PCE appeared to be consistent with the step-wise band alignment mechanism suggested for the type-II heterojunction of CdSe/CdS/ZnO structures, our microstructural analysis of the cell structure yielded results strongly indicating that the reduction of both interface defects and misfit strain in the CdS lattices plays an additional role on the PCE improvement. Analyses on the interface and the CdS crystallinity using high-resolution electron microscopy (HRTEM) combined with the geometric phase analysis (GPA) revealed that the addition of CdSe effectively reduced the lattice strain in the CdS without introducing misfit dislocations at CdS/CdSe interface, probably owing to Se anion diffusion (or exchange) to the defective SILAR CdS layer during the CBD process. Although an entire enhancement in PCE by the addition of CdSe layer seen in our study cannot be attributed solely to the interface defect/strain reduction, our observations suggest that control of misfit dislocation and lattice nano-strain is equally significant to the step-wise band alignment in affecting the performance of the heterojunction solar cell.

The mechanical stability of Sn-Ag-Cu interconnects with low and high silver content against mechanical shock at room and elevated temperatures was investigated. With a heating element-embedded printed circuit board design, a test temperature from room temperature to 80°C was established. High impact shock tests were applied to isothermally pre-conditioned ball-grid array interconnects. Under cyclic shock testing, degradation and improved shock performances were identified associated with test temperature variation and non-solder mask defined and solder-mask defined pad design configuration differences. Different crack propagation paths were observed, induced by the effect of the elevated temperature test conditions and isothermal aging pre-conditions.

Cooling system based on recirculating water has a long history of application in various industrial sectors with its configurational/material simplicity, cost effectiveness and extreme cooling efficiency. While such systems are in use with many variations in system specifics such as the cooling media and component materials, the one with emerging interest is the configuration using microchannel Cu coldplate as a heat-exchanger because of its demand in high performance computing facilities. These facilities have to remove the intense amount of heat generated by a number of CPUs in parallel connection, forcing them to rely on the water based fluid and microchannel Cu coldplate attached to CPU as heat exchanger. One of the critical but less well address concern in such system is the danger of losing reliability by the process of corrosion. The concern exists especially because the Cu coldplate is likely to be subjected to a galvanic corrosion due to the brazing used during the assembly process. The brazing alloy is chemically different from pure Cu and may produce electrochemical potential sufficient to result in accelerated corrosion by the way of galvanic reaction. While such possibility has been noted at early stage of the system development, its risk to the system reliability and practical method of its suppression is not well known with lack of fundamental studies. Motivated by the growing need to better understand the corrosion mechanism active in such system, we have investigated the nature of corrosion in galvanic pair created by the braze. The braze is likely to suffer from galvanic corrosion due to multiple galvanic pairs. As shown in Fig.1-1, where microstructure of braze created by Cu-Ag-P alloy is shown, the braze consists of multiple phases. The inter-phase galvanic potential is expected. Also, the braze and surrounding Cu can form a galvanic pair. The existence of multiple galvanic pairs makes the study of corrosion to be extremely difficult and require new characterization strategies. For this, we have employed two new measurement techniques in order to isolate the galvanic corrosion mechanism from other background corrosion. Using these techniques we carried out extensive studies on braze/Cu corrosion with variation in coolant, filler materials and temperature. The first method was to measure the corrosion current under the configuration of ZRA (zero-resistance-analysis). The ZRA uses the Cu and the joint as the counter/working electrode as shown in Fig.1.2 and measures the galvanic potential and current across the joint. The second is to make a mini-galvanic cell consisting of a pure Cu plate and a Cu plate with the brazed part. Spaced by an O-ring, these two plates are forced to make a galvanic cell with a liquid filled the gap and an external short circuit. We were able to measure the resulting corrosion current, direction and the amount of which can be related to the galvanic corrosion created by the braze. This study presents key findings of ours evidencing that the galvanic corrosion can be better characterized by the use of techniques employed in our study. An example of such evidence can be found in Fig.1-3 where the Cu plate with the braze strip after microcell testing is shown. Note the corrosion happened along the interface between the strip and Cu plate. It is also found that the cell current corelates very well to the degree of corrosion as evidenced in Fig.1-4, enabling quantitative measurement of corrosion rate of the galvanic pair formed between Cu and the braze. A detailed description of such findings along with theoretical explanation will be provide in this paper. Figure 1

The mechanism behind corrosion of Cu and Cu alloys has been the subject of extensive studies within their applications in various engineering components. These studies explore the electrochemical mechanisms along with the contributing factors impacting the corrosion behaviors such as pH, temperature, and solution chemistry. Thanks to these studies, the corrosion behavior of Cu and Cu alloys is reasonably well understood to a degree to predict their reliability under conditions that have been a concern of related industries. However, it is noted that studies are very limited to the impact of a microscopic structure that forms a galvanic pair with Cu and Cu alloys. One of the most obvious but least studied examples of such a case is the corrosion located in the joint that is created by brazing material. The Cu or Cu alloy components with a brazed joint is commonly found in water handling devices and facilities (such as a heat exchanger). Since the filler material used for the brazing of Cu and Cu alloys contains a substantial number of elements other than Cu the joint changes chemically and microstructurally different. It then becomes a source of a galvanic couple. In this case, the most corrosion prone part is the area near or at the joint as the corrosion is accelerated by the galvanic potential. While such joints are abundantly placed in various engineering components (and therefore the concern for corrosion should be focused more on those parts), studies on the corrosion mechanism with the brazing joint are extremely limited. This is probably due to sufficient material redundancy in the joint and thus causing immunity to failure by corrosion. However, a recent development of an advanced cooling system, where heat from electronic devices such as a microcontroller is removed by the use of recirculating coolant, the redundancy is no longer valid and the corrosion at the brazed joint becomes a matter of critical and practical concern. Motivated by the growing need to better understand the corrosion mechanism active in advanced cooling system, we have investigated the nature of corrosion in Cu plate with a variation in filler materials (Cu-Ag eutectic alloy, Cu-Ag-P alloys), liquid type (water, water mixed with PG), and temperatures. One of the most challenging part of our investigation is the fact that there are multiple sources of galvanic potential due to the existence of multiple phases in the sample. For example, when a Cu-Ag-P alloy was chosen as a filler material, the joint contained three phases, Cu-rich, Ag-rich and Cu3P intermetallic phases. An example of such a microstructure is shown in Fig.1, where an SEM micrograph of the joint is displayed. In this case, the galvanic potential exists not only between the filler materials as a whole and the surrounding Cu, but also among these three phases. In order to overcome the difficulty of studying corrosion behaviors in such a structure, we adapted two different experimental approaches. The first is to measure the corrosion current under the configuration of ZRA (zero-resistance-analysis). The ZRA uses the Cu and the joint as the counter/working electrode as shown in Fig.2 and measures the galvanic potential and current across the joint. The second is to make a mini-galvanic cell consisting of a pure Cu plate and a Cu plate with the brazed part. Spaced by an O-ring, these two plates are forced to make a galvanic cell with a liquid filled the gap and an external short circuit. We were able to measure the resulting corrosion current, direction and the amount of which can be related to the galvanic corrosion created by the braze. This study presents some of our highlighting findings suggesting that the galvanic corrosion can be better characterized by the use of techniques employed in our study. An example of such evidence can be found in Fig.3 where the ZRA scan signal (voltage and current) in 1M KCl solution is shown with the filler area as the working and surrounding Cu as the counter electrode. Note the instability in the galvanic potential with time, which is attributable to the lack of sustainable passivation layer on top of Cu3P and is responsible for the complex corrosion response of the brazed part to the solution chemistry. A detailed description of such findings along with theoretical explanation will be provide in this paper. Figure 1

The mechanical stability and thermo-mechanical fatigue performance of solder joints with low silver content Sn-1.0Ag-0.5Cu (wt.%) (SAC105) alloy based on different cooling rates are investigated in high G level shock environment and thermal cycling conditions. The cooling rate-controlled samples ranging from 1°C/min to 75°C/min cooling rate, not only show differences in microstructure, where a fine poly-granular microstructure develops in the case of fast cooling versus normal cooling, but also show various shock performances based on the microstructure changes. The fast cooling rate improves the high G shock performance by over 90% compared to the normal cooled SAC105 alloy air-cooling environment commonly used after assembly reflow. The microstructure effect on thermal cycling performance is also discussed, which is analyzed based on the Sn grain orientation, interconnect stability, and solder joint bulk microstructure.

This paper presents highlights of an investigation to develop methods of monitoring corrosion risk of a water-based, closed-loop cooling system and also to identify the mechanism of reliability threats by corrosion. The current investigation finds that an array of multiple, in-line sensors is necessary to track the change in stability of the cooling system against corrosion. The coolant pressure and the flow-rate sensors are necessary to detect clogging resulting from scale deposition. On the other hand, pH and electrical conductivity sensors are needed to measure the cumulative corrosion activity. It is further found that the corrosion activity is alarmingly high in a closed-loop cooling system with a high Cu surface area relative to the coolant volume. Slowly developing failure both by flow blockage and a localized corrosion within a cold plate is observed. The rate of failure is seen to be fastest, when the starting coolant is untreated, while it decreases with deionization and inhibition treatment of water.

A model-inspired phenomenology constitutive equation was developed from the first principles to study the temperature dependence of flow stress at confined dimension. The model was limited in the range of temperature and strain-rate where diffusion is insignificant. It was assumed that flow stress was predominantly governed by the thermal activation of dislocation lines overcoming short-range barriers. A simple sound model was developed from the established principles. Data from relevant experiments were fitted into the model to evaluate and reveal key parameters. Normalization of the data and linearization of the model were performed prior to the evaluation and analysis. The proposed models were generally well fitted to the experimental data as indicated by the correlation factors of >0.85, which could be principally accepted by the criteria of R2=0.90. Of the candidate models, Model III and Model I are particularly recommended to study the temperature-dependent behavior of Cu at confined dimension in the space of interest related to the intended applications (2M/T

The impact of isothermal aging and recrystallized grain structure distribution on mechanical shock and thermal cycling performance of solder joints with 1% and 3% silver content Sn-Ag-Cu interconnects were investigated. Localized recrystallized grain structure distributions were analyzed to identify correlations between the microstructure evolution and shock performance. The results reveal that the shock tolerance depends on the amount of shock energy that can be absorbed during each shock cycle, which depends on microstructural features. Based on the recrystallized grain distribution, additional isothermal aging in 1% silver Sn-Ag-Cu interconnects shows improved shock performance, whereas degraded shock performance was observed in 3% Sn-Ag-Cu interconnects. Using the same grain boundary distribution analysis on thermally cycled samples, relationships between the particle size distribution, localized recrystallized grain structure development, shock, and thermomechanical performance were identified: finer particle spacing is beneficial for thermal cycling as it resists grain boundary generation, while conversely, wider particle spacing facilitates recrystallization and grain boundary mobility that allows Sn to absorb shock energy.

This article summarizes the mechanics of two mechanical fatigue methods, cyclic bending fatigue and shear fatigue, in inducing failure in solder joints in package assemblies, and it presents the characteristics of fatigue failures resulting from these methods using example cases of Sn-Pb eutectic and Sn-rich Pb-free solder alloys. Numerical simulation suggests that both testing configurations induce fatigue failure by the crack-opening mode. In the case of bending fatigue, the strain induced by the bending displacement is found to be sensitive to chip geometry, and it induces fatigue cracks mainly at the solder matrix adjacent to the printed circuit board interface. In case of shear fatigue, the failure location is firmly fixed at the solder neck, created by solder mask, where an abrupt change in the solder geometry occurs. Both methods conclude that the Coffin–Manson model is the most appropriate model for the isothermal mechanical fatigue of solder alloys. An analysis of fatigue characteristics using the frame of the Coffin–Manson model produces several insightful results, such as the reason why Pb-free alloys show higher fatigue resistance than Sn-Pb alloys even if they are generally more brittle. Our analysis suggests that it is related to higher work hardening. All these results indicate that mechanical fatigue can be an extremely useful method for fast screening of defective package structures and also in gaining a better understanding of fatigue failure mechanism and prediction of reliability in solder joints.

The improved control of lattice strain in the quantum-dot (QD) region of p – i – n structures using a modified epitaxial growth procedure has been observed and analyzed. Strain in the QD region was managed by (a) inserting a correction layer (CL) with a lattice constant that was intermediate between the lattice constant of the QD region and the lattice constant of the underlying substrate, (b) capping the QD islands with a layer that had the same lattice constant as the CL, and (c) the utilization of only three atomic elements in the growth of the QD intrinsic region. These results were demonstrated in In x Ga 1− x As/InAs/In x Ga 1− x As p–i–n devices with five InAs QD layers and then compared with In x Ga 1− x As p–n (HOM) devices with identical ternary alloy compositions and no quantum dot layers. The layers in all of the devices were grown by molecular beam epitaxy. X-ray diffraction (XRD) measurements showed the interface dislocations in the QD samples were fewer than in the HOM samples and were isolated at the In x Ga 1− x As–GaAs interface, away from the optically active QD region. Cross-sectional, high-resolution transmission electron microscopy (HRTEM) images showed no evidence of threading dislocations in the QD region. Post-growth calculations of the average lattice constant of the QD region, using atomic force microscopy, XRD, and HRTEM data, indicated the QD region experienced a ∼3× reduction in its lattice misfit while increasing its critical thickness by more than 3×. Although the total misfit in the QD samples increased with the insertion of the CL and the average lattice constant of the QD region was not matched to the CL, the strain energy nevertheless was absorbed successfully without creating deleterious dislocations as seen in QD devices exhibiting lower dark current densities than in HOM control devices.

This paper concerns the new method of detecting the integration failures in porous low-k (PLK)/Cu interconnects using simple voltammetry-based techniques. In essence, the technique takes advantage of the fact that pores in PLK allow permeation of liquid, including electrolyte, into interconnect structures. The infiltration of electrolyte allows the formation of a micro-cell, consisting of two mating Cu interconnect electrodes and the electrolyte in PLK, where simple linear voltammetry can examine various integration reliability issues pertinent to PLK/Cu interconnects. Specifically, the technique is proven to be effective in detection of 1) failure in Ta barrier, 2) cracks in the capping layer, and 3) trapped impurity in pores in PLK. The working principle of the voltammetry technique and demonstration of its effectiveness is introduced in this paper.

This paper examines the mechanics of ball shear testing with the objective of understanding the mechanism by which the maximum shear force and the rate of crack growth is dependent on the solder bump size. For this, Pb-Sn solder bumps with diameters between 460 lm and 760 lm are soldered to 400 lm-diameter Cu pads and subjected to ball shear testing. In spite of the constant interface area, the bump size significantly impacts the measured shear fracture force and the crack growth rate. Both the fracture force and the crack growth rate increase with bump size, and in the case of the fracture force, the increase is almost linear. Our analysis finds that the linear increase in the fracture force is a result of the bump deformation force, which increases with bump size. A simple model that accounts for the deformation force component is developed and used to extract the true interface fracture force. The estimated true interface fracture force is found to vary little with bump size, tightly converging to the 40 MPa to 48 MPa range. On the other hand, the dependence of crack growth rate on bump size is found to result from the higher degree of rotational moment associated with larger bumps.

This paper is concerned with the mechanics of interfacial fracture that are active in two common testing configurations of solder joint reliability. Utilizing eutectic Pb-Sn/Cu as a reference system and assuming the presence of a predefined crack size in the intermetallic compound (IMC) layer, stress intensity factors (K I and K II) at the crack are numerically calculated for the two given configurations. The analysis of the tensile test configuration reveals that the fracture occurs by the crack-opening mode (K I mode), as anticipated, but that it is greatly assisted by the viscoplasticity of the solder. With nonuniform viscoplastic deformation across the joint, K I is found to increase much more rapidly than it would without the solder, decreasing the critical crack size to the micron scale. The same mechanism is also responsible for the development of a K II comparable to K I at the crack tip, that is, |K I /K II| ~ 1. It is also found that the predominant fracture mode in the bump shear configuration is crack opening, not crack shearing. This is an unexpected result, but numerical analyses as well as experimental observations provide consistent indications that fracture occurs by crack opening. During shear testing, bump rotation due to nonzero rotational moment in the test configuration is found to be responsible for the change in the fracture mode because the rotation makes K I become dominant over K II. With rotational moment being affected by the geometry of the bump, it is further found that the fracture behavior may vary with bump size or shape.

We demonstrate a large-scale placement of nanoparticles through a scheme named "electrostatic funneling", in which charged nanoparticles are guided by an electrostatic potential energy gradient and placed on targeted locations with nanoscale precision. The guiding electrostatic structures are defined using current CMOS fabrication technology. The effectiveness of this scheme is demonstrated for a variety of geometries including one-dimensional and zero-dimensional patterns as well as three-dimensional step structures. Placement precision of 6 nm has been demonstrated using a one-dimensional guiding structure comprising alternatively charged lines with line width of approximately 100 nm. Detailed calculations using DLVO theory agree well with the observed long-range interactions and also estimate lateral forces as strong as (1-3) x 10(-7) dyn, which well explains the observed guided placement of Au nanoparticles.

This paper reports experimental observations made on the mechanism of electromigration (EM) voiding and its kinetics active in low-profile solder joint consisting of long Cu-post (∼300×110µm), thin solder layer (15µm), and Ni coated Cu-lead frame assembly. When EM testing on the samples was conducted at accelerated conditions it revealed that although they are immune to EM failure, they contained a small amount of residual void as a result of EM. Kinetic analysis of EM voiding was done based on the analysis of resistance change and post EM microstructural characterization. Our analysis suggests that the origin of the residual voids is EM in Sn and it occurred prior to conversion of Sn to Cu 6 Sn 5 phase. It further suggests that the extent of voiding is determined by the two competing kinetic factors that are producing opposite effect on the voiding. The first is the kinetics of Sn EM because it determines the amount of vacancy responsible for voiding while the second is the Cu EM as it promotes the growth of Cu 6 Sn 5 phase that interrupts voids growth by removing Sn phase in the joint. The kinetic interplay between these two factors makes EM voiding kinetics of the low-profile solder joint to be different from the more conventional solder joint where Cu EM has a rather insignificant influence.

This paper illustrates the influence of Au addition on the phase equilibria of Sn-Ag-Cu (SAC) near-eutectic alloys and on the interface reaction with the Cu substrate. From the thermal and microstructural characterization of Sn-3.8Ag-0.7Cu alloys containing various amounts of Au, it is found that the Au promotes the formation of a quaternary-eutectic reaction at 204.5°C ± 0.3°C. The equilibrium phases in the quaternary-eutectic microstructure are found to be AuSn4, Ag3Sn, βSn, and Cu6Sn5. While the addition of Au to Sn-3.8Ag-0.7Cu alloys is also found to increase liquidus temperature and the temperature ranges of the phase equilibria field for primary phases, such influences from Au are found to be less pronounced when the alloys were reacted with the Cu substrate. Because of the formation of the Au-Cu-Sn-ternary interface intermetallic, it is found that a majority of Au added to the solder is drained from the melt. The drainage of Au reduces the impact of Au on the phase equilibria of the solder alloys in the joint. It is further found that the involvement of Au in the interface reaction results in a change of the interface phase morphology from the conventional scallop structure to a compositelike structure consisting of (AuCu)6Sn5 grains and finely dispersed, βSn islands.

This paper is a study of the phase equilibria of the Sn-3.8Ag-0.7Cu alloy investigated by a differential cooling method. The difficulty in assessing phase equilibria of the Sn-Ag-Cu (SAC) system because of the insufficient resolution of conventional characterization techniques is solved by inducing preferential growth of a solid phase in a melt by holding the alloy at the solid-liquid phase-equilibrium field. Application of the technique to Sn-3.8Ag-0.7Cu with varying holding temperatures yielded results that the alloy is slightly off eutectic composition. The phase-formation sequence of the alloy during solidification was found to be Ag3Sn, β-Sn, and finally the ternary eutectic microstructure.

We report findings related to the HgTe nanodots fabricated from the precipitation phenomenon in the HgTe-PbTe quasi-binary system. With homogeneous nucleation and growth of HgTe phase, a well defined nanostructure is obtained, where the nanodots are three-dimensionally dispersed within the PbTe matrix. The nanodots prefer to take disk shape to relax a strong strain energy that resulted from the formation of disordered HgTe phase. With this facile and effective technique, the nanodots with various morphologies can be realized within any form of matrix, such as powder, bulk, or thin film, providing a new way to tailor the properties of the nanodots.

This paper presents a study of electron scattering in damascene-processed Cu interconnects. To understand the leading electron-scattering mechanism responsible for the size effect, Cu interconnects with varying physical widths, 80–750 nm, were made, and their resistivity characterized as a function of temperature, ranging from liquid He temperature (4.2 K) to 500 K. The resulting data suggest that surface scattering, contrary to expectations, was not the primary cause of the size effect observed in this investigation. Surface scattering was found to weaken with decreasing line width. Further analysis leads to the conclusion that a substantial fraction of the size effect originates from impurity content scaling inversely with width in these samples.

This paper presents experimental evidence suggesting that electromigration (EM) can be a serious reliability threat when the dimension of Cu interconnects approaches the nanoscale range. To understand the failure mechanism prevailing in nanoscale Cu interconnects, single-level, 400-µm long interconnects with various effective widths, ranging from 750 nm to 80 nm, were made, EM tested, and characterized in this investigation. The results indicate that interface EM (Cu/barrier) may be the predominant EM mechanism in all line widths. The evidence supporting the active Cu/barrier interface EM includes the fact that the EM lifetime is inversely proportional to the interface area fraction. Microscopic analysis of the failure sites also supports the conclusion of interface EM because voids and hillocks are found at the ends of the test strip, which is not possible if lines fail by grain-boundary EM in the test structure used in this study. In addition, our study finds evidence that failure is assisted by a secondary mechanism. The influence of this factor is particularly significant when the feature size is small, resulting in more uniform distribution of failure time in narrower lines. Although limited, evidence suggests that the secondary factor is probably attributed to pre-existing defects or grain boundaries.

This report studies the electromigration performance of W-plug via structures under the reservoir effect. The lifetime improvement factor M was observed to be a weak function of the stressing current and approximately equal to 2. A Simple model is included in the report to explain this observation. The model also predicts the most effective reservoir length for electromigration lifetime improvement.

The thermodynamic driving forces that govern non-equilibrium solidification and subsequent microstructural evolution of solder joint microstructures are discussed in detail, including formation of intermetallic phases within, and at the interface that creates the bond between the solder, package, and board.