Effect of temperature on the fracture behavior of lead-free solder joints

The objective of the present study is to examine the effect of temperature on the fracture behavior of Cu-SAC305-Cu joints. To this end, double cantilever beam (DCB) specimens, consisting of a thin layer of Sn96.5Ag3.0Cu0.5 (SAC305) solder sandwiched between two copper bars, fabricated under standard surface mount (SMT) processing conditions are fractured under various temperatures with a MTS machine equipped with an environment chamber. The load-displacement behavior corresponding to crack initiation and the subsequent toughening before ultimate failure and the displacements near crack tip are recorded and used to calculate the fracture energy release rates. The fracture surfaces and the crack path analyses are conducted with a scanning electron microscope to understand the effect of temperature on the mechanism of fracture. These results provide new information to manufacturers of microelectronics packages, which could be used to further improve package reliability

Dynamic Mechanical and Failure Properties of Solder Joints

Ph.DDOCTOR OF PHILOSOPH

Infrared testing of electronic components Final report, 5 Apr. 1965 - 5 Jun. 1966

Infrared radiation nondestructive test technique for electrical/electronic equipmen

Mission and spacecraft support functions of the Materials Engineering Branch: A space oriented technology resource

The capabilities of the Materials Engineering Branch (MEB) of the Goddard Space Flight Center, Greenbelt, Maryland, are surveyed. The specific functions of spacecraft materials review, materials processing and information dissemination, and laboratory support, are outlined in the Activity Report. Further detail is provided by case histories of laboratory satellite support and equipment. Project support statistics are shown, and complete listings of MEB publications, patents, and tech briefs are included. MEB staff, and their respective discipline areas and spacecraft liaison associations, are listed

Finite Element Modelling for the Investigation of Edge Effect in Acoustic Micro Imaging of Microelectronic Packages

In acoustic micro imaging of microelectronic packages, edge effect is often presented as artifacts of C-scan images, which may potentially obscure the detection of defects such as cracks and voids in the solder joints. The cause of edge effect is debatable. In this paper, a two-dimensional finite element model is developed on the basis of acoustic micro imaging of a flip-chip package using a 230 MHz focused transducer to investigate acoustic propagation inside the package in attempt to elucidate the fundamental mechanism that causes the edge effect. A virtual transducer is designed in the finite element model to reduce the coupling fluid domain, and its performance is characterised against the physical transducer specification. The numerical results showed that the Under Bump Metallization (UBM) structure inside the package has a significant impact on the edge effect. Simulated wavefields also showed that the edge effect is mainly attributed to the horizontal scatter, which is observed in the interface of silicon die-to-the outer radius of solder bump. The horizontal scatter occurs even for a flip-chip package without the UBM structure

Development of Al, Mn, & Zn doped Sn-Ag-Cu-X solders for electronic assembly

The global electronic assembly community is striving for a robust replacement for leaded solders due to increased environmental regulations. A family of Pb-free solder alloys based on Sn-Ag-Cu (SAC) compositions has shown promise; but reliability issues in certain assembly and operating environments have arisen. Elemental (X) additions (Al, Mn, Zn) to SAC3595 were developed recently for better control of heterogeneous nucleation in solder joint solidification. Cu substrate solderability of these SAC+X alloys was investigated at concentrations between 0.01-0.25 wt. % using globule wetting balance tests due to concern about increased oxidation during reflow. Asymmetric four point bend (AFPB) tests were conducted on as-soldered and thermally aged specimens to investigate correlation between decreased shear strength and extended aging time; a common phenomenon seen in solder joints in service. Composition dependence of these X additions also was explored in simplified Cu joints by differential scanning calorimetry (DSC) and joint microstructure analysis to determine the coupling between undercooling and solidification morphology on single and multiple reflow cycles. Interesting observations by methods such as x-ray diffraction (XRD) and nano-indentation of SAC solder joints with aluminum elemental additions led to promising results and provided a possible solution to promoting heterogeneous nucleation and high reliability in these solder alloys

Nucleation and grain refinement of tin alloys: the role of nucleant particles and solute

This thesis develops the understanding of heterogeneous nucleation and grain refinement during the solidification of Pb-free solder alloys, and explores the potential of grain refining ball grid array (BGA)-scale solder joints by catalysing nucleation during solidification in the reflow process. A review of the literature revealed that numerous past studies have identified dilute alloying additions to Sn-Ag-Cu solders that suppress the nucleation undercooling, but no past study has explored in detail whether these additions can be used to grain refine solder alloys. Of the additions that suppress undercooling, the mechanisms of nucleation catalysis have been proved in past work only for Co, Pd and Pt alloying. In this thesis, the mechanisms of βSn heterogeneous nucleation in alloys containing Zn and Ti are explored. Zn additions are shown to introduce a variety of phases by reaction of the Zn with impurities such as Cu, Fe and the atmosphere. A lattice matching analysis shows that there is a reasonable lattice match between βSn and ZnO and it is suggested that ZnO is the heterogeneous nucleant in Zn-microalloyed solders. In Ti-microalloyed SAC305 solder, past research has found that Ti-microalloying introduces the Ti2Sn3 IMC (Intermetallic Compound) phase but, in this thesis, it has been shown that Ti-microalloying introduces two IMC phases: Ti2Sn3 with the V2GaSn2-structure type and (Ti,Cu,Fe)Sn2 with Mg2Ni-structure type, where the Fe is present as an impurity in commercial purity SAC305. Reproducible orientation relationships (ORs) have been measured between βSn and both primary Ti2Sn3 and (Ti,Cu,Fe)Sn2, although it is unclear whether these ORs are due to nucleation or particle pushing and engulfment. The potential of grain refining solder alloys by combining nucleant particles with solute is then explored in both large (60g) samples and 500μm solder balls. For large samples, Sn-based solders were grain refined effectively with this approach. Solute played a strong role in grain refinement and the grain size was approximately inversely proportional to the growth restriction factor, similar to past work on Mg-, Al-, and Ti-based casting alloys. For 500μm BGA balls, the key factors affecting the number of nucleation events during solidification are shown to be: (i) the addition of nucleant particles such as PtSn4 or αCoSn3, (ii) the addition of solute that generates a high growth restriction factor, and (iii) a high cooling rate. The most nucleation events were triggered when combining (i), (ii) and (iii) and up to ~12 independent βSn orientations formed in 500μm BGA balls compared with 1 independent grain in typical solder alloys. Thus, the approaches used in this thesis can grain refine solder balls, however, even if these approaches are further optimised and better nucleants are develop. At this stage it appears they are unlikely to be able to generate the hundreds of different βSn orientations in a BGA solder ball that would be required to give a near-isotropic joint.Open Acces

Micro-mechanical characteristics and dimensional change of Cu-Sn interconnects due to growth of interfacial intermetallic compounds

Sn-based solder alloys are extensively used in electronic devices to form interconnects between different components to provide mechanical support and electrical path. The formation of a reliable solder interconnects fundamentally relies on the metallurgic reaction between the molten solder and solid pad metallization in reflowing. The resultant IMC layer at the solder/pad metallization interface can grow continuously during service or aging at an elevated temperature, uplifting the proportion of IMCs in the entire solder joint. However, the essential mechanical properties of interfacial IMC (i.e. Cu6Sn5, Cu3Sn) layers, such as Young s modulus and hardness, are drastically different in comparison with Sn-based solder and substrate. Therefore, the increasing fraction of interfacial IMCs in the solder joint can lead to significant deformation incompatibility under exterior load, which becomes an important reliability concern in the uses of solder joints for electronic interconnects. In the past decades, extensive research works were implemented and reported regarding the growth of interfacial IMC layers and its effect on the mechanical integrity of solder joints. But, the following fundamental issues in terms of mechanical and microstructural evolution in the uses of solder joints still remain unclear, demanding further research to elaborate: (1) The protrusion of IMCs: Though the growth of interfacial IMC layers along the diffusion direction in solder joints were studied extensively, the growth of IMCs perpendicular to the diffusion direction were reported in only a few papers without any further detailed investigation. This phenomena can crucially govern the long-term reliability of solder interconnects, in particular, in the applications that require a robust microstructural integrity from a solder joint. (2) Fracture behaviour of interfacial IMC layers: The fracture behaviour of interfacial IMC layers is a vital factor in determining the failure mechanism of solder joints, but this was scarcely investigated due to numerous challenges to enable a potential in-situ micro-scale tests. It is therefore highly imperative to carry out such study in order to reveal the fracture behaviour of interfacial IMC layers which can eventually provide better understanding of the influence of interfacial IMC layers on the mechanical integrity of solder joints. (3) Volume shrinkage: The volume shrinkage (or solder joint collapse) induced by the growth of interfacial IMC layers was frequently ascribed as one of the main causes of the degradation of mechanical reliability during aging due to the potentially resulted voids and residual stress at the solder/substrate interface. However, very few experimental works on the characterisation of such type of volume shrinkage can be found in literatures, primarily due to the difficulties of observing the small dimensional changes that can be encountered in the course of IMCs growth. (4) Residual stress: The residual stress within solder joints is another key factor that contributes to the failure of solder joints under external loads. However, the stress evolution in solder joints as aging progresses and the potential correlation between the residual stress and the growth of interfacial IMC layers is yet to be fully understood, as stress/strain status can fundamentally alter the course of total failure of a solder joint. (5) Crack initiation and propagation in solder joints: Modelling on the mechanical behaviour of solder joints is often undertaken primarily on the stress distribution within solder joints, for instance, under a given external loading. But there is lack of utilising numerical analysis to simulate the crack initiation and propagation within solder joints, thus the effect of interfacial IMC layers on the fracture behaviour of the solder joints can be elaborated in further details. In this thesis, the growth of interfacial IMCs in parallel and perpendicular to the interdiffusion direction in the Sn99Cu1/Cu solder joints after aging was investigated and followed by observation with SEM, with an intention of correlating the growth of IMCs along these two directions with aging durations based on the measured thickness of IMC layer and height of perpendicular IMCs. The mechanism of the protrusion of IMCs and the mutual effect between the growth of IMCs along these two directions was also discussed. The tensile fracture behaviour of interfacial Cu6Sn5 and Cu3Sn layers at the Sn99Cu1/Cu interface was characterised by implementing cantilever bending tests on micro Cu6Sn5 and Cu3Sn pillars prepared by focused ion beam (FIB). The fracture stress and strain were evaluated by finite element modelling using Abaqus. The tensile fracture mechanism of both Cu6Sn5 and Cu3Sn can then be proposed and discussed based on the observed fracture surface of the micro IMC pillars. The volume shrinkage of solder joints induced by the growth of interfacial IMC layers in parallel to the interdiffusion direction in solder joint was also studied by specifically designed specimens, to enable the collapse of the solder joint to be estimated by surface profiling with Zygo Newview after increased durations of aging. Finite element modelling was also carried out to understand the residual stress potentially induced due to the volume shrinkage. The volume shrinkage in solder joints is likely to be subjected to the constraint from both the attached solder and substrate, which can lead to the build-up of residual stress at the solder/Cu interface. Depth-controlled nanoindentation tests were therefore carried out in the Sn99Cu1 solder, interfacial Cu6Sn5 layer, Cu3Sn layer and Cu with Vickers indenter after aging. The residual stress was then evaluated in the correlation with aging durations, different interlayers and the locations in the solder joint. Finally, finite element models incorporated with factors that may contribute to the failure of solder joints, including microstructure of solder joints, residual stress and the fracture of interfacial IMC, were built using Abaqus to reveal the effect of these factors on the fracture behaviour of solder joints under applied load. The effect of growth of IMC layer during aging on the fracture behaviour was then discussed to provide a better understanding of the degradation of mechanical integrity of solder joints due to aging. The results from this thesis can facilitate the understanding of the influence of interfacial IMC layers on the mechanical behaviour of solder joints due to long-term exposure to high temperatures

The ATLAS TRT barrel detector

Çetin, Serkant Ali (Dogus Author)The ATLAS TRT barrel is a tracking drift chamber using 52,544 individual tubular drift tubes. It is one part of the ATLAS Inner Detector, which consists of three sub-systems: the pixel detector spanning the radius range 4 to 20 cm, the semiconductor tracker (SCT) from 30 to 52 cm, and the transition radiation tracker (TRT) from 56 to 108 cm. The TRT barrel covers the central pseudo-rapidity region |η| 1, and the TRT while endcaps cover the forward and backward eta regions. These TRT systems provide a combination of continuous tracking with many measurements in individual drift tubes (or straws) and of electron identification based on transition radiation from fibers or foils interleaved between the straws themselves. This paper describes the recently-completed construction of the TRT Barrel detector, including the quality control procedures used in the fabrication of the detector

Spring contact probes: wear characteristics testing for electrical and mechanical parameters

The study considers the development and evaluation of spring contact probes used for automated testing of printed circuit boards (PCBs) and assemblies. It considers the evolution of circuit technology which originated from the introduction of the thermionic valve at the beginning of the century. Since the introduction of the integrated circuit in the 1960's, the industry has seen considerable advances in integrated and printed circuit miniaturisation with its associated effect on the testability of the completed assembly. The close spacing between the tracks and pads within the printed circuit board, which is possibly loaded on both sides with integrated circuits and other components with fine pitch termination spacings, has initiated the rapid development of a specialised electronic test industry to ensure product quality. [Continues.