Sn-Ag-Cu Solder Joint Microstructure and Orientation Evolution as a Function of Position and Thermal Cycles in Ball Grid Arrays Using Orientation Imaging Microscopy

Thermally cycled plastic ball grid array (PBGA) packages with full arrays of 196 solder joints after various preconditions were examined to observe the microstructure evolution of Sn-Ag-Cu solder joints during aging and thermal cycling, focusing on Sn grain orientation. Each PBGA package was polished to obtain a plan-view cross-section of every solder joint, and characterized using both polarized optical microscopy and orientation imaging microscopy (OIM). By OIM observations, distribution maps were obtained based on Sn crystal c-axis orientations. Each precondition showed a characteristic distribution related to the combined thermal aging and thermal cycling history. This study on Sn grain orientation using OIM provides further understanding about deformation and microstructure evolution processes that occur during thermal cycling, and the impact of isothermal aging as a precondition.     

Microstructure and Orientation Evolution of the Sn Phase as a Function of Position in Ball Grid Arrays in Sn-Ag-Cu Solder Joints

The microstructure evolution of Sn-Ag-Cu solder joints during aging and thermal cycling is studied, with a focus on the Sn grain orientation in plastic ball grid array (PBGA) packages. Thermally cycled PBGA packages with a full array of 196 solder joints were examined after being subjected to various pre-conditions. Each PBGA package was polished to obtain plan-view cross- sections of each solder joint. Solder joints were characterized using both polarized optical microscopy and orientation imaging microscopy (OIM). The observations reveal that the distribution of single and multigrain Sn microstructure as a function of position in the package is dependent on the sample’s preconditions and thermal cycle history. Based on distribution maps from polarized optical microscopy observation, thermal aging has a relatively small impact on the overall fraction of single-grained solder joints. Thermal cycling, however, can cause many single-grained joints to transform into multigrained solder joints. The dependence of the grain structure distribution on different preconditions and evolution of the grain structures during thermal cycling are discussed.     

Microstructural evolutions of Sn-3.0Ag-0.5Cu solder joints during thermal cycling

Temperature-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under harsh loadings. Due to the RoHS directive which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free thermomechanical behavior when subjected to temperature variations. As solder joints mechanical properties are dependent of their microstructural characteristics, developing accurate solder joint fatigue models means to correctly capture the microstructural changes that undergo the solder alloy during thermal cycling. This study reports the Sn3.0Ag0.5Cu (SAC305) solder joints microstructural evolution during damaging temperature cycles. Electron BackScatter Diffraction (EBSD) analysis was conducted to assess the SAC305 microstructure corresponding to a specific damage level. Investigated microstructural features included the β-Sn grain size and crystallographic orientation, as well as the grain boundary misorientation and Ag₃Sn intermetallic compound (IMC) size. As-reflowed and damaged components were also mechanically characterized using nanoindentation technique. The microstructural analysis of SAC305 solder joints prior to thermal cycling showed a highly textured microstructure characteristic of hexa-cyclic twinning with two β-Sn morphologies consisting of preferentially orientated macrograins known as Kara's beach ball, along with smaller interlaced grains. The main observation is that recrystallization systematically occurred in SAC305 solder joints during thermal cycling, creating a high population of misoriented grain boundaries leading to intergranular crack initiation and propagation in the high strain regions. The recrystallization process is accompanied with a progressive loss of crystallographic texture and twinning structure. Ag₃Sn IMCs coalescence is another strong indicator of SAC305 solder damage since the bigger and more spaced the IMCs are the less dislocation pinning can prevent recrystallization from occurring.     

Methodology for Analyzing Strain States During <Emphasis Type="Italic">In Situ</Emphasis> Thermomechanical Cycling in Individual Lead-Free Solder Joints Using Synchrotron Radiation

To examine how a lead-free solder joint deforms in a thermal cycling environment, both the elastic and plastic stress and strain behavior must be understood. Methods to identify evolution of the internal strain (stress) state during thermal cycling are described. A slice of a package containing a single row of solder joints was thermally cycled from 0°C to 100°C with a period of about 1 h with concurrent acquisition of transmission Laue patterns using synchrotron radiation. These results indicated that most joints are single crystals, with some being multicrystals with no more than a few Sn grain orientations. Laue patterns were analyzed to estimate local strains in different crystal directions at different temperatures during a thermal cycle. While the strains perpendicular to various crystal planes all vary in a similar way, the magnitude of strain varies. The specimens were subsequently given several hundred additional thermal cycles and measured again to assess changes in the crystal orientations. These results show that modest changes in crystal orientations occur during thermal cycling.     

Observations of microstructural coarsening in micro flip-chip solder joints

Coarsening of solder microstructures dramatically affects fatigue lifetimes. This paper presents a study of microstructural evolution due to thermal cycling and aging of small solder joints. The lead-tin solder joints in this study have a height of 55 5 m and a tin content of 65–70 wt.%, with a degenerate eutectic microstructure. The joint microstructure coarsens more rapidly during aging at 160°C than cycling from 0–160°C. No coarsened bands are observed. The cycling data scales with standard coarsening equations, while the aging data fits to an enhanced trend. The joints experiencing 2.8% strain during cycling fail by 1000 cycles.     

Localized Recrystallization Induced by Subgrain Rotation in Sn-3.0Ag-0.5Cu Ball Grid Array Solder Interconnects During Thermal Cycling

The evolution of microstructures and grain orientations of a Pb-free solder interconnect during thermal cycling significantly affects its mechanical properties and failure modes. Thus, Sn-3.0Ag-0.5Cu ball grid array assemblies were subjected to thermal cycling to study the thermomechanical responses of the solder interconnects. The orientations and microstructures of the solder interconnects were studied by optical microscopy with cross-polarized light and scanning electron microscopy with an electron backscattered diffraction analysis system. Localized recrystallization behavior was observed in Pb-free solder interconnects during thermal cycling. Closer examination of the very early stage of recrystallization in the same solder interconnect revealed that the subgrains appeared before the formation of the recrystallized grains, and the orientations of the small recrystallized grains separated by high-angle grain boundaries evolved from the initial orientations by subgrain rotation. The localized recrystallization produced fine-grained microstructures during thermal cycling, providing an additional deformation mechanism for the solder interconnects, i.e., grain boundary sliding, which would have been impossible prior to recrystallization. The grain orientation has a strong effect on damage generation and the subsequent failure mode; initiation and propagation of cracks could be facilitated by the intrinsic anisotropic thermomechanical responses of the differently oriented grains, leading to a change in the crack propagation path and corresponding failure mode.     

倒装焊SnPb焊点热循环失效和底充胶的影响

采用实验方法,确定了倒装焊SnPb焊点的热循环寿命．采用粘塑性和粘弹性材料模式描述了SnPb焊料和底充胶的力学行为,用有限元方法模拟了SnPb焊点在热循环条件下的应力应变过程．基于计算的塑性应变范围和实验的热循环寿命,确定了倒装焊SnPb焊点热循环失效Coffin-Manson经验方程的材料参数．研究表明,有底充胶倒装焊SnPb焊点的塑性应变范围比无底充胶时明显减小,热循环寿命可提高约20倍,充胶后的焊点高度对可靠性的影响变得不明显．     

倒装焊SnPb焊点热循环失效和底充胶的影响

采用实验方法 ,确定了倒装焊 Sn Pb焊点的热循环寿命 .采用粘塑性和粘弹性材料模式描述了 Sn Pb焊料和底充胶的力学行为 ,用有限元方法模拟了 Sn Pb焊点在热循环条件下的应力应变过程 .基于计算的塑性应变范围和实验的热循环寿命 ,确定了倒装焊 Sn Pb焊点热循环失效 Coffin- Manson经验方程的材料参数 .研究表明 ,有底充胶倒装焊 Sn Pb焊点的塑性应变范围比无底充胶时明显减小 ,热循环寿命可提高约 2 0倍 ,充胶后的焊点高度对可靠性的影响变得不明显     

A microstructural study of the thermal fatigue failures of 60sn-40Pb solder joints

When an electronic package encounters thermal fluctuations, cyclical shear strain is imposed on the solder joint interconnections. The thermal cycling leads to a condition of thermal fatigue and eventual solder joint failure. This study was performed in order to understand the microstructural mechanisms that lead to solder joint failures in thermal fatigue. Thermal cycling tests were performed on 60Sn-40Pb joints using a -55° C to 125° C cycle and 19% imposed shear strain. A heterogeneously coarsened region of both Pb and Sn-rich phases develops within the 60Sn-40Pb solder joints. Cracks initiate in the heterogeneously coarsened Sn-rich phase at the Sn-Sn grain boundaries. Heterogeneous coarsening and failure occurs in both high (35 to 125° C) and low (-55 to 35° C) thermal cycles. The elevated temperature portion of the thermal cycle was found to be the most significant factor in the heterogeneous coarsening and failure of the solder joints.     

热循环过程中焊点残余应变的研究

采用激光云纹干涉法,测量了不同热循环规范下焊点内的残余应变分布及最后失效的的焊点内最大的累积残余应变(即累积塑性变形),结果表明:材料热膨胀系数的不匹配导致焊点中存在很大的剪切变形,而且焊点内的残余应变的分布是很不均匀的;对应于同一种焊料,不同的热循环规范下焊点失效时的累积塑性变形基本上相同,可以认为对于焊点来说,失效时的累积塑性变形是一个常数,这可以作为热循环过程中焊点失效的判据。     

Thermal cycling aging effects on microstructural and mechanicalproperties of a single PBGA solder joint specimen

This paper reports on an experimental study on how thermal cycling aging exposure changes the solder joint microstructure, intermetallic layer thickness and the residual shear strength and fatigue life in a single plastic ball grid array (PBGA) solder joint specimen. The single BGA solder joint specimen was specially designed to evaluate the microstructure and mechanical properties of three different batches of solder joint after subjected to 0, 500, 1000, and 2000 cycles of thermal cycling aging (-40°C to 125°C). It is important to relate the effects of thermal cycling aging on the changes of the microstructural and intermetallic layer thickness to the residual shear strength and fatigue life of solder joints subjected to thermal cycling aging exposure. The results of this study shows that the microstructural and intermetallic development due to thermal cycling aging has a major impact on the residual mechanical and fatigue strength of the solder joint. It was noted that the solder joint shear strength and residual fatigue life degrades with exposure to thermal cycling aging     

PBGA封装热可靠性分析

对PBGA封装体建立了有限元数值模拟分析模型。模型采用无铅焊点,完全焊点阵列形式。研究了封装体在经历IPC9701标准下的五种不同温度循环加载后,受到的热应力、应变,以及可能的失效形式。结果表明,焊点是封装体结构失效的关键环节,焊点所受应力大小与焊点位置有关。比较了不同温度循环下封装体的疲劳寿命。其结果为提高封装体的可靠性和优化设计提供了理论依据。     

Inhomogeneous deformation and microstructure evolution of Sn–Ag-based solder interconnects during thermal cycling and shear testing

Orientation imaging microscopy was adopted to characterize the microstructural changes in Sn–Ag-based solder interconnects during thermal cycling and shear testing. The deformation and microstructure evolution of Sn–Ag-based solder interconnects are inhomogeneous, depending on the orientations of β-Sn grains in the as-solidified microstructure. Recovery or recrystallization can take place even under pure shear stress at room temperature, and it tends to occur at high-angle grain boundaries in multi-grained solder interconnects, while it localizes in near-interface region in solder interconnects with only one grain inside. During thermal cycling, the hardness of recrystallized microstructure decreased significantly due to the segregation of Ag₃Sn IMC particles towards the newly-formed recrystallized boundaries, increasing the ease of localized deformation in this weakened microstructure. As a consequence, cracks were propagated intergranularly in the recrystallized microstructure.     

Influence of High-G Mechanical Shock and Thermal Cycling on Localized Recrystallization in Sn-Ag-Cu Solder Interconnects

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.     

Thermally induced deformation of solder joints in real packages: Measurement and analysis

The paper presents a hybrid experimental and analytical approach to track the deformation of solder joints in an electronic package subject to a thermal process. The solder joint strain is directly measured using a computer vision technique. The strain measurement is analyzed following an approach that is devised based on an established solder constitutive relation. The analysis leads to the determination of the solder joint stress and in turn, to the separation of the elastic, plastic and creep strain from the measured total strain. The creep strain rate and stress–strain hysteresis loop are also obtained. Two case studies are presented to illustrate the applications and to show the viability of the approach. Each case involves a resistor package with SAC (Sn95.5Ag3.8Cu0.7) solder joints, which is subjected to a temperature variation between ambient and 120 °C. The results confirm that shear is a dominant strain component in such solder joints. The shear strain varies nearly in phase with the temperature whereas the shear stress exhibits a different trend of variation due to stress relaxation. The peak shear stress of around 10 MPa to 15 MPa are found, which occur at near 70 °C in both cases, when the temperature ramps up at approximately 3 °C/min. The creep shear strain goes up to 0.02 and accounts for over 80% of the total shear strain. The creep strain rate is in the order of magnitude of 10⁻⁵ s⁻¹. Responding to the temperature cycling with such moderate rate, the creep strain shows modest ratcheting while the stress–strain hysteresis stabilizes in two cycles.     

Comprehensive board-level solder joint reliability modeling and testing of QFN and PowerQFN packages

For quad flat non-lead (QFN) packages, board-level solder joint reliability during thermal cycling test is a critical issue. In this paper, a parametric 3D FEA sliced model is established for QFN on board with considerations of detailed pad design, realistic shape of solder joint and solder fillet, and non-linear material properties. It has the capability to predict the fatigue life of solder joint during thermal cycling test within ±34% error. The fatigue model applied is based on a modified Darveaux’s approach with non-linear viscoplastic analysis of solder joints. A solder joint damage model is used to establish a connection between the strain energy density (SED) per cycle obtained from the FEA model and the actual characteristic life during thermal cycling test. For the test vehicles studied, the maximum SED is observed mostly at the top corner of peripheral solder joint. The modeling predicted fatigue life is first correlated to thermal cycling test results using modified correlation constants, curve-fitted from in-house QFN thermal cycling test data. Subsequently, design analysis is performed to study the effects of 17 key package dimensions, material properties, and thermal cycling test condition. Generally, smaller package size, smaller die size, bigger pad size, thinner PCB, higher mold compound CTE, higher solder standoff, and extra soldering at the center pad help to enhance the fatigue life. Comparisons are made with thermal cycling test results to confirm the relative trends of certain effects. Another enhanced QFN design with better solder joint reliability, PowerQFN, is also studied and compared with QFN of the same package size.     

Characterization of the Effect of Shape Memory Alloy on Solder Joint Strength Through Modeling and Testing

Currently some of the most common problems that surface mount technology encounters are warpage, delamination, and inelastic strain concentration accumulated in the solder joint during thermal cycling because of mismatch of thermal expansion coefficient between the package and chip side. Material as well as package structure are the critical issues with respect to these problems. The objective of this research is to investigate how shape memory alloy (SMA) applied in the under bump metallization (UBM) can affect solder joint reliability under thermal mechanical stress. Joint strength tests revealed the better strength of solder joints with SMA UBM after accelerated thermal cycling test. Finite element modeling as well as multilayer stress calculations revealed less strain accumulated in the solder and more stress concentrated in Si in the solder joint with SMA UBM. A mechanism by which the SMA accommodates most of the stress and strain caused by the mismatch of the thermal expansion coefficients was proposed to explain the reinforcement of the solder joint by the SMA UBM.     

LGA焊点形态对焊点寿命影响的有限元分析

通过Surface Evolver软件对LGA焊点进行了三维形态预测,利用有限元数值模拟对LGA焊点在热循环条件下寿命进行了分析。研究了热循环条件下LGA焊点的应力应变分布规律,随着焊点远离元件的中心位置焊点所受到的等效应力、等效应变和塑性应变能密度逐渐增大,从而得出处于外面拐角的焊点最先发生失效的结论。基于塑性应变范围和Coffin-Manson公式计算了焊点热疲劳寿命;找出了LGA焊点形态对焊点寿命的影响规律,模板厚度一定时PCB焊盘尺寸小于上焊盘时LGA焊点的热疲劳寿命与PCB焊盘尺寸成正比,大于上焊盘时成反比,大约相等时焊点寿命最大。当PCB焊盘和模板开孔尺寸固定时,通过增大模板厚度来增加焊料体积在一定程度上可提高LGA焊点的热疲劳寿命,但是模板厚度增大到一定值时LGA焊点寿命会逐渐降低。     

Crack Development in a Low-Stress PBGA Package due to Continuous Recrystallization Leading to Formation of Orientations with [001] Parallel to the Interface

Thermal cycling was imposed on plastic ball grid array (PBGA) packages with a small die, a package design that does not impose a large strain on solder joints. Less cracking was observed after 2500 cycles from 0°C to 100°C (with 10 min dwell times and 10 min ramps) than in a prior study with a higher-stress package design, so these samples were thermally cycled (TC) to 6400 cycles to investigate the relationship between cracks, microstructure, and grain crystal orientation. Cracked joint locations within the package were identified using the dye and pry method, indicating that cracks were most often found in joints near the perimeter of the die. Using orientation imaging microscopy (OIM), cracks were observed in many joints having a variety of dominant crystal orientations where the c-axis was between 0° and ∼50° from the package interface. Continuous recrystallization processes occurred and caused gradual rotations of initial orientations that reduced the angle between the c-axis and the package interface. While cracks were observed in joints with a variety of orientations, cracks were highly correlated with recrystallized grains having the [001] c-axis nearly parallel to the interface (“red” orientations) in those joints that did not initially have this orientation.     

Board level solder joint reliability modeling and testing of TFBGA packages for telecommunication applications

For thin-profile fine-pitch BGA (TFBGA) packages, board level solder joint reliability during the thermal cycling test is a critical issue. In this paper, both global and local parametric 3D FEA fatigue models are established for TFBGA on board with considerations of detailed pad design, realistic shape of solder joint, and nonlinear material properties. They have the capability to predict the fatigue life of solder joint during the thermal cycling test within ±13% error. The fatigue model applied is based on a modified Darveaux’s approach with nonlinear viscoplastic analysis of solder joints. A solder joint damage model is used to establish a connection between the strain energy density (SED) per cycle obtained from the FEA model and the actual characteristic life during the thermal cycling test. For the test vehicles studied, the maximum SED is observed at the top corner of outermost diagonal solder ball. The modeling predicted fatigue life is first correlated to the thermal cycling test results using modified correlation constants, curve-fitted from in-house BGA thermal cycling test data. Subsequently, design analysis is performed to study the effects of 14 key package dimensions, material properties, and thermal cycling test condition. In general, smaller die size, higher solder ball standoff, smaller maximum solder ball diameter, bigger solder mask opening, thinner board, higher mold compound CTE, smaller thermal cycling temperature range, and depopulated array type of ball layout pattern contribute to longer fatigue life.