片式电阻混合焊点热循环负载可靠性研究

对在不同工艺参数下形成的、并且经过不同周数热循环负载的片式电阻混合焊点、有铅焊点和无铅焊点进行了外观检测和剪切测试。结果显示,在不同工艺参数下形成的混合焊点的剪切力,随热循环周数的变化趋势有所不同,但是在保证片式电阻焊端和焊料充分熔融的情况下,部分混合焊点的平均剪切力比有铅焊点高,热循环1 000周后,为9.1～11.1 N。     

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。     

元件焊端镀层对无铅焊点可靠性的影响

电子产品无铅化的转变加速了对元件焊端镀层的无铅化研究，目前候选无铅镀层有纯锡、锡铋或锡铜合金。当然,Sn—Ag—Cu焊料和以上这些镀层结合而构成的焊点的可靠性是大家关注的重点。Sn—Ag—Cu焊料和元件焊端镀层若不相容会导致焊点变脆、强度降低、缺乏热疲劳抵抗力，特别在产品生命周期的后期。对焊点的可靠性影响尤为明显。在本中我们研究了由Sn-3．8Ag-0．7Cu焊料和不同的元件焊端镀层：纯锡、Sn-3Cu、锡铋合金(铋的重量百分比分别为1％、3％、6％)组成的SMT焊点可靠性。中给出了焊点金相分析、焊点老化前和老化后的引脚拉伸测试的试验结果，首次发表了SMT焊点加速热循环试验的结果，也提到了Sn—Bi和Sn—Pb镀层对波峰焊焊点可靠性的少量研究结果。通过我们的研究发现，所有试验元件的无铅镀层和Sn—Ag—Cu焊料构成的焊点性能至少和常规的锡铅焊点一样好。     

球栅阵列封装中SnPb焊点的应力应变分析

基于SnPb焊料的统一粘塑性Anand本构模型,运用ANSYS有限元软件分析了球栅阵列封装中复合SnPb焊点在热循环过程中的应力、应变的分布,观察到SnPb焊料的蠕变行为和应力松弛现象,结果证明:外侧焊点经受的应力、应变范围比内侧焊点大;焊点的最高应力区域出现在Sn60Pb40焊料的最外缘处,最高应变区域出现在Pb90Sn10焊料与UBM层接触面的最上缘处.     

无铅PBGA组装的有限元分析与寿命预测

本文采用粘塑性hyperbolic-sine本构方程描述了Sn95．5Ag3．8Cu0．7焊料的材料模式。使用通用有限元软件模拟了PBGA（Plastic Ball Grid Array）封装器件焊球阵列在-55℃-125℃热循环作用下应力应变的分布。并采用修正后的热疲劳寿命预测coffin—Masson公式分析预测了该焊料焊点的热疲劳寿命。     

铜焊盘与锡合金焊点界面物相分析及可靠性探讨

Sn60Pb40焊料与铜焊盘的焊接界面中金属间化合物Cu6Sn5的形成与长大以及在热循环过程中的组织粗化是影响焊点可靠性的重要因素。作者根据SMT工艺的实际情况，使用Au—Sn共晶焊料、Sn60Pb40焊料分别涂覆在铜合金基板表面，并分别在320℃、240℃下保温1min，冷却形成焊点，利用X射线研究分析了两种不同焊盘基材与Sn60Pb40钎料、Au—Sn共晶钎料的钎焊界面的物相。运用经典相变理论、低周疲劳失效的机理以及“柯肯达尔”效应，就优异焊点的形成、物相产生、温度循环后组织粗化与增加Ni阻挡层，对提高焊接接点的温度循环可靠性的作用进行了分析与探讨。     

倒装焊中复合SnPb焊点形态模拟

本文给出了倒装焊(flip-chip)焊点形态的能量控制方程,采用Surface Evolver软件模拟了倒装焊复合SnPb焊点(高Pb焊料凸点,共晶SnPb焊料焊点)的三维形态.利用焊点形态模拟的数据,分析了芯片和基板之间SnPb焊点的高度与焊点设计和焊接工艺参数的关系.研究表明:共晶SnPb焊料量存在临界值,当共晶SnPb焊料量小于临界值时,焊点的高度等于芯片上高Pb焊料凸点的半径值;当共晶SnPb焊料量大于临界值时,焊点的高度随共晶SnPb焊料量的增加而增加.另外,采用无量纲的形式给出了焊点高度与共晶焊料量、焊盘尺寸、芯片凸点的尺寸,芯片重量之间的关系模型,研究结果对倒装焊焊点形态的控制、工艺参数的优化和提高焊点可靠性具有指导意义.     

随机振动加载条件下QFN焊点的优化设计

采用基于最小能量原理和有限元数值分析方法的Surface Evolver软件，建立了四方扁平无引脚器件（QFN：Quad Flat No-lead）焊点三维形态预测模型；选取焊盘长度、焊盘宽度、焊料体积和间隙高度作为四个关键因素，采用水平正交表设计了9种不同的QFN焊点工艺参数水平组合，建立了这9种焊点的三维形态预测模型，得到了不同工艺参数水平组合下的QFN焊点形态；分析在随机振动加载条件下，焊盘长度、焊盘宽度、焊料体积和间隙高度四个工艺参数的改变对QFN焊点的应力应变的影响；通过对因子趋势图分析表明：在随机振动加栽下焊盘长度和焊盘宽度对焊点应力应变影响较大，间隙高度和焊料体积对应力应变影响较小；使QFN焊点应变值最小的参数细合为：焊盘长度为0．8mm；焊盘宽度为0．37mm；间隙高度为0．15mm；焊料体积为0.014mm^3。     

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     

The influence of dwell time on reliability of SMT solder joints under thermal cycling

The influence of dwell time on mechanical behaviour and fatigue life of SMT solder joints under thermal cycling has been investigated. The dwell time has two effects on the mechanical behaviour of SMT solder joints under thermal cycling: first, in the dwell time of high-temperature part, the stress in SMT solder joints will notably relax, and secondly, as the dwell time increase, the stress in solder joints in the low-temperature part of thermal cycling increases. With the increase of dwell time, the life of SMT solder joints under thermal cycling exponentially decreases.     

Improved reliability of copper-cored solder joints under a harsh thermal cycling condition

This study simulated the performance of Cu-cored solder joints in microelectronic components subjected to the extreme thermal cycling conditions often encountered in the automobile industry by comparing the thermal cycling behavior of Cu-cored solder joints containing two different coating layers of Sn–3.0Ag and Sn–1.0In with that of a baseline Sn–3.0Ag–0.5Cu solder joint under a severe temperature cycling range of ?55 to +150 °C. Both Cu-cored solder joints can be considered a potential solution to interconnects in microelectronic semiconductor packages used under harsh thermal conditions on account of their greater resistance to thermal stress caused by the severe temperature cycling than the baseline Sn–3.0Ag–0.5Cu solder joint.     

The deformation behavior of Sn62Pb36Ag2 and itsimplications on the design of thermal cycling tests for electronicassemblies

The introduction of new packages as well as the ongoing miniaturization in SMT make the evaluation of the reliability of solder joints a permanent task. Since solder joints fail due to low cycle fatigue caused by cyclic thermomechanical stress passive thermal cycling is an important test to evaluate the lifetime of solder joints. Since a reliability prediction with the thermal cycle encountered in reality would take years to complete the tests, methods to accelerate the test cycle are to be used. However, due to the viscoplastical deformation behavior of tin-lead solder it is mandatory to take the metallurgical behavior of the solder into account when designing accelerated tests. Two different deformation mechanisms occur, depending on the temperatures of the test as well as the temperature gradient: grain boundary sliding (GBS) and dislocation climb (DC) each one having its own influence on the damage occurring in the solder. Therefore, one is not free in choosing the parameters of a test cycle. In this paper the deformation behavior of tin lead solder is explained, A constitutive equation extracted from experiments is presented, describing the deformation behavior of Sn62Pb36Ag2. Results of simulations of the deformation behavior of Sn62Pb36Ag2 on the base of the constitutive equation are shown. Suggestions for the design of accelerated lifetime tests for solder joints are given     

Coupling effects of mechanical vibrations and thermal cycling on reliability of CCGA solder joints

The microstructures and crack propagation behavior of CCGA (ceramic column grid array) solder joints after sinusoidal vibration loading, random vibration loading, and thermal cycling test have been discussed in this study. The failure mechanism of solder joints was analyzed using an experimental method and finite element analysis. It was found that the failed solder joints mainly distributed at the peripheral area in the solder column arrays and the crack initiation was mainly caused by mechanical vibrations. The deformation of PCB (printed circuit board) introduced by mechanical vibrations brought the outermost solder columns in CCGA devices with significant stress concentration and induced the initiation of cracks. Furthermore, cracks propagated during the process of mechanical vibrations and thermal cycling. The cracks propagated rapidly and the solder joints finally failed. The structure of the PCB holder was improved to relieve the vibration response from the peripheral joints. No visible crack was found in the solder joints after the same mechanical vibrations and thermal cycling test. The reliability of solder joints have been greatly improved with the new PCB holder.     

Impact of laminate cracks under solder pads on the fatigue lives of ball grid array solder joints

This paper reports how the solder joint fatigue lives of three types of lead free plastic BGA components were affected by cracks formed in the printed PCB laminate during a thermal cycling test. The investigation showed that cracks were formed in the laminate for all three tested components. For one of the components having a large chip with solder joints located under the chip, very large cracks were formed in the PCB laminate beneath some solder pads.For lead-free solder joints to BGA components consisting of near eutectic solders based on tin, silver and copper, a large fraction of the solder joints may consist of one single tin grain. Due to anisotropy of tin grains, each solder joint to a BGA component will experience a unique stress condition which will make laminate cracking more likely under certain solder joints.The laminate cracks increased the flexibility of the joints and thereby improved the fatigue lives of the solder joints. Therefore, an estimation of the fatigue lives of solder joints to BGA components based on the results from a thermal cycling test may lead to an overestimation of the fatigue lives if products will be exposed to smaller temperature changes in the field than in the test.If cracks are not formed in the PCB laminate, or if the extent of cracking is small, single-grained solder joints can be expected to result in a high spread in failure distribution with some quite early failures.     

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

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

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.     

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.     

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.