板级跌落碰撞下无铅焊点的可靠性研究

文中引入一块不同于JEDEC标准测试板的圆形测试板,探究板级跌落碰撞下无铅焊点的可靠性.首先做模态试验了解测试板的动力学特性,接着做跌落测试,同时测量板的应变和加速度历程.并用ABAQUS软件进行模拟,模拟焊点在跌落碰撞条件下焊点的应力应变等.结果表明有限元模拟得到的应变、板中心的加速度响应和试验吻合,而且用模拟预测的失效焊点的位置与试验一致.失效模式是靠近封装一侧的金属间化合物(IMC)界面的脆性断裂.     

BGA焊点在板级跌落实验中的疲劳寿命估计

按照JEDEC标准对板级跌落实验的要求测试了有铅和无铅焊点的球栅阵列封装.用ANSYS软件建立了有限元分析模型,并用ANSYS/LS-DYNA直接求解器计算了典型结点的应力和应变,以及每次跌落时积累在焊点中的平均应变能密度.利用实验和模拟的结果重新计算了Darveaux模型中的常数,将这个模型的应用范围扩展到了跌落环境,并计算了各种条件下焊点的疲劳寿命.     

基于热循环与跌落冲击的BGA焊点可靠性研究

研究了BGA焊点在受到热循环冲击与跌落冲击综合作用下的可靠性,研究了基于不同热循环冲击条件下的电子产品的板级跌落寿命、失效机理与失效模式、断裂行为与Kirkendall空洞的关系。结果表明,有铅焊点比无铅焊点可靠性略高,弹性模量低的BGA与PCB制造材料耐热不耐跌落冲击,并提出了改善电子产品可靠性的办法。     

随机振动载荷下板级焊点失效模式分析

通过搭建振动加速失效实验平台,完成了芯片中板级焊点的失效实验。在此基础上,对焊点进行电镜观察分析,以确定焊点在振动载荷下的失效模式。结果表明,内部焊点基本没有裂纹扩展迹象,裂纹集中于最外侧角的焊点,焊点的损伤程度分布从内至外呈递增趋势。裂纹的分布集中于PCB板侧和芯片侧的IMC层区域,即在焊点两侧的钎料疏松部位,并沿着IMC层区域的Cu₆Sn₅晶粒方向进行穿晶、沿晶扩展。裂纹的扩展接近于贯穿时,易发生瞬间脆性断裂。     

圆片级封装的板级跌落可靠性研究

圆片级封装(WLP)具有尺寸小、散热性能好、封测成本低等优点,广泛应用于便携式电子产品,其在跌落、碰撞等环境下的可靠性越来越受到重视。将WLP器件组装到PCB基板上,按照JEDEC电子产品板级跌落实验标准进行实验,研究了WLP元件引脚节距、焊球尺寸、PCB焊盘工艺等因素对样品可靠性的影响。对失效样品进行了切片制样,通过金相显微镜、能量色散X射线光谱(EDX)和扫描电子显微镜进行了分析,研究了WLP器件失效机理及其与器件焊球尺寸、节距之间的关系,讨论了底部填充料对WLP封装可靠性的改进作用。     

随机振动下板级组件焊点可靠性研究进展

针对板级组件焊点在随机振动下的可靠性问题,对与之相关的实验研究成果和仿真结果进行了综述。首先介绍了球栅阵列(BGA)封装芯片焊点失效的位置,BGA焊点的材料以及BGA参数和印制电路板(PCB)参数对焊点可靠性的分析,并介绍了疲劳寿命预测方法和子模型法、特殊结构的随机振动分析。最后,提出了随机振动情况下的BGA结构的优化方法。     

振动载荷下板级焊点退化状态表征与分析

为研究振动载荷下板级微互连焊点的退化规律,设计了等幅定频正弦振动试验。采集焊点两端的电压信号,利用退化数据的统计特征对焊点失效过程的电压信号进行阶段划分和趋势拟合。结果表明,焊点的退化数据表征具有健康、轻度失效、重度失效和完全断裂四个阶段。轻度失效和重度失效阶段具有相似的表征形式,均可以分为陡变、缓变和平坦三个区间。采用监测数据计算样本的均值方差,建立了单调递增的退化特征模型。通过数据拟合选取误差较低的表征量,实现了焊点的健康状况评估和失效预警。     

BGA焊点失效分析

文章通过对BGA焊点失效实例的分析,介绍了对BGA焊接失效的分析过程与分析方法,同时得出了引起该BGA焊点失效的主要原因,展现了失效分析对BGA封装的质量控制作用。     

A review of board level solder joints for mobile applications

The reliability of electronics under drop-shock conditions has attracted significant interest in recent years due to the widespread use of mobile electronic products. This review focuses on the drop-impact reliability of lead-free solder joints that interconnect the integrated circuit (IC) component to the printed circuit board (PCB). Major topics covered are the physics of failure in drop-impact; the use of board level and component level test methods to evaluate drop performance; micro-damage mechanisms; failure models for life prediction under drop-impact; modelling and simulation techniques; and dynamic stress–strain properties of solder joint materials. Differential bending between the PCB and the IC component is the dominant failure driver for solder joints in portable electronics subjected to drop-impact. Board level drop-shock tests correlate well with board level high speed cyclic bending tests but not with component level ball impact shear tests. Fatigue is the micro-damage mechanism responsible for the failure of solder joints in the drop-shock of PCB assemblies and the fatigue strength of solder joints depends strongly on the strain rate, test temperature, and the sequence of loading. Finally, tin-rich lead-free solders exhibit significantly higher strain rate sensitivity than eutectic SnPb solder.     

振动载荷下特种设备中电路板级焊点疲劳寿命预测

基于内聚力模型,提出一种用于振动载荷下特种设备中电路板级焊点疲劳寿命预测的介观尺度模型。将单调载荷与振动周期载荷相结合,建立焊点累积损伤参数来表征焊点的剩余疲劳寿命,利用焊点损伤累积率来表征焊点损伤演化规律。以Sn3.0Ag0.5Cu(SAC305)细间距无铅焊点为例分析了模型参数确定方法,并通过振幅为5 mm与10 mm的定频振动试验对模型的预测精度进行了验证,寿命预测结果误差小于10%。由于模型参数为焊点钎料累积塑性应变的固有函数,与焊点尺寸与几何形态无关,通过小样本试验,数据一经确定即可应用于同种钎料不同尺寸焊点在不同振动等级下的疲劳寿命预测,节约了时间与试验成本,该模型能为特种设备中的电子组件提供一种评估板级焊点疲劳寿命的简洁、实用方法。     

基于SolidWorkS的基座有限元分析

通过Solidworks三维绘图软件建立基座的三维模型,在COSMOSWorks仿真环境下,对基座进行了受力分析,获得了工况下基座周围的应力应变图,为其进行结构优化设计提供了重要的依据。     

Modeling and analysis of 96.5Sn-3.5Ag lead-free solder joints ofwafer level chip scale package on buildup microvia printed circuit board

In this study, time-temperature-dependent nonlinear analyses of lead-free solder bumped wafer level chip scale package (WLCSP) on microvia buildup printed circuit board (PCB) assemblies subjected to thermal cycling conditions are presented. The lead-free solder considered is 96.5Sn-3.5Ag. The 62Sn-2Ag-36Pb solder is also considered to establish a baseline. These two solder alloys are assumed to obey the Garofalo-Arrhenius steady-state creep constitutive law. The shear stress and shear creep strain hysteresis loops, shear stress history, shear creep strain history, and creep strain density range at the corner solder joint are presented for a better understanding of the thermal-mechanical behavior of the lead-free solder bumped WLCSP on microvia buildup PCB assemblies     

基于电阻应变临界点的无铅焊点失效分析

利用特制的焊点在线测试系统,测试了不同载荷条件下单个无铅焊点的电阻应变曲线,推导出电阻应变和损伤量之间的定量关系式,分析了焊点失效特性。结果表明:无铅焊点电阻应变曲线包括线性变化区和指数变化区;两个区域临界点处的电阻应变值为0.05左右,临界点至失效的时间约占焊点寿命的20.00%～30.00%;热循环条件下,40℃的电阻应变曲线临界点滞后于125℃的临界点,滞后时间约占焊点寿命的7.50%。     

Nanoindentation on SnAgCu lead-free solder joints and analysis

The lead-free SnAgCu (SAC) solder joint on copper pad with organic solderability preservative (Cu-OSP) and electroless nickel and immersion gold (ENIG) subjected to thermal testing leads to intermetallic growth. It causes corresponding reliability concerns at the interface. Nanoindentation characterization on SnAgCu solder alloy, intermetallic compounds (IMCs), and the substrates subjected to thermal aging is reported. The modulus and hardness of thin IMC layers were measured by nanoindentation continuous stiffness measurement (CSM) from planar IMC surface. When SAC/Ni(Au) solder joints were subject to thermal aging, the Young’s modulus of the NiCuSn IMC at the SAC/ENIG specimen changed from 207 GPa to 146 GPa with different aging times up to 500 h. The hardness decreased from 10.0 GPa to 7.3 GPa. For the SAC/Cu-OSP reaction couple, the Young’s modulus of Cu₆Sn₅ stayed constant at 97.0 GPa and hardness about 5.7 GPa. Electron-probe microanalysis (EPMA) was used to thermal aging. The creep effect on the measured result was analyzed when measuring SnAgCu solder; it was found that the indentation penetration, and thus the hardness, is loading rate dependent. With the proposed constant P/P experiment, a constant indentation strain rate h/h and hardness could be achieved. The log-log plot of indentation strain rate versus hardness for the data from the constant P/P experiments yields a slope of 7.52. With the optimized test method and CSM Technique, the Modulus of SAC387 solder alloy and all the layers in a solder joint were investigated.     

Finite element modeling on electromigration of solder joints in wafer level packages

This work studies the electromigration of solder joints in an encapsulated copper post wafer level package (WLP) by finite element modeling. Experimental data showed that the electromigration failure occurs in solder joints on the printed circuit board (PCB) side due to the current crowding. In order to improve the electromigration performance on the PCB side with a copper post WLP, two new line-to-bump geometry designs are proposed. Coupled electro-thermal finite element modeling is performed to obtain the electrical and thermal fields simultaneously. The ionic flux from electron wind and thermal response is calculated based on finite element solutions. The divergence of the total flux, which is the sum of the divergence of electromigration and thermomigration, is extracted at the critical locations in solder joints. Results show that the new proposed design structures can reduce the maximum current density by 19%, and the divergence of the total ionic flux by 42%. Thermal gradient is very small in solder joints, therefore, the main driving force for electromigration failures comes from the electron wind. The finite element results on mesh dependency are discussed in this paper.     

Impact of board and component metallizations on microstructure and reliability of lead-free solder joints

Aging and accelerated thermal cycling (ATC) have been performed on 2512 chip resistors assembled with Sn3.8Ag0.7Cu (wt.%) solder. The boards were finished with immersion Ag (IAg), electroless nickel/immersion gold (ENIG), and hot air solder leveling Sn–Pb eutectic solder (HASL), and the components’ terminations were finished with 100% Sn and Sn8.0Pb (wt.%). The boards were reflowed with an average cooling rate of 1.6 °C/s. It was found that the microstructure and reliability of the solder joints depended on the board surface finish. The boards containing small amounts of Pb (from board/component terminations) were the most reliable. Solder joints to copper showed a significantly higher number of cycles to first failure than the joints on nickel. Better reliability of the Sn3.8Ag0.7Cu/Cu joints was attributed to an increased copper content in the bulk due to substrate dissolution.     

Tensile behavior of pb-sn solder/cu joints

Solders of nominal 95Pb-5Sn and 60Sn-40Pb were used to join Cu plates. The effect of ternary additions of In, Ag, Sb, and Bi to the near-eutectic solder were also investigated. Bulk solder and interfacial joint microstructures were characterized for each solder alloy. The solder joints were strained to failure in tension; joint strength and failure mode were determined. 95Pb-5Sn/Cu and 60Sn-40Pb/Cu specimens were tested both as-processed and after reflow. 95Pb-5Sn/Cu as-processed and reflow specimens failed in tension in a ductile mode. Voids initiated at β-Sn precipitates in the as-processed specimens and at the Cu₃Sn intermetallic in the reflow specimens. 60Sn-40Pb/Cu failed transgranularly through the Cu₆Sn₅ intermetallic in both the as-processed and reflow conditions. The joint tensile strength of the reflow specimens was approximately half that of the as-processed specimens for both the high-Pb and near-eutectic alloys. The Cu₆Sn{5} intermetallic dominated the tensile failure mode of the near-eutectic solder/Cu joints. The fracture path of the near-eutectic alloys with ternary additions depended on the presence of Cu₆Sn₅ rods in the solder within the Cu plates. Specimens with ternary additions of In and Ag contained only interfacial intermetallics and exhibited interfacial failure at the Cu₆Sn₅. Joints manufactured with ternary additions of Sb and Bi contained rods of Cu₆Sn₅ within the solder. Tensile failure of the Sb and Bi specimens occurred through the solder at the Cu₆Sn₅ rods.     

Evaluating board level solder interconnects reliability using vibration test methods

In this paper, three types of accelerated test methods based on vibration loadings are conducted and compared for board level mechanical reliability evaluation. The first type is fixed frequency sine vibration. The second type is swept sine vibration within a narrow-band of frequency. And the third type is swept random vibration within a narrow-band of frequency. The PCB responses were recorded using a high speed strain data acquisition system. The eigenfrequency of test boards were obtained with the FFT (Fast Fourier Transform) of the strain data of the PCBs during vibration. The PCBs' responses under different tests are compared. The failure processes were monitored and characterized. Results show that the vibrating amplitude is highly dependent upon the frequency ratio. The variation of PCBs’ eigenfrequency may cause the difference of loading amplitudes for fixed frequency vibration, which reduced the repeatability and comparability. The other two vibration methods within a narrow-band frequency could eliminate the influence from the frequency variation of the test boards. The differences of these methods are the loading density and repetitions. The failure processes of the three types of test methods are similar. Four failure stages were found from collected failure data. Weibull plot results show the characteristic life of the solder interconnects which are verified with loading repetition.