倒装焊电子封装底充胶分层研究

使用裂缝尖端附近小矩形路径J积分方法计算电子封装分层传播的能量释放率，并在热循环加载下使用高频超声显微镜技术测定了B型和D型两种倒装焊封装在焊点有无断裂时苡片/底充胶界面的分层裂缝扩展速度，由有限元模拟给出的能量释放率的实验测得的裂缝扩展速率得到可作为倒装焊封装可靠性计算依据的Paris半经验方程。     

热循环条件下SnAgCu/Cu焊点金属间化合物生长及焊点失效行为

研究了热循环过程中SnAgCu/Cu焊点界面金属间化合物的生长规律及焊点疲劳失效行为。提出了热循环条件下金属间化合物生长的等效方程以及焊点界面区不均匀体模型,并用有限元模拟的方法分析了热循环条件下焊点界面区的应力应变场分布及焊点失效模式。研究结果表明:低温极限较低的热循环,对应焊点的寿命较低。焊点的失效表现为钎料与金属间化合物的界面失效,且金属间化合物厚度越大,焊点中的累加塑性功密度越大,焊点越容易失效。     

不同填胶方式对细间距器件焊点可靠性影响

为了获得一种较好的填胶方式来增加3D PLUS组件焊点的可靠性,采用非线性有限元分析方法和统一型粘塑性Anand本构方程,分析热循环载荷下无胶、端封、底封三种状况焊点的应力、应变分布情况及危险点位置,得到焊点应力和塑性应变周期性累积叠加并逐渐趋于平缓的规律.比较三种状态下焊点的应力、塑性应变的最大值及其变化规律,分析结果表明底封方式能有效的改善焊点的应力、应变状况.改变底封方式胶粘剂的线膨胀系数,找到线膨胀系数对焊点应力、应变影响规律,选出较优线膨胀系数的胶粘剂.     

不同尺寸参数对QFP封装热应力场的影响

基于ABAQUS有限元分析软件,利用子模型法,对四边扁平封装(QFP)器件在热循环载荷下的应力场进行了研究,比较了不同引线宽度、引线间距、引线高度等参数对QFP封装器件应力场的影响规律。结果表明,在热循环载荷条件下,QFP封装应力集中区域位于焊点的根部以及焊趾部位,且最大应力位置出现在焊点根部,即焊点最内侧的尖角处;QFP封装的最大应力值与引线间距成正比,与引线宽度和引线高度成反比;利用子模型法,可以方便地对复杂模型中的关键部件进行较准确的分析。     

板级跌落碰撞下无铅焊点的有限元分析

为了预测跌落碰撞下球栅阵列(BGA)封装中无铅焊点的失效,采用ABAQUS软件来模拟跌落碰撞过程中焊点的应力分布.首先建立圆形电路板(PCB)组件的有限元模型,接着用模态试验和有限元模拟相结合的方法确定有限元模型的边界条件和PCB的阻尼参数,然后运用ABAQUS有限元软件模拟PCB组件从三种高度下跌落碰撞过程中BGA封装中无铅焊点的拉应力分布.结果表明:封装最外圈四个拐角焊点的拉应力最大,最大拉应力出现在焊点靠近封装的一侧.由此预测最外圈拐角的焊点最易失效,焊点失效的位置在靠近封装一侧.     

板级跌落冲击载荷下无铅焊点形状对BGA封装可靠性的影响

焊点高度和焊盘尺寸相同情况下,分析焊点形状（桶形、柱形、沙漏形）对BGA封装在板级跌落冲击载荷下可靠性的影响。根据不同焊点形状建立3种3D有限元模型,采用Input-G方法将加速度曲线作为数值模型的载荷输入,对BGA封装件在板级跌落冲击载荷下的可靠性进行分析。结果表明：在跌落冲击过程中,在0.1ms左右PCB板出现最大弯曲变形;焊点形状对BGA封装件在跌落冲击过程中的可靠性有较大的影响;以最大剥离应力作为失效准则对三种焊点进行寿命预测, 沙漏形焊点的平均碰撞寿命值最大,其次是柱形焊点,桶形焊点最小,表明沙漏形焊点在跌落测试中表现出较好的抗跌落碰撞性能。     

随机振动载荷下塑封球栅阵列含铅焊点疲劳寿命模型

对塑封球栅阵列(PBGA)封装器件Sn37Pb焊点进行了正弦振动、随机振动实验,得到各个载荷下焊点的疲劳寿命结果.建立了三维有限元模型,进行与实验条件一致的有限元分析,计算焊点的应力;将实验结果与有限元计算相结合,并基于Steinberg寿命预测模型,发展了随机振动载荷下焊点疲劳寿命预测方法.结果表明,疲劳寿命模型预测...     

微尺度CSP焊点弯振耦合应力应变分析与优化

建立了微尺度芯片尺寸封装(chip scale package, CSP)焊点三维有限元模型,对其进行了弯振复合加载应力应变仿真分析。分析了焊点材料、焊点直径、焊点高度和焊盘直径对微尺度CSP焊点弯振耦合应力应变的影响;选取焊点直径、焊点高度和焊盘直径为设计变量,设计了17组不同水平组合的焊点模型并获取了相应焊点最大弯振耦合应力,采用响应曲面法建立了焊点弯振耦合应力与焊点结构参数的回归方程,结合粒子群算法对焊点结构参数进行了优化。结果表明:焊点材料为SAC387时弯振耦合应力最大,最大弯振耦合应力应变随焊点高度和焊盘直径增大而减小、随焊点直径增大而增大;最优焊点结构参数水平组合为焊点直径0.18 mm、焊点高度0.16 mm和焊盘直径0.15 mm;优化后CSP焊点最大弯振耦合应力下降了8.49%。     

热循环对SnAgCu(纳米Al)/Cu焊点界面与性能影响

研究了纳米0.1%(质量分数)Al颗粒对SnAgCu无铅钎料与铜基板之间界面反应的影响,研究两种无铅钎料界面在-55~125℃热循环过程中的生长行为及其焊点力学性能变化。结果表明:随着热循环次数的增加,界面层金属间化合物的厚度明显增加,焊后界面层金属间化合物为Cu6Sn5相,在热循环过程中在Cu6Sn5和Cu基板之间出现Cu3Sn相。发现纳米Al颗粒的添加,界面层金属间化合物的厚度明显减少,纳米颗粒对界面层的生长具有明显的抑制作用。同时对焊点在热循环过程中的可靠性进行分析,发现焊点的拉伸力随着循环次数的增加逐渐降低,含纳米Al颗粒的焊点具有明显的优越性,在焊点服役期间,焊点失效路径为Cu6Sn5/Cu3Sn的界面处。     

热循环作用下单向炭纤维/环氧树脂复合材料的热应力

利用二维有限元模型对单向T700炭纤维/3234环氧树脂复合材料内部热应力分布进行了数值模拟,同时测试了T700/3234复合材料经不同次数真空热循环后的拉伸强度。研究结果表明,随热循环次数增加,基体应力单调下降,界面应力先下降后上升,25次热循环后变化趋于平缓。界面区域内热应力最大,且产生明显的应力集中。部分界面出现脱粘是复合材料在热循环作用下产生损伤的主要原因。     

硅通孔转接板封装结构多尺度问题的有限元模型

三维硅通孔转接板封装结构中,存在大量的微凸点与微焊球,尺寸相差3个数量级,这种结构多尺度给有限元分析模型的建立带来困难。以板级封装焊锡接点热疲劳寿命的有限元计算为目标,采用均匀化方法将芯片与转接板间的微凸点/下填料层以及转接板与基板间的微焊点/下填料层等效为均匀介质,以解决结构多尺度带来的网格划分困难。在对比分析了几种均匀化方案的基础上,建议在计算三维硅通孔转接板板级封装焊锡接点的热疲劳寿命时,芯片与转接板间的微凸点/下填料层以及转接板与基板间的微焊点/下填料层可采用各自的下填料层替代建模。     

Thermal stress analysis and design optimization of direct chip attach (DCA) and chip scale package (CSP) in flip chip technology

Underfill encapsulation is a technique used to reinforce the solder bumps between the chip and the substrate in flip chip technology. To determine the optimal geometrical parameters and material properties for the package and candidate underfill materials is an important strategy for improving the thermo-mechanical reliability of flip chip packages. In this study, a stress-function-based energy method was developed to evaluate the interfacial peel and shear stress distributions in multilayered packaging structures. The stress functions were expressed in terms of sine and cosine trigonometric series. Simple programming and short CPU time lead to accurate stress distributions. After comparisons with other proposed numerical methods and results, the developed model was then coupled with a Genetic Algorithm to optimize the design of the direct chip attach (DCA) and chip scale package (CSP) in order to diminish the interfacial stresses and the possibility of crack initiation. The results revealed that the maximum peel and shear stress values were productively decreased and their peaks moved toward the center after conducting the optimizations in both cases. Improved geometrical and material parameters of the flip chip package were determined.     

Reliability of lead-free solder joints in CSP device under thermal cycling

Finite element method and Garofalo–Arrheninus creep model were combined and used to evaluate the reliability of different lead-free solder joints (SnAgCu, SnAg, SnSb and SnZn) and SnPb solder joints in chip scale package (CSP) 14 × 14 device under thermal cyclic loading. The results show that von Mises stress and equivalent creep strain in each of the four lead-free solder joints and SnPb solder joints were strongly different, increasing in the order SnPb < SnAg < SnSb < SnZn < SnAgCu. It is found that maximum stress–strain concentrates on the top-surface of corner solder joints in the CSP device for all solder joints, and SnAgCu solder joints shows the highest fatigue life among those five kinds of solder joints.     

An Optimization Analysis of UBM Thicknesses and Solder Geometry on A Wafer Level Chip Scale Package Using Robust Methods

Wafer level chip scale package (WLCSP) has been recognized providing clear advantages over traditional wire-bond package in relaxing the need of underfill while offering high density of I/O interconnects. Without the underfill, the solder joint reliability becomes more critical. Adding to the reliability concerns is the safety demand trend toward "green' products on which unleaded material, e.g. lead-free solders, is required. The requirement of lead-free solders on the packages results in a higher reflow temperature profile in the package manufacturing process, in turn, complicating the reliability issue. This paper presents an optimization study, considering the fatigue reliability, for a wafer level chip scale IC package in which a Ti/Cu/Ni UBM is involved. A finite element model is developed for the package. The model employs Sn3.8Ag0.7Cu lead-free solders built on build-up layers with micro-vias. Finite element analyses are performed to study the mechanical behaviors of the package elements in which the solder as well as the UBM is of interest. Firstly, a Surface Evolver program is used to construct the solder based non-solder mask defined (NSMD) pad. Then, multi-purpose finite element software, ANSYS^tm, is used to create a double symmetric 3-D numerical model to investigate the mechanical behaviors including deformation, stress-strain relation as well as hysteresis loops for temperature cycles. The Garofalo-Arrhenius Creep Model is employed. A modified Coffin-Manson formula is also employed to estimate the fatigue life for the package. Finally, the Taguchi robust analysis is adopted for optimization analysis of UBM thicknesses and solder geometry. Our results show that thicker UBM layers tend to increase the fatigue life while a small solder pad will prolong the fatigue life and as volume increases so does the fatigue life. From the results of Taguchi robust analysis, it is shown that among the factors of UBM layer thickness, solder pad radius and solder volume, the solder volume is the most dominating factor on the fatigue life of the package. The optimal combination of UBM thickness set at 0.0066 mm (level 3), solder pad radius set at 0.10 mm (Level 1), and solder volume set at 0.020 mm³ (Level 3) contributes the greatest fatigue life of 1229 cycles which is 448% gained over our reference package model.     

Analytical thermal stress model for a typical flip-chip (FC) package design

A simple analytical thermal stress model is suggested for a typical flip-chip (FC) lidded package design. The model is based on the concept of the interfacial compliance. The addressed design consists of a silicon FC bonded to an organic substrate and covered by a lid. The lid is configured in such a way that its mid-portion is bonded to the back side of the chip using a thermal interface material (a heat sink is intended to be subsequently mounted on the outer surface of the lid) and the lid’s peripheral portions are adhesively bonded to the same substrate using compliant attachments around the lid’s perimeter. A copper lid and a (hypothetical) organic lid are considered to develop a general feeling of the possible stress relief that could be expected if an organic lid is employed. The in-plane compliances of all the attachments, including the effective compliance of the encapsulated solder joint interconnections, are taken into account. A numerical example shows how the model could be used in practical computations. It shows also that the application of an organic lid, although is less attractive from the standpoint of the thermal management of the design, might result in appreciably lower thermal stresses. This is true for both the normal stresses in the chip’s cross-sections and the maximum interfacial shearing stresses at the chip’s ends. The developed model can be employed in the analysis of a FC package design of the type in question. Future work should include FEA verifications, and the suggested analytical stress model can be of help when developing a FEA preprocessing simulation model.     

Influence of Thermal Cycling on the Microstructure and Shear Strength of Sn3.5Ag0.75Cu and Sn63Pb37 Solder Joints on Au/Ni Metallization

The influence of thermal cycling on the microstructure and joint strength of Sn3.5Ag0.75Cu （SAC） and Sn63Pb37 （SnPb） solder joints was investigated. SAC and SnPb solder balls were soldered on 0.1 and 0.9 μm Au finished metallization, respectively. After 1000 thermal cycles between -40℃ and 125℃, a very thin intermetallic compound （IMC） layer containing Au, Sn, Ni, and Cu formed at the interface between SAC solder joints and underneath metallization with 0.1 μm Au finish, and （Au, Ni, Cu）Sn4 and a very thin AuSn-Ni-Cu IMC layer formed between SAC solder joints and underneath metallization with 0.9 μm Au finish. For SnPb solder joints with 0.1 μm Au finish, a thin （Ni, Cu, Au）3Sn4 IMC layer and a Pb-rich layer formed below and above the （Au, Ni）Sn4 IMC, respectively. Cu diffused through Ni layer and was involved into the IMC formation process. Similar interfacial microstructure was also found for SnPb solder joints with 0.9μm Au finish. The results of shear test show that the shear strength of SAC solder joints is consistently higher than that of SnPb eutectic solder joints during thermal cycling.     

Electrical properties and interfacial reaction of BGA package with underfill

This study evaluated the electrical properties and interfacial reaction of the ball grid array (BGA) packages with various underfills under thermal shock and isothermal aging conditions. The BGA packages were composed of two surface finishing materials on the bottom substrate, viz. bare Cu and electroless nickel-immersion gold (ENIG). The thickness of the intermetallic compounds (IMCs) formed at the interface between the solder and substrate increased with increasing number of thermal shock cycles and isothermal aging times. Also, the growth rate of the IMCs during the isothermal aging condition was faster than that during the thermal shock condition. The electrical resistance of the BGA package with the underfill was lower than that of the BGA package without the underfill under thermal shock condition. The electrical resistance of the underfill with silica particles was lower than that of the underfill without silica particles.     

Comparative study on stress–strain hysteresis response of SAC solder joints under thermal cycles

The present study attempts to evaluate the stress-strain hysteresis responses of SAC solder joints in Resistor and FleXBGA144 packages subjected to thermal cyclic loading using several constitutive models. The total deformation of the solder material consists of elastic, rate-independent plastic and rate-dependent creep components. The constitutive models discussed in this study each weighted elastic, plastic and creep deformations differently. At low stresses SAC solder alloys were found to be creep resistant, where at higher stresses, the influence of different microstructures disappears as matrix-creep dominates in this region. Thus, the proper constitutive model requires all the three ingredients of the elastic, the creep, and the time-independent plastic data for different stress levels to effectively predict the hysteresis behavior of the SAC solder alloys. The hysteresis loops predicted by constitutive models were also found in close agreement with the loops generated by FEM for the SAC solder joint subjected to thermal cycling.     

覆晶封装底胶充填流动研究

在IC封装中,覆晶封装拥有低成本、低交介口及体积小的特色.文中主要探讨了覆晶封装底胶充填时,锡球、芯片及基板间的流动状况.所使用的制程参数为进浇型式、射出压力、充填时间及锡球尺寸.进浇型式有单点、一字、L型和U型.研究结果显示,在覆晶封装底胶充填时,实验观察和仿真分析所得的平面方向的自由液面形状非常一致.在厚度方向,实验观察的自由液面形状为凹形.不同射出压力下,自由液面的接触角均相同.由此而知,在底胶充填时,表面张力为主要作用力.在相同射出压力下,0.8 mm锡球的自由液面接触角大于1.0 mm锡球的自由液面接触角.     

Fracture mechanics analysis of the effect of substrate flexibility on solder joint reliability

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue life significantly. The reliability of solder joints in real flip chip assembly with both rigid and compliant substrates was evaluated by the accelerated temperature cycling test and thermal mechanical analysis. The mechanism of substrate flexibility on improving solder joint thermal fatigue lifetime was investigated by fracture mechanics methods. Two different methods (crack tip opening displacement, CTOD and virtual crack closure technique, VCCT) are used to determine the crack tip parameters which are considered as the indices of reliability of solder joints, including the strain energy release rate and phase angle for the different crack lengths and temperatures. It was found that the thermal fatigue lifetime of solder joints in flip chip on flex assembly (FCOF) was much longer than that of flip chip on rigid board assembly (FCOB). The flex substrates could dissipate energy that otherwise would be absorbed by solder joints, that is, substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during thermal cycling.