Geometry effect on mechanical performance and fracture behavior of micro-scale ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints

Solder joint integrity has long been recognized as a key issue affecting the reliability of integrated circuit packages. In this study, both experimental and finite element simulation methods were used to characterize the mechanical performance and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints with different standoff heights (h, varying from 500 to 100 μm) and constant pad diameter (d, d = 480 μm) and contact angle under shear loading. With decreasing h (or the ratio of h/d), results show that the stiffness of BGA solder joints clearly increases with decreasing coefficient of stress state and torque. The stress triaxiality reflects the mechanical constraint effect on the mechanical strength of the solder joints and it is dependent on the loading mode and increases dramatically with decreasing h under tensile loading, while the change of h has very limited influence on the stress triaxiality under shear loading. Moreover, when h is decreased, the concentration of stress and plastic strain energy along the interface of solder and pad decreases, and the fracture location of BGA solder joints changes from near the interface to the middle of the solder. Both geometry and microstructure greatly affect the shear behavior of joints, the average shear strength shows a parabolic trend with decreasing standoff height. Furthermore, the brittle fracture of BGA solder joints after long-time isothermal aging was investigated. Results obtained show that, under the same shear force, the stress intensity factors, K_I and K_II, and the strain energy release rate, G_I, at the Sn–3.0Ag–0.5Cu/Cu₆Sn₅ interface and in the Cu₆Sn₅ layer obviously decrease with decreasing h, hence brittle fracture is more prone to occur in the joint with a large standoff height.     

基于再流焊冷却过程应力分析的板极组件BGA焊点参数优化

建立了板级组件BGA（Ball Grid Array）焊点有限元分析模型,对BGA焊点进行了再流焊冷却过程应力仿真分析,设计并完成了验证性实验以验证仿真分析方法的有效性,分析了焊点结构参数和材料变化对焊点再流焊冷却过程应力应变的影响,采用响应面法建立了焊点应力与结构参数的回归方程,结合遗传算法对焊点结构参数进行了优化.结果表明：实验结果证明了仿真分析的有效性;焊点应力随着焊点高度的增加而增大,随着焊点直径的增加而减小;最优焊点结构参数水平组合为：焊点高度0.44mm、焊点直径0.65mm、焊盘直径0.52mm和焊点间距1.10mm;对该最优焊点仿真验证表明最大应力下降了0.1101MPa.     

影响BGA封装焊接技术的因素研究

虽然表面贴装制造工艺已经纯熟,但是随着BGA封装的广泛应用以及焊球间距的逐步减小,给表面贴装制造工艺带来了新的挑战.基于BGA封装在表面贴装技术焊接中的应用,从印制电路板焊盘设计、印制电路板板材选取和保护、BGA封装选取和保护、印刷工艺、回焊炉温度曲线设定与控制等方面,阐述了影响BGA封装焊接技术的各个因素,进而提升B...     

Investigation of soldering process and interfacial microstructure evolution for the formation of full Cu3Sn joints in electronic packaging

Within electronic products, solder joints with common interfacial structure of Cu/IMCs/Sn-based solders/IMCs/Cu cannot be used under high temperature for relatively low melting points of Sn-based solders (200–300 °C). However, there is a trend for solder joints to service under high temperature because of the objective for achieving multi-functionality of electronic products.With the purpose of ensuring that solder joints can service under high temperature, full Cu₃Sn solder joints with the interfacial structure of Cu/Cu₃Sn/Cu can be a substitute due to the high melting point of Cu₃Sn (676 °C). In this investigation, soldering process parameters were optimized systematically in order to obtain such joints. Further, interfacial microstructure evolution during soldering was analyzed. The soldering temperature of 260 °C, the soldering pressure of 1 N and the soldering time of 5 h were found to be the optimal parameter combination. During soldering of 260 °C and 1 N, the Cu₆Sn₅ precipitated first in a planar shape at Cu-Sn interfaces, which was followed by the appearance of planar Cu₃Sn between Cu and Cu₆Sn₅. Then, the Cu₆Sn₅ at opposite sides continued to grow with a transition from a planar shape to a scallop-like shape until residual Sn was consumed totally. Meanwhile, the Cu₃Sn grew with a round-trip shift from a planar shape to a wave-like shape until the full Cu₃Sn solder joint was eventually formed at 5 h. The detailed reasons for the shape transformation in both Cu₆Sn₅ and Cu₃Sn during soldering were given. Afterwards, a microstructure evolution model for Cu-Sn-Cu sandwich structure during soldering was proposed. Besides, it was found that no void appeared in the interfacial region during the entire soldering process, and a discuss about what led to the formation of void-free joints was conducted.     

Low cycle fatigue performance of ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints

Low cycle fatigue performance of ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu joints with different standoff heights (h, varying from 100 to 500 μm) and two pad diameters (d, d = 320 and 480 μm) under displacement-controlled cyclic loading was studied by experimental method and finite element (FE) simulation. A prediction method based on the plastic strain energy density and continuum damage mechanics (CDM) framework was proposed to evaluate the initiation and propagation of fatigue crack in solder joints. The results show that fatigue failure of solder joints is a process of damage accumulation and the plastic strain energy density performs a power function correlation with the cycle numbers of crack initiation and propagation. Crack propagation rate is affected by the stress triaxiality, which is dependent on the loading mode and increases dramatically with decreasing h under tensile loading, while the change of standoff height has very limited influence on the stress triaxiality under shear loading mode. Moreover, crack growth correlation constants identified in Cu/Sn–3.0Ag–0.5Cu/Cu joints with a specific geometry (h = 100 μm and d = 480 μm) can be well used to predict the fatigue life of BGA joints with other geometries. Furthermore, the results have also shown that the fatigue life of solder joints increases with decreasing the geometric ratio of h/d under the same nominal shear strain amplitude, while it drops with decreasing h/d under the same shear displacement amplitude in cyclic loading. When the geometric ratio (i.e., h/d ratio) is unchanged, the miniaturization of BGA joints brings about a decrease in fatigue life of the joints.     

基于回归分析和遗传算法的BGA焊点功率载荷热应力分析与优化

建立了球栅阵列BGA（Ball Grid Array）焊点有限元分析模型,选取焊点高度、焊点最大径向尺寸、上焊盘直径和下焊盘直径作为设计变量,以焊点应力作为目标值,采用响应曲面法设计了29组不同水平组合的焊点模型并建模进行仿真计算,建立了焊点应力与结构参数的回归方程,基于回归方程结合遗传算法对焊点结构参数进行了优化,获得了焊点应力最小的结构参数最优水平组合.结果表明：对于无铅焊料SAC387,焊点应力随焊点的高度增加而减小,随最大径向尺寸的减小而减小;应力最小的焊点水平组合为：焊点高度0.38mm、最大径向尺寸0.42mm、上焊盘直径0.34mm和下焊盘直径0.35mm;对最优水平组合仿真验证表明优化后焊点最大应力下降了4.66MPa,实现了BGA焊点的结构优化.     

Optimization of Pb-Free Solder Joint Reliability from a Metallurgical Perspective

To obtain the desired performance of Pb-free packages in mechanical tests, while the solder composition should be carefully selected, the influence of metals dissolved from the soldering pad or under bump metallization (UBM) should also be taken into account. Dissolved metals such as Cu can alter the intermetallic compound (IMC) formation, not only at the local interface but also on the other side of the joint. The high rate of interfacial cracking of Sn-Ag-Cu solder joints on Ni/Au-plated pads is attributed to the high stiffness of the solder and the dual IMC structure of (Cu,Ni)₆Sn₅ on Ni₃Sn₄ at the interface. Approaches to avoid this dual IMC structure at the interface are discussed. A rule for selecting the solder alloy composition and the pad surface materials on both sides of the joints is proposed for ball grid array (BGA) packages.     

BGA焊点的形态预测及可靠性优化设计

制定了BGA(球栅阵列)焊点的形态预测以及可靠性分析优化设计方案，对完全分布和四边分布的两种BGA元件，通过改变下焊盘的尺寸得到不同钎料量的焊点，并对其形态进行了预测，建立了可靠性分析的三维力学模型。采用有限元方法分析了元件和焊点在热循环条件下的应力应变分布特征，预测了不同种类和不同形态的BGA焊点的热疲劳寿命，由此给出了最佳的上下焊盘比例范围。     

A novel bumping process for fine pitch Sn–Cu lead-free plating-based flip chip solder bumps

Electroplating is the best process for the manufacture of fine pitch flip chip solder bumps. However, certain unstable electroplating parameters usually cause poorer coplanarity, which affects packaging reliability and yield. This paper attempts to utilize a CMP-like polisher to reduce the nonuniform height deviation after electroplating. The optimization of three major polishing parameters—pad hardness, loading pressure, and polishing speed—enables the polisher to have a higher material removal rate (MRR) and an easier manipulation as compared with chemical mechanical polishing (CMP). After polishing at a pitch size of 100 μm, the overall coplanarity could be decreased sharply from 33±2.5 μm (coplanarity=7.5%) to 28±1 μm (coplanarity=3%) and it even reached 26±0.5 μm (coplanarity=1%) after reflow.     

BGA器件焊接合格率的预测模型

在表面贴装技术(SMT)大规模生产过程中,如果能够对焊接合格率进行预测,无疑对提高SMT产品的生产率、产品可靠性及成本控制具有重要意义.以球栅阵列(BGA)器件为例,研究SMT焊接合格率的预测方法.通过统计分析,结合焊点成形软件的方法,建立了BGA器件焊接合格率的预测模型,运用该模型可以找出影响焊接合格率的制约因素.结合仿真技术模拟焊点形态,发现引起焊接缺陷各参数之间的关系,并提出相应的解决方案.     

A theoretical yield model for assembly process of area array solder interconnect packages with experimental verification

This paper presents a theoretical yield model for area array solder interconnect process. To achieve a successful solder joint, contact between the solder ball and its associated wettable pad area is essential because without contact, the solder ball cannot initiate wetting its associated pad and, finally, is found an open defect. When an area array solder joints are made simultaneously, it may happen that some of the solder joints in a chip cannot make contact with their associated pads because of the variations of design parameters such as solder ball size, pad size and height, substrate warpage, etc. The yield model provides the relationships of the interconnect yield to the statistical variations of the design parameters. A series of experiments were performed with specially designed area array flip-chips and substrates to verify the model, focusing on the effects of the solder ball size variation and the number of solder joints on interconnect yield. The experimental observations agree well with the model prediction.     

Sn-9Zn/Cu焊点剪切强度及断口形貌分析

研究了BGA直径分别为750μm、1 000μm、1 300μm的Sn-9Zn/Cu焊点剪切强度及其变化规律。采用SEM和EDX对剪切断口进行观察和元素成分分析。试验结果表明,随着焊球直径的增大,焊点剪切强度先减小后增大。剪切断裂位置大部分位于钎料内部,局部位于界面化合物Cu5Zn8处。在相同的剪切高度与剪切速率下,随...     

片式元件焊点的热循环应力应变模拟技术研究

采用ANSYS软件,以0402片式元件焊点为对象,系统探讨了焊点热应变损伤的有限元仿真方法,分析了焊点在热循环过程中的应力应变响应,并基于修正的Coffin-Manson方程,预测了焊点的热疲劳寿命。结果显示:焊点应力集中区域和应变最大区域均位于焊点与PCB焊盘的交界面,基于应变失效原则,推断焊点裂纹将在此界面萌生和扩展,直至失效。指出了焊点有限元热应变损伤模拟技术的不足及未来的研究方向。     

BGA无铅焊点零空洞缺陷控制研究

在无铅BGA封装工艺过程中,通过不同组分的BGA焊球合金与焊膏合金组合焊接、焊膏助焊剂活性剂不同配比及其不同再流焊接条件等实验,对焊料合金和助焊剂配比、再流焊接峰值温度、再流保温时间等参数变化,以降低BGA焊点空洞缺陷进行了研究。结果表明选用相同或相似的BGA焊球和焊膏合金组合焊接、选用活性强的焊膏、选择焊接保温时间较长均有助于降低BGA焊点空洞缺陷产生的几率和空洞面积,BGA焊点最佳再流焊接峰值温度为240℃,当峰值温度设置为250℃时,BGA焊点产生空洞缺陷几率会比240℃高出25%～30%。     

Power electronic assemblies: Thermo-mechanical degradations of gold-tin solder for attaching devices

The eutectic Au₈₀Sn₂₀ solder alloy has been applied in semiconductor assemblies and other industries for years. Due to some superior physical properties, Au/Sn alloy gradually becomes one of the best materials for soldering in electronic devices and components packaging but the voids growth in AuSn solder joints is one of the many critical factors governing the solder joint reliability. Voids may degrade the mechanical robustness of the die attach and consequently affect the reliability and thermal conducting performance of the assembly. Severe thermal cycles [− 55 °C/+175 °C] have highlighted degradations in AuSn die attach solder. The inspection of as-prepared die-attachments by X-ray and SEM (observation of cross-section) shows that the initial voids sizes were increased and a propagation of transverse cracks inside the joint between voids has appeared after ageing, it was featured also the existence of the IMC typical scallop-shape morphology with the phase structure of (Ni, Au)₃Sn₂ on as-reflowed joints. In this paper, we evaluate the origin of these degradations and ways to address them.     

Using Taguchi method to obtain the optimal design of heat dissipation mechanism for electronic component packaging

Packaging technology developments in semiconductor chips are moving towards miniaturization, thinner products, lighter weights, and higher performance. However, in the process of packaging, warpage and residual stress have always been major problems, such as pin deviation, breakage, and weak signals. Further, the distinctive properties of the numerous materials that comprise a semiconductor chip demand different molding temperatures; thus, excessive internal thermal stresses are produced within the packaging structure which ultimately results in colloid warpage. This study used a 3D coordinate measuring machine to determine the levels of warpage produced in electronic packaging products and to verify the amount of warpage simulated by the finite element method. Then, Taguchi method was also utilized to analyze and discuss the four critical control factors namely: (1) shape of the heat sink; (2) thickness of molding; (3) molding temperature; and (4) thickness of soldering tin. Thus, the minimum thermal stress for electronic packaging components was obtained, which meant the optimal parameter combination for the packaging was a triangle-shaped heat sink, with a molding compound of 1.175 mm thick, a molding temperature of 170 °C, and a soldering tin that was 0.03 mm thick.     

Discrete phase method particle simulation of ultra-fine package assembly with SAC305-TiO2 nano-reinforced lead free solder at different weighted percentages

This paper presents a 3D numerical simulation of nano-reinforced lead (Pb)-free solder at the ultra-fine joint component for 01005 capacitor with dimension of 0.2 × 0.2 × 0.4 mm³. The nano-reinforced particles introduced in the Sn-3.0Ag-0.5Cu (SAC305) solder is titanium oxide (TiO₂) nanoparticles with approximate diameter of ≈ 20 nm at different weight percentages of 0.01, 0.05 and 0.15 wt% respectively. The 3D model developed is based on the reflow thermal profile of nano-reinforced Pb-free solder in the wetting zone temperature of 217 °C–239 °C. A two way interactions utilizing both volume of fluid method (VOF) and discrete phase method (DPM) are introduced in the current study. The study effectively shows the distribution of the nanoparticles as it is being doped in the molten solder after undergoing soldering process. Based on the findings, it was shown that good agreement can be seen between experimental data obtained using High Resolution Transmission Electron Microscope (HRTEM) system as compared to multiphase DPM based simulation. At weight percentage of SAC305 + 0.05% TiO₂ nanoparticles, the nanoparticles are well distributed. The fillet height of nano-reinforced solder also meets the minimum requirement for 01005 capacitor. Additionally, as the weight percentage of the doped nanoparticles increases, the time required for the formation of wetted solder also increases. In terms of the velocity and pressure distribution of the nano-reinforced lead (Pb)-free solder, higher weight percentage of doped nanoparticles have higher velocity distribution and lower pressure distributions.     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

再流焊焊点可靠性自动管理系统

加热因子Q是描述SMT再流焊过程的一个量化参数,决定了再流焊工艺以及焊点的可靠性,其大小直接反映了焊点的吸热量以及焊接界面形成的金属间化合物的形态.通过自动再流焊管理系统(ARM)对温度曲线进行监控和优化,调整加热因子,实现可靠的产品焊接.     

Interfacial behaviors of Sn-Pb,Sn-Ag-Cu Pb-free and mixed Sn-Ag-Cu/Sn-Pb solder joints during electromigration

SnPb-SnAgCu mixed solder joints with Sn-Pb soldering Sn-Ag-Cu Pb-free components are inevitably occurred in the high reliability applications. In this study, the interfacial behaviors in Sn-37Pb and Sn-3.0Ag-0.5Cu mixed solder joints was addressed and compared with Sn-37Pb solder joints and Sn-3.0Ag-0.5Cu solder joints with the influence from isothermal aging and electromigration. Considering the difference on the melting point between Sn-3.0Ag-0.5Cu and Sn-37Pb solder, two mixed solder joints: partial mixing and full mixing between Sn-Pb and Sn-Ag-Cu solders were reached with the peak reflowing temperature of 190 and 250 °C, respectively. During isothermal aging, the intermetallic compound (IMC) layer increased with aging time and its growth was diffusion controlled. There was also no obvious affect from the solder composition on IMC growth. After electromigration with the current density of 2.0 × 10³ A/cm², Sn-37Pb solder joints showed the shortest lifetime with the cracks observed at the cathode for the stressing time < 250 h. In Sn-3.0Ag-0.5Cu Pb-free solder joints, current stressing promoted the growth of IMC layer at the interfaces, but the growing rate of IMC at the anode interface was far faster than that at the cathode interface. Therefore, there existed an obvious polarity effect on IMC growth in Sn-Ag-Cu Pb-free solder joints. After Sn-37Pb was mixed with Sn-3.0Ag-0.5Cu Pb-free solder, whether the partial mixing or the full mixing between Sn-Pb and Sn-Ag-Cu can obviously depress both the crack formation at the cathode side and the IMC growth at the anode.