Study on Cu particles-enhanced SnPb composite solder

The Sn-Pb solder is widely used in the electronics industry. With the development of surface mount technology and miniaturization of elements, mechanical properties of the solder are critical. Creep resistance and size stability of soldered joints are important for optical electronics. In the present work, Cu particles with a size of about 8 μm were added to the eutectic 63Sn-37Pb solder to improve the creep property of the soldering alloy. The contents of the Cu particles are 5 vol.% and 10 vol.% separately. The solder matrix is 63Sn37Pb particles with a normal size of 43 μm. The composite solder pastes are manufactured from a mixture of these particles with no-clean flux. Under reflow soldering, the metal particles were uniformly dispersed in the Sn-Pb alloy, and very thin intermetallic compounds were formed between the particles and matrix. To simulate practical soldering of printed circuit boards, a specially designed mini specimen with lap joint is used for the creep-rupture test. For the condition of ambient temperature, the creep-rupture lifetime of the soldered joint can be increased by one order quantitatively using the composite solder compared to the 63Sn37Pb eutectic solder. Other mechanical properties are measured also. In addition, the wetting property of the enhanced solder is good through the wettability test.     

The microstructures and mechanical properties of the Sn-Zn-Ag-Al-Ga solder alloys—The effect of Ga

The microstructures and mechanical properties of Sn-8.55Zn-0.5Ag-0.45Al-yGa (wt.%) lead-free solders were investigated. The y content of the solders investigated was 0.5–3.0 wt.%. The results indicate that Ga exhibits prominent influence in the microstructure as well as mechanical properties of the solders. By increasing Ga, the fraction of the Sn/Zn eutectic region decreases and the Sn-matrix region increases. An increase in the Ga content from 0.5 wt.% to 2.0 wt.% enhances the tensile strength while degrading the ductility. The mechanical properties and differential scanning calorimetry (DSC) behavior have been compared with that of the 63Sn-37Pb solder. Gallium lowers the melting point of the Sn-8.55Zn-0.5Ag-0.45Al-yGa solders. The Sn-8.55Zn-0.5Ag-0.45Al-0.5Ga solders exhibit greater tensile strength and better ductility than the 63Sn-37Pb solder.     

陶瓷封装倒装焊器件Sn-Pb微焊点的电迁移行为

高密度陶瓷封装倒装焊器件的焊点尺寸已降低至100μm以下,焊点电流密度达到10~4 A/cm~2以上,由此引发的电迁移失效成为不可忽视的问题。以陶瓷封装菊花链倒装焊器件为研究对象,开展了Sn10Pb90、Sn63Pb37焊点热电环境可靠性评估试验,通过电连接检测及扫描电子显微镜(SEM)等方法对焊点互连情况进行分析。结果表明,Sn63Pb37焊点阴极侧金属间化合物(IMC)增长明显,表现出明显的极化现象,IMC厚度的平方与通电时间呈线性关系。通电时间达到576 h后Sn63Pb37焊点阴极侧产生微裂纹,而Sn10Pb90焊点在通电576 h后仍未出现异常,表现出优异的电迁移可靠性。研究结果对于直径100μm微焊点的陶瓷封装倒装焊器件的应用具有重要的意义。     

高密度LED焊点微空洞的X射线检测和分析

利用高精度X射线检测设备分别对用Sn37Pb焊膏和Sn3.0Ag0.5Cu焊膏组装的高密度LED灯板进行焊后和老化后的微空洞检测,观察了焊点的微空洞缺陷,并计算微空洞尺寸。结果表明:老化前微空洞面积与焊点面积比在10%~25%的,Sn3.0Ag0.5Cu焊点中约含25.5%,略大于Sn37Pb焊点的23.5%,且明显小于Sn3.0Ag0.5Cu焊点老化后的31.4%。两种焊点老化前后微空洞所占面积比都在<25%的合格范围内,但Sn3.0Ag0.5Cu焊点更易形成微空洞。     

Characterization of 63Sn37Pb and 80Au2OSn solder sealed opticalfiber feedthroughs subjected to repetitive thermal cycling

A highly accurate prediction of hermeticity lifetime is made for eutectic 63Sn37Pb and 80Au20Sn alloy solder sealed optical fiber-Kovar ^TM nosetube feedthroughs subjected to repetitive thermal cycling. Thermal fatigue fracture of the Sn-Pb solder/Kovar^TM interface develops when cracks, initially generated from creep deformation of the solder, propagate gradually through the junction in the axial direction. A nonlinear axisymmetric finite element analysis of the 63Sn37Pb fiber feedthrough seal is performed using a thermo-elastic creep constitutive equation, and solder joint fatigue based on accumulated strain energy associated with solder creep imposed by temperature cycling is analyzed. Additionally, thermal effective stress and plastic strain is studied for alternative 80Au20Sn solder by the finite element method with results indicating significant increase in useful life as compared to 63Sn37Pb. SEM/EDX metallurgical analysis of the solder/Ni-Au plated Kovar^TM nosetube interface indicates that AuSn₄ intermetallic formed during soldering with 63Sn37Pb also contributes to joint weakening, whereas no brittle intermetallic is observed for 80Au20Sn. Hermetic carbon coated optical fibers metallized with Ni,P-Ni underplate and electrolytic Au overplating exhibit correspondingly similar metallurgy at the solder/fiber interface. Combined hermeticity testing and metallurgical analysis carried out on 63Sn37Pb and 80Au20Sn alloy solder sealed optical fiber feedthroughs after repetitive temperature cycling between -65 and +150°C, and -40 and +125°C validated the analytical approach     

Effects of palladium and solder aging on mechanical and fatigue properties of tin-lead eutectic solder

The effects of (a) 0.5 wt.% of Pd addition, and (b) aging on mechanical and fatigue properties of eutectic solder (63Sn37Pb) were investigated. The creep rate of eutectic solder at room temperature is not affected by Pd addition. However, at 80°C, solder containing Pd creeps slower than Sn-Pb eutectic. Strain rate dramatically affects yield and tensile stress of eutectic solder with Pd as it does for the binary solder. Isothermal fatigue life of solder at 25°C is essentially not changed by Pd addition. The microstructure of Pd-containing solder consisted of polyhedral grains of (Pb), (Sn), and a dispersion of PdSn₄ intermetallic. Significant microstructural changes and interphase interface phenomena take place during creep deformation at 25 and 80°C. Ambient aging for seven years leads to solder softening and to moderate increase in isothermal fatigue life.     

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.     

铜颗粒增强对锡铅钎料蠕变寿命的影响

颗粒增强是提高合金性能的重要手段之一.增强体的不同含量对基体的性能会产生不同的影响.主要分析和讨论了铜的不同含量对铜颗粒增强的锡铅基复合钎料蠕变性能的影响.测定6种铜颗粒增强的锡铅基复合钎料的蠕变寿命.试验结果表明:在相同的条件下,复合钎料的蠕变寿命与锡铅钎料相比,均有不同程度的提高.当铜的质量分数约为1%时,复合钎料的蠕变性能最好,约为锡铅钎料的17倍.     

The microstructures and mechanical properties of the Sn-Zn-Ag-Al-Ga solder alloys—the effect of Ag

The microstructures and mechanical properties of Sn-8.55Zn-xAg-0.45 Al-0.5Ga (wt.%) lead-free solders were investigated. The x content of the solders investigated were 0.5–3.0 wt.%. The results indicate that Ag plays an important role not only in the structure but also in the mechanically properties. The mechanical properties and differential scanning calorimetry (DSC) behavior has been compared with that of 63Sn-37Pb solder. Small additions of Ag decreased the melting point of the Sn-8.55Zn-xAg-0.45Al-0.5Ga solders while maintaining the same strength and ductility as the 63Sn-37Pb solder.     

Formation of Pb/63Sn solder bumps using a solder droplet jetting method

Formation processes of Pb/63Sn solder droplets using a solder droplet jetting have not been sufficiently reported. Solving problems such as satellite droplets and position errors are very important for a uniform bump size and reliable flip-chip solder bump formation process. First, this paper presents the optimization of jet conditions of Pb/63Sn solder droplets and the formation process of Pb/63Sn solder bumps using a solder droplet jetting method. Second, interfacial reactions and mechanical strength of jetted Pb/63Sn solder bumps and electroless Ni-P/Au UBM joints have been investigated. Interfacial reactions have been investigated after the second solder reflow and aging, and results were compared with those of solder bumps formed by a solder screen-printing method. Third, jetted solder bumps with variable bump sizes have been demonstrated by a multiple jetting method and the control of waveform induced to a jet nozzle. Multiple droplets jetting method can control various height and size of solder bumps. Finally, real applications of jetted Pb/63Sn solder bumps have been successfully demonstrated on conventional DRAM chips and integrated passive devices (IPDs).     

动态拉伸载荷下SMT焊点可靠性研究

对Sn37Pb和Sn3.0Ag0.5Cu(SAC305)钎料贴装的SMT组件进行了动态拉伸载荷下的机械疲劳试验研究,结果表明,Sn37Pb焊点的疲劳寿命要高于SnAgCu305.在峰值应力为40MPa时寿命差距达到最大,前者是后者2倍～3倍.同时,还分别得出了两种焊点在50%和10%破坏率下的S-N寿命曲线.通过对两种焊点试样横截面的显微观察,发现两种焊点均主要沿着三处位置开裂,然后随着疲劳周次的增加直至裂纹贯穿整个焊点.     

Solid-state annealing behavior of two high-Pb solders, 95Pb5Sn and 90Pb10Sn,on Cu under bump metallurgy

The solid-state annealing behavior of two high-lead solders, 95Pb5Sn and 90Pb10Sn (in wt.%), was examined. After reflow, Cu₃Sn intermetallics formed on the Cu under bump metallurgy (UBM) for both solder alloys. However, solidstate annealing produced significantly different reaction morphologies for the two solder compositions. The Cu₃Sn intermetallics spalled off faster at higher temperatures in the 95Pb5Sn solder. In the case of 90Pb10Sn solder, the Cu₃Sn intermetallics continued to grow even after 1500 h at 170°C. The difference was explained by a two-step phenomenon—Sn diffusion from the bulk solder region to the solder/Cu₃Sn interface (J_Sn), and subsequent intermetallic formation (I_Cu3Sn) by interdiffusion of Cu and Sn. For 95Pb5Sn, the relation, J_Sn < I_Cu3Sn was postulated because of insufficient supply of Sn. The relation, J_Sn > I_Cu3Sn was suggested for the continuous intermetallic growth of the 90Pb10Sn solder. Although a small difference was expected between the two quantities in both solder alloys, the difference in the solid-state annealing behavior was dramatic.     

Constitutive modeling on creep deformation for a SnPb-based composite solder reinforced with microsized Cu particles

In the present work, the creep strain of solder joints is measured using a stepped load creep test on a single specimen. Based on the creep strain tests, the constitutive modeling on the steady-state creep rate is determined for the Cu particle-reinforced Sn37Pb-based composite solder joint and the Sn37Pb solder joint, respectively. It is indicated that the activation energy of the Cu particle-reinforced Sn37Pb-based composite solder joint is higher than that of Sn37Pb solder joint. In addition, the stress exponent of the Cu particle-reinforced Sn37Pb-based composite solder joint is higher than that of the Sn37Pb solder joint. It is expected that the creep resistance of the Cu particle-reinforced Sn37Pb-based composite solder joint is superior to that of the Sn37Pb solder. Finally, the creep deformation mechanisms of the solder joint are discussed.     

Experimental and statistical analyses of surface-mount technologyPLCC solder-joint reliability

The mechanical integrity of surface-mount technology (SMT) plastic leaded chip carrier (PLCC) solder joints has been studied by a four-point mechanical flexure fatigue test. The effects of printed circuit board (PCB) pad surface composition and testing temperature on solder-joint reliability are emphasized. Three sets of PCBs have been tested, one with Cu-Ni-Sn pad surface metallurgy, one with Cu-Ni-Au, and one with SMOBC/SSC (solder mask over bare copper selective solder coating). The solder composition was the 63 wt.%Sn/37 wt.%Pb eutectic. A two-parameter Weibull distribution was used for the lifetime model for these three products. The uniformity, quality, reliability, and a comparison of these products are discussed. The joints formed on Cu-Ni-Au and SMOBC boards were appreciably more reliable than those formed on the Cu-Ni-Sn board     

Viscoplastic Creep Response and Microstructure of As-Fabricated Microscale Sn-3.0Ag-0.5Cu Solder Interconnects

The viscoplastic behavior of as-fabricated, undamaged, microscale Sn-3.0 Ag-0.5Cu (SAC305) Pb-free solder is investigated and compared with that of eutectic Sn-37Pb solder and near-eutectic Sn-3.8Ag-0.7Cu (SAC387) solder from prior studies. Creep measurements of microscale SAC305 solder shear specimens show significant piece-to-piece variability under identical loading. Orientation imaging microscopy reveals that these specimens contain only a few, highly anisotropic Sn grains across the entire joint. For the studied loads, the coarse-grained Sn microstructure has a more significant impact on the scatter in primary creep compared to that in the secondary creep. The observed lack of statistical homogeneity (microstructure) and joint-dependent mechanical behavior of microscale SAC305 joints are consistent with those observed for functional microelectronics interconnects. Compared with SAC305 joints, microscale Sn-37Pb shear specimens exhibit more homogenous behavior and microstructure with a large number of small Sn (and Pb) grains. Creep damage in the Pb-free joint is predominantly concentrated at highly misoriented Sn grain boundaries. The coarse-grained Sn microstructure recrystallizes into new grains with high misorientation angles under creep loading. In spite of the observed joint-dependent behavior, as-fabricated SAC305 is significantly more creep resistant than Sn-37Pb solder and slightly less creep resistant than near-eutectic SAC387 solder. Average model constants for primary and secondary creep of SAC305 are presented. Since the viscoplastic measurements are averaged over a wide range of grain configurations, the creep model constants represent the effective continuum behavior in an average sense. The average secondary creep behavior suggests that the dominant creep mechanism is dislocation climb assisted by dislocation pipe diffusion.     

新型Sn-Ag-Bi-Cu-In系无铅钎料合金研究

通过正交试验设计形成Sn-Ag-Bi-Cu-In系无铅钎料合金,并对钎料合金的熔化温度、可焊性、密度及剪切强度等进行研究.研究发现,Sn-Ag-Bi-Cu-In系钎料合金熔化温度接近传统的熔化温度,固-液温度差小;钎料密度约为传统Sn63Pb37的87%;试样铺展面积为72.02mm2,与Sn63Pb37焊锡的铺展面积76.85 mm2非常接近;钎料的剪切强度远大于传统Sn63Pb37钎料的剪切强度,其断裂形貌为沿晶脆断.     

Sn3.8Ag0.7Cu和Sn37Pb焊点界面显微结构研究

利用扫描电子显微镜（SEM）和透射电子显微镜（TEM）研究了Sn3.8Ag0.7Cu（Sn37Pb）/Cu焊点在时效过程中的界面金属间化合物（IMC）形貌和成份。结果表明：150℃高温时效50、100、200、500h后,Sn3.8Ag0.7Cu（Sn37Pb）/Cu焊点界面IMC尺寸和厚度增加明显,IMC颗粒间的沟槽越来越小。50h时效后界面出现双层IMC结构,靠近焊料的上层为Cu6Sn5,邻近基板的下层为Cu3Sn。之后利用透射电镜观察了Sn37Pb/Ni和Sn3.8Ag0.7Cu/Ni样品焊点界面,结果显示,焊点界面清晰,IMC晶粒明显。     

Fundamental Study of the Intermixing of 95Pb-5Sn High-Lead Solder Bumps and 37Pb-63Sn Pre-Solder on Chip-Carrier Substrates

This study investigated the intermixing of 95Pb-5Sn solder bumps and 37Pb-63Sn pre-solder in flip-chip solder joints. The reaction conditions included multiple reflows (up to ten) at 240°C, whereby previously solder-coated parts are joined by heating without using additional solder. We found that the molten pre-solder had an irregular shape similar to a calyx (i.e., a cup-like structure) wrapped around a high-lead solder bump. The height to which the molten pre-solder ascended along the solid high-lead solder bump increased with the number of reflows. The molten pre-solder was able to reach the under bump metallurgy (UBM)/95Pb-5Sn interface after three to five reflows. The molten pre-solder at the UBM/95Pb-5Sn interface generated two important phenomena: (1) the molten solder dewetted (i.e., flowed away from the soldered surface) along the UBM/95Pb-5Sn interface, particularly when the number of reflows was high, and (2) the molten pre-solder transported Cu␣atoms to the UBM/95Pb-5Sn interface, which in turn caused the Ni-Sn compounds at the chip-side interface to change into (Cu_0.6Ni_0.4)₆Sn₅.     

Electroless Ni-Cu-P barrier between Si/Ti/Al pad and Sn-Pbflip-chip solder bumps

This work investigates the barrier effect of the electroless Ni-Cu-P deposit between Al conductor and Sn-Pb solder bump, as well as the interfacial reaction with the Sn-Pb solder. For the Ni-Cu-P/63Sn-37Pb system, a (Ni, Cu)₃Sn₄ compound with three different morphologies: fine-grain, whisker, and polygon are formed at the Ni-Cu-P/63Sn-37Pb interface after reflow at 220°C for 15 s. These (Ni, Cu)₃Sn₄ crystals transform into polygon shape with smooth appearance during 150°C aging. For the Ni-Cu-P/95Pb-5Sn system, equiaxial (Ni, Cu)₃Sn₄ crystals are formed at the Ni-Cu-P/95Pb-5Sn interface after reflow at 350°C for 15 s, and they also transform into polygon shape during 150°C aging. In addition, the Ni-Cu-P deposit will crystallize to form Ni₅ P₂ during 350°C reflow. The growth of the (Ni, Cu)₃ Sn₄ compound by solid state reaction is a diffusion controlled process for both Ni-Cu-P/63Sn-37Pb and Ni-Cu-P/95Pb-5Sn systems. A 4 μm Ni-Cu-P deposit can provide adequate barrier function between an Al conductor and two Sn-Pb solders under 150°C aging for 1000 h