Correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps

The correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps has been studied. Upon solid-state aging, a diffusion-controlled process was observed for the interfacial Ni-Sn compound formation of the Sn/Ni(P) reaction couple and the activation energy is calculated to be 42 KJ/mol. For the Sn0.7Cu/Ni(P), in the initial aging, a needle-shaped Ni-Sn compound layer formed on Ni(P). Then, it was gradually covered by a layer of the Cu-Sn compound in the later aging process. Hence, a mixture layer of Ni-Sn and Cu-Sn compounds formed at the interface. For the Sn3.0Cu/Ni(P), a thick Cu-Sn compound layer quickly formed on Ni(P), which retarded the Ni-Sn compound formation and resulted in a distinct Cu-Sn compound/Ni(P) interface. The shear test results show that the mixture interface of Sn0.7Cu bumps have fair shear strengths against the aging process. In contrast, the distinct Cu-Sn/Ni(P) interface of Sn3.0Cu solder bumps is relatively weak and exhibits poor resistance against the aging process. Upon the reflowing process, the gap formation at the Ni(P)/Cu interface caused a fast degradation in the interfacial strength for Sn solder bumps. For Sn0.7Cu and Sn3.0Cu solder bumps, Ni₃P formation was greatly retarded by the self-formed Cu-Sn compound layer. Therefore, Sn(Cu) solder bumps show better shear strengths over the Sn solder bump.     

SnAgCu无铅焊点的电迁移行为研究

电迁移引发的焊点失效已经成为当今高集成度电子封装中的最严重的可靠性问题之一。应用SnAgCu无铅焊膏焊接微米级铜线,进行电迁移实验。结果表明:焊点形貌从原来的光滑平整变得凹凸不平,阴极处出现了裂纹和孔洞,并且在铜基板和Cu6Sn5金属间化合物(IMC)之间出现薄薄的一层Cu3Sn金属间化合物,由ImageJ软件测量其平均厚度约为2.11μm;而在阳极附近没有明显的Cu3Sn金属间化合物形成。     

Electromigration-Induced Failure of Ni/Cu Bilayer Bond Pads Joined with Sn(Cu) Solders

The effect of electromigration (EM) on Sn(Cu)/Ni/Cu solder joint interfaces under current stressing of 10⁴ A/cm² at 160°C was studied. In the pure Sn/Ni/Cu case, the interfacial compound layer was mainly the Cu₆Sn₅ compound phase, which suffered serious EM-induced dissolution, eventually resulting in serious Cu-pad consumption. In the Sn-0.7Cu case, a (Cu,Ni)₆Sn₅ interfacial compound layer formed at the joint interface, which showed a strong resistance to EM-induced dissolution. Thus, there was no serious consumption of the Cu pad under current stressing. In the Sn-3.0Cu case, we believe that the␣massive Cu₆Sn₅ phase in the solder matrix eased possible EM-induced dissolution at the interfacial compound layer due to current stressing.     

不同加载速率对SAC/Cu焊点剪切强度的影响

通过对Sn0.3Ag0.7Cu/Cu和Sn3.0Ag0.5Cu/Cu焊点进行剪切测试结果表明:两种钎料焊点的剪切强度与加载速率有着明显的相关性,即焊点的剪切强度都随着加载速率的增加而增加。当加载速率为0.01 mm/s时,断裂模式为韧脆混合断裂,随着加载速率的增加,两种钎料焊点断口的韧窝数量不断增加,呈现韧性断裂特征,断口以韧窝为主。另外在相同加载速率下,Sn3.0Ag0.5Cu/Cu焊点断口的韧窝数量和分布情况都优于Sn0.3Ag0.7Cu/Cu焊点,即其韧性断裂的趋势更加明显,剪切强度更大。     

室温下Cu/Sn3.5Ag焊点/Cu体系中电迁移引发裂纹的形成

Electromigration （EM） behavior of Cu/Sn3.5Ag/Cu solder reaction couple was investigated with a high current density of 5 × 10^3 A/cm^2 at room temperature. One dimensional structure, copper wire/solder ball/copper wire SRC was designed and fabricated to dissipate the Joule heating induced by the current flow. In addition, thermomigration effect was excluded due to the symmetrical structure of the SRC. The experimental results indicated that micro-cracks initially appeared near the cathode interface between solder matrix and copper substrate after 474 h current stressing. With current stressing time increased, the cracks propagated and extended along the cathode interface. It should be noted that the continuous Cu6Sn5 intermetallic compounds （IMCs） layer both at the anode and at the cathode remained their sizes. Interestingly, tiny cracks appeared at the root of some long columntype Cu6Sn5 at the cathode interface due to the thermal stress.     

Retarding Electromigration in Lead-Free Solder Joints by Alloying and Composite Approaches

The voids induced by electromigration (EM) can trigger serious failure across the entire cathode interface of solder joints. In this study, alloying and composite approaches showed great potential for inhibiting EM in lead-free solder joints. Microsized Ni, Co, and Sb particles were added to the solder matrix. Cu and Sn particles were added to the melting solder to form in situ Cu₆Sn₅, which formed a barrier layer in the underbump metallization of flip-chip solder joints. The polarity effect induced by EM was observed to be significantly inhibited in the alloyed and composite solder joints. This indicates that the Sn-Ni, Sn-Co, Sn-Sb, and Cu₆Sn₅ intermetallic compounds may act as barriers to obstruct the movement of the dominant diffusion species along phase boundaries, which in turn improves the resistance to EM. However, Sb particles could induce crack formation and propagation that might lead to joint fracture.     

Design Envelopes and Optical Feature Extraction Techniques for Survivability of SnAg Lead-free Packaging Architectures Under Shock and Vibration

In this paper, a design-envelope approach based on optical feature extraction techniques has been investigated for drop and shock survivability of electronic packaging has been presented for 6-lead-free solder alloy systems. Solder alloy systems investigated include, Sn1Ag0.5Cu, Sn3Ag0.5Cu, Sn0.3Ag0.7Cu, Sn0.3Ag0.7Cu0.1Bi, Sn0.2Ag0.7Cu0.1Bi-0.1Ni, 96.5Sn 3.5Ag. Previously, digital image correlation (DIC) has been used for measurement of thermally induced deformation and material-characterization. In this paper, DIC has been used for transient dynamic measurements, and optical feature extraction. Board assemblies have been subjected to shock-impact in various orientations including the JEDEC zero-degree drop and the vertical free-drop. Transient deformation has been measured using both digital image correlation and the strain gages. Measurements have been taken on both the package and the board side of the assemblies. Accuracy of high-speed optical measurement has been compared with that from discrete strain gages. Package architectures examined include-flex ball-grid arrays, tape-array ball-grid arrays, and metal lead-frame packages. Explicit finite-element models have been developed and correlated with experimental data. Models developed include, smeared property models, and Timoshenko-beam models. The potential of damage identification and tracking for various solder alloys has been investigated. Data on identification of damage proxies for competing failure mechanisms at the copper-to-solder, solder-to-printed circuit board, and copper-to-package substrate has been presented. Design envelopes have been developed based on statistical pattern recognition. The design-envelope is intended for component integration to ensure survivability in shock and vibration environments at a user-specified confidence level.     

Electromigration-induced cracks in Cu/Sn3.5Ag/Cu solder reaction couple at room temperature

Electromigration (EM) behavior of Cu/Sn3.5Ag/Cu solder reaction couple was investigated with a high current density of 5× 103 A/cm2 at room temperature. One dimensional structure, copper wire/solder ball/copper wire SRC was designed and fabricated to dissipate the Joule heating induced by the current flow. In addition, thermomigration effect was excluded due to the symmetrical structure of the SRC. The experimental results in-dicated that micro-cracks initially appeared near the cathode interface between solder matrix and copper substrate after 474 h current stressing. With current stressing time increased, the cracks propagated and extended along the cathode interface. It should be noted that the continuous Cu6Sn5 intcrmetallic compounds (LMCs) layer both at the anode and at the cathode remained their sizes. Interestingly, tiny cracks appeared at the root of some long column-type Cu6Sn5 at the cathode interface due to the thermal stress.     

The Hillock Formation in a Cu/Sn-9Zn/Cu Lamella upon Current Stressing

The electromigration (EM) that occurs in a Cu/Sn-9Zn/Cu lamella was investigated for hillock formation at room temperature with a current density of 10³ A/cm² for up to 230 h. Hillocks and cavities grew in the middle of the bulk solder and at the cathode, respectively. The formation of hillocks was ascribed to a compressive stress resulting from the diffusion of Sn atoms driven by electromigration and Cu-Zn compound formation.     

Constitutive and Aging Behavior of Sn3.0Ag0.5Cu Solder Alloy

We describe double-lap shear experiments on Sn3.0Ag0.5Cu solder alloy, from which fits to Anand's viscoplastic constitutive model, power-law creep model, and to time-hardening primary-secondary creep model are derived. Results of monotonic tests for strain rates ranging from 4.02E-6 to 2.40E-3 s^-1, and creep response at stress levels ranging from 19.5 to 45.6 MPa are reported. Both types of tests were conducted at temperatures of 25degC, 75degC , and 125degC. Following an earlier study where Anand model and time hardening creep parameters for Sn3.8Ag0.7Cu and Sn1.0Ag0.5Cu solder alloys were reported, here we report power law model parameters so as to enable a comparison between all three alloys. Primary creep in Sn3.0Ag0.5Cu solder alloy is shown to be significant and are considered in addition to secondary creep and monotonic behavior. Aging influence on behavior is also shown to be significant. On the basis of experimental data, the following four aspects are discussed: 1) difference between testing on bulk versus joint specimen; 2) consistency between the creep and monotonic behaviors; 3) comparison against behaviors of Sn1.0Ag0.5Cu and Sn3.8Ag0.7Cu alloys as well as aganist Sn40Pb, 62Sn36Pb2Ag and 96.5Sn3.5Ag alloys; and 4) comparison of Sn3.0Ag0.5Cu and Sn3.8Ag0.7Cu relative to their aging response.     

Maximum entropy fracture model and its use for predicting cyclic hysteresis in Sn3.8Ag0.7Cu and Sn3.0Ag0.5 solder alloys

Appropriate constitutive, damage accumulation and fracture models are critical to accurate life predictions. In this study, we utilize the maximum entropy fracture model (MEFM) to predict and validate cyclic hysteresis in Sn3.8Ag0.7Cu and Sn3.0Ag0.5 solder alloys through a damage enhanced Anand viscoplasticity model. MEFM is a single-parameter, information theory inspired model that aims to provide the best estimate for accumulated damage at a material point in ductile solids in the absence of detailed microstructural information. Using the developed model, we predict the load drop during cyclic fatigue testing of the two chosen alloys. A custom-built microscale mechanical tester was utilized to carryout isothermal cyclic fatigue tests on specially designed assemblies. The resultant relationship between load drop and accumulated inelastic dissipation was used to extract the geometry and temperature-independent damage accumulation parameter of the maximum entropy fracture model for each alloy. The damage accumulation relationship is input into the Anand viscoplastic constitutive model, allowing prediction of the stress–strain hysteresis and cyclic load drop. The damage accumulation model is validated by comparing predicted and measured load drops after 55 and 85 cycles respectively for Sn3.8Ag0.7Cu and Sn3.0Ag0.5 solder alloys. The predictions agreed to within 10% and 20% of the experimental observations respectively for the two alloys. The damage enhanced Anand model developed in this study will enable the tracking of crack fronts during finite element simulations of fatigue crack initiation and propagation in complex solder joint geometries.     

Properties of Sn3.8Ag0.7Cu Solder Alloy with Trace Rare Earth Element Y Additions

In the current research, trace rare earth (RE) element Y was incorporated into a promising lead-free solder, Sn3.8Ag0.7Cu, in an effort to improve the comprehensive properties of Sn3.8Ag0.7Cu solder. The range of Y content in Sn3.8Ag0.7Cu solder alloys varied from 0 wt.% to 1.0 wt.%. As an illustration of the advantage of Y doping, the melting temperature, wettability, mechanical properties, and microstructures of Sn3.8Ag0.7CuY solder were studied. Trace Y additions had little influence on the melting behavior, but the solder showed better wettability and mechanical properties, as well as finer microstructures, than found in Y-free Sn3.8Ag0.7Cu solder. The Sn3.8Ag0.7Cu0.15Y solder alloy exhibited the best comprehensive properties compared to other solders with different Y content. Furthermore, interfacial and microstructural studies were conducted on Sn3.8Ag0.7Cu0.15Y solder alloys, and notable changes in microstructure were found compared to the Y-free alloy. The thickness of an intermetallic compound layer (IML) was decreased during soldering, and the growth of the IML was suppressed during aging. At the same time, the growth of intermetallic compounds (IMCs) inside the solder was reduced. In particular, some bigger IMC plates were replaced by fine, granular IMCs.     

Constitutive relations on creep for SnAgCuRE lead-free solder joints

Taking the most promising substitute of the Sn-3.8Ag-0.7Cu solder as the research base, investigations were made to explore the effect of rare earths (REs) on the creep performance of the Sn-3.8Ag-0.7Cu solder joints. The SnAgCu-0.1RE solder with the longest creep-rupture life was selected for subsequent research. Creep strain tests were conducted on Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints in the intermediate temperature range from 298 K to 398 K, corresponding to the homologous temperatures η=0.606, 0.687, 0.748, and 0.809 and η = 0.602, 0.683, 0.743, and 0.804, respectively, to acquire the relevant creep parameters, such as stress exponent and activation energy, which characterize the creep mechanisms. The final creep constitutive equations for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints were established, demonstrating the dependence of steady-state creep rate on stress and temperature. By correcting the apparent creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints from the experiments, the true creep-activation energy is obtained. Results indicated that at low stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the lattice self-diffusion activation energy, so the steady-state creep rates of these two solder joints are both dominated by the rate of lattice self-diffusion. While at high stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the dislocation-pipe diffusion activation energy, so the steady-state creep rates are dominated by the rate of dislocation-pipe diffusion. At low stress, the best-fit stress exponents n of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are 6.9 and 8.2, respectively, and the true creep-activation energy of them both is close to that of lattice self-diffusion. At high stress, it equals 11.6 and 14.6 for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints, respectively, and the true creep-activation energy for both is close to that of the dislocation-pipe diffusion. Thus, under the condition of the experimental temperatures and stresses, the dislocation climbing mechanism serves as the controlling mechanism for creep deformation of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints. The creep values of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are both controlled by dislocation climbing. Dislocation glide and climb both contribute to creep deformation, but the controlling mechanism is dislocation climb. At low stress, dislocation climbing is dominated by the lattice self-diffusion process in the Sn matrix and dominated by the dislocation-pipe diffusion process at high stress.     

Creep Properties of Composite Solders Reinforced with Nano- and Microsized Particles

In the present work the creep properties of Sn37Pb- and Sn0.7Cu-based composite solders reinforced with metallic nano- and microsized Cu and Ag particles have been studied. First, a series of volume percentages of reinforcements were selected to optimize the content of reinforcing particles. Then, the composite solder with optimum volume fraction of reinforcement particles, corresponding to the maximum creep rupture lifetime, was selected to investigate the effect of applied stress and temperature on the creep rupture lifetime of the composite solder joints. In the creep rupture lifetime test, small single-lap tensile-shear joints were adopted. The results indicate that composite solders reinforced with microsized particles exhibit better creep strengthening than composite solders reinforced with nanosized particles, although the mechanical tensile shear strength of composite solder joints reinforced with nanosized particles may be higher than those reinforced with microsized particles. Moreover, the creep strengthening action of the reinforcement particles is more obvious under conditions of lower applied stress or lower test temperature. Strengthening by metallic Cu or Ag reinforcement particles decreases with increasing temperature or applied stress. The Sn0.7Cu-based composite solder reinforced with microsized Ag particles is a low-cost lead-free solder that is easy to process and may have good market potential.     

温度对无铅焊锡膏活性剂与Sn0.3Ag0.7Cu反应规律的影响

以MPEG400为溶剂,将定量锡块放入添加不同活性剂的溶液中加热保温,保温时间为1 h,考察锡块的质量变化,以研究不同温度下活性剂与焊锡材料的反应程度,进而用SEM进行反应形貌验证。实验结果表明,苹果酸在150℃以后与Sn0.3Ag0.7Cu焊锡材料反应强烈;丙二酸在100℃时与Sn0.3Ag0.7Cu焊锡材料的反应强于其他温度,125℃以后,随着温度的升高,反应反而减弱;三乙醇胺与Sn0.3Ag0.7Cu焊锡材料之间有微弱反应,随温度升高,反应强度增大。探讨温度对无铅焊锡膏用活性剂与Sn0.3Ag0.7Cu焊锡材料之间反应规律的影响,对助焊剂的研究具有重要指导意义。     

Electromigration Behaviors and Effects of Addition Elements on the Formation of a Bi-rich Layer in Sn58Bi-Based Solders

This study investigates the electromigration (EM) behaviors and effects of the addition elements on the formation of a Bi-rich layer in Sn58Bi-based solders including Sn58Bi (SB), Sn58Bi0.5Ag (SBA) and Sn58Bi0.5Ag0.1Cu0.07Ni0.01Ge (SBACNG) solders. The EM tests were conducted at a relatively high temperature of 373 K and at a current density of 30 kA/cm². Although the dominant diffusing atom was Bi, hillocks were formed from Sn more easily than from Bi. The electrical resistance increased in the solder during the current stressing, and the dominant factor was attributed to the formation of a Bi-rich layer. SBACNG solder showed the highest resistance to the formation of a Bi-rich layer, followed by SBA, and then SB solder. The possible addition elements enhancing the resistance of SBACNG solder are Ag, Ni and Ge. The effects of the addition elements are summarized as follows: (1) Ag distributes in the Sn phase as Ag₃Sn intermetallic compounds (IMCs) that enhance the mechanical strength of Sn; (2) Ni distribution in Bi as Ni-Bi IMCs stabilizes Bi and suppresses its migration; and (3) Ge may distribute in Bi, stabilizing Bi, or Ge exists at the phase boundaries as a precipitate that inhibits Bi migration.     

An investigation of effects of Sb on the intermetallic formation in Sn-3.5Ag-0.7Cu solder joints

This study investigates the microstructural evolution and kinetics of intermetallic (IMC) formation in Sn-3.5Ag-0.7Cu lead-free solder joints with different percentages of Sb element, namely, Sn-3.5Ag-0.7Cu-xSb (x=0, 0.2, 0.5, 0.8, 1.0, 1.5, and 2.0). To investigate the elemental interdiffusion and growth kinetics of IMC formation, isothermal aging test is performed at temperatures of 100/spl deg/C, 150/spl deg/C, and 190/spl deg/C, respectively. Scanning electron microscope (SEM) is used to measure the thickness of intermetallic layer and observe the microstructural evolution of solder joint. The IMC phases are identified by EDX and XRD. Results show that some of the antimony powders are dissolved in the /spl beta/-Sn matrix (Sn-rich phase), some of them participate in the formation of Ag/sub 3/(Sn,Sb) and the rest dissolves in the Cu/sub 6/Sn/sub 5/ IMC layer. There is a significant drop in IMC thickness when Sb is added to 0.8 wt%. Over this amount the thickness of the IMC increases slightly again. The activation energy and growth rate of the IMC formation are determined. Results reveal that adding antimony in Sn-3.5Ag-0.7Cu solder system can increase the activation energy, and thus reduce the atomic diffusion rate, so as to inhibit the excessive growth of the IMC. The solder joint containing 0.8 wt% antimony has the highest activation energy. SEM images reveal that the number of small particles precipitating in the solder matrix increases with the increase in Sb composition. Based on the observation of the microstructural evolution of the solder joints, a grain boundary pinning mechanism for inhibition of the IMC growth due to Sb addition is proposed.     

Microstructure Evolution of Cu-Cored Sn Solder Joints Under High Temperature and High Current Density

This work investigated the microstructure evolution of Cu-cored Sn solder joints under high temperature and high current density. The Cu₆Sn₅ phase formed at both the Cu core/Sn interface and Cu wire/Sn interface right after reflow and grew with increasing annealing time, while the Cu₃Sn phase formed and grew at the Cu/Cu₆Sn₅ interfaces. Intermetallic compound (IMC) growth followed a linear relationship with the square root of annealing time due to a diffusion-controlled mechanism. Under high current density, the thickness of the interfacial IMCs of the Cu core/Sn interface at the cathode side increased and the Cu core/Sn interface at the anode side exhibited an irregular and serrated morphology with prolonged current stressing time. Finite-element simulation was carried out to obtain the distribution of current density in the solder joint. Since Cu has lower resistivity, the electrical current primarily selected the Cu core as its electrical path, resulting in current crowding at the Cu core and the region between the Cu core and Cu wire. Compared with the conventional solder joint, the electromigration (EM) lifetime of the Cu-cored solder joint was much longer.     

Development of lead-free wave soldering process

Lead-free wave soldering was studied in this work using a 95.5Sn/3.8Ag/0.7Cu alloy. A process DOE was developed, with three variables (solder bath temperature, conveyor speed, and soldering atmosphere), using a dual wave system. Four no-clean flux systems, including alcohol- and water-based types, were included in the evaluation. A specially designed "Lead-Free Solder Test Vehicle", which has various types of components, was used in the experiments. Both organic solderability preservative (OSP) and electroless nickel/immersion gold (Ni/Au, or ENIG) surface finishes were studied. Soldering performance (bridging, wetting and hole filling) was used as the responses for the DOE. In addition, dross formation was measured at different solder bath temperatures and atmospheres. Dross formation with Sn/Ag/Cu bath was compared to that with eutectic Sn/Pb bath. Regarding the connector-type component, a pad design giving the best soldering performance was evaluated based on the DOE results. Finally, a confirmation run with the optimum flux and process parameters was carried out using the Sn/Ag/Cu solder, and a comparative run was made with the Sn/Pb solder alloy and a no-clean flux used in production. The soldering results between the two runs indicate that with optimum flux and process parameters, it is possible to achieve acceptable process performance with the Sn/Ag/Cu alloy.     

Surface Morphology of Sn-Rich Solder Interconnects After Electrical Loading

Morphological changes from electromigration were examined on microsized Sn-Ag-Cu, pure Sn, and single-crystal Sn solder interconnects. It was found that both grain structure and alloying had a strong influence on the form of electromigration damage. In polycrystal Sn, grain boundary grooves were the primary form of electromigration damage, while in single-crystal Sn interconnects wavy surface relief appeared following electromigration. Alloying with Ag and Cu encouraged formation of Sn hillocks and Cu₆Sn₅ intermetallic compound (IMC) segregation. The grain boundary grooves were related to the divergence of the vacancy concentration at grain boundaries, which induced Sn grain tilting or sliding. Removal of the grain boundaries in the single-crystal interconnect made surface diffusion the primary electromigration mechanism, resulting in wavy surface relief after long electromigration time. In Sn-Ag-Cu alloy, directional flow of Cu caused Cu₆Sn₅ IMC segregation, which produced large compressive stress, driving the stressed grains to grow into hillocks.