Ag元素含量对SnAgCuX无铅钎料性能的影响

分析了SnAgCuX系无铅钎料中Ag异元素含量的变化对熔化温度、润湿性能的影响,同时研究了时效前和高温时效后钎焊接头的抗剪强度和显微组织,其中X包含Ni,P和Ce三种元素.结果表明,添加微量X元素能够在一定程度上弥补snAgCu钎料中Ag元素含量的降低引起的性能下降.X元素的添加对SnAgCu钎料的熔化温度影响不大,但能改善钎料合金的润湿性能,提高钎焊接头的抗剪强度,并抑制时效引起的接头强度下降.这与微量X元素的添加改善了钎料的显微组织和金属间化合物的形貌有很大的关系.     

Predicting the Drop Performance of Solder Joints by Evaluating the Elastic Strain Energy from High-Speed Ball Pull Tests

Despite being expensive and time consuming, board-level drop testing has been widely used to assess the drop or impact resistance of the solder joints in handheld microelectronic devices, such as cellphones and personal digital assistants (PDAs). In this study, a new test method, which is much simpler and quicker, is proposed. The method involves evaluating the elastic strain energy and relating it to the impact resistance of the solder joint by considering the Young’s modulus of the bulk solder and the fracture stress of the solder joint during a ball pull test at high strain rates. The results show that solder joints can be ranked in order of descending elastic strain energy as follows: Sn-37Pb, Sn-1Ag-0.5Cu, Sn-3Ag-0.5Cu, and Sn-4Ag-0.5Cu. This order is consistent with the actual drop performances of the samples.     

Influence of interfacial reaction between molten SnAgCu solder droplet and Au/Ni/Cu pad on IMC evolution

Intermetallic compounds（IMC） formed at Sn-Ag-Cu solder droplet/pad interface during wetting reaction were investigated. Comparative studies of the IMC evolution during reflow and aging were also conducted. The results show that the wetting reaction between molten solder droplet and pad leads to the formation of Au-Sn compound at interface, but Au element is not fully consumed during wetting reaction. After reflow, all Au layer disappears from the interface, Au element is dissolved into solder and Au-Sn intermetallic compounds are precipitated in the bulk. Reaction between Ni layer and Sn-Ag-Cu solder leads to the formation of （CuxNi1-x）6Sn5 layer at interface during reflow. According to the thermodynamic-kinetic of interfacial reaction, the wetting reaction at solder droplet/pad interface influences the phase selectivity of IMC evolution during reflow and aging process.     

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.     

Degradation of solderability of electroless nickel by phosphide particles

Wetting of electroless nickel by the eutectic SnAgCu solder alloy was investigated in the as-deposited and annealed conditions. In the as-deposited state, P content in the nickel had a minimal effect on the solderability of the electroless nickel. Upon annealing, numerous nickel phosphide particles precipitated out of the electroless nickel with a high-P content. The presence of these phosphide precipitates reduced the solderability of the electroless nickel both in terms of the wetting force and wetting kinetics. While weakening in the wetting force is related to the inferior contact condition, the slower wetting kinetics is explained in terms of the reduction in the dissolution rate of the electroless nickel from non-soluble phosphide particles.     

Mechanical and microstructural properties of SnAgCu solder joints

Mechanical and microstructural properties of SnAgCu solder joints with hypoeutectic, eutectic and hypereutectic compositions were studied. Eutectic SnPb joints were used as the reference. Reflowed lap shear specimens made of FR-4 glass epoxy printed circuit boards with OSP and NiAu surface finishes were used in the tests. Mechanical properties and microstructural features of the joints were examined in the as-reflowed condition and after isothermal aging at 85 °C for 1000 h. Both the composition and PCB surface finish had a notable effect on the mechanical behaviour of the SnAgCu solder joints. The shear strength value of SnAgCu solder joints was mainly dependent on the size and distribution of Ag₃Sn dispersions. The coarseness of the dispersions depends strongly on the amount of Ag in the solder alloy, the cooling rate after the reflow and the aging history of the solder joints.     

Sn3.0Ag0.5Cu0.05Cr/Cu焊点界面IMC层热时效形貌及生长行为研究

研究了Sn3.0Ag0.5Cu0.05Cr焊料(SACCr)制成的Cu/Solder/Cu焊点在150℃时效0、168、500及1000 h下界面金属间化合物(IMC)层的形貌及生长行为,并与Sn3.0Ag0.5Cu(SAC)焊料的焊点进行了比较。结果表明,相对于SAC的焊点,SACCr中弥散或固溶分布的微量Cr延缓了焊点界面IMC层的生长。时效时间越长,Cr的阻抑效果越明显。150℃时效1000 h的Cu/SACCr/Cu焊点界面IMC层的平均厚度是Cu/SAC/Cu的45%,仅为5.13μm。     

纳米氧化物颗粒掺杂对SnAgCu无铅钎料性能的影响

研究了纳米颗粒掺杂对SnAgCu无铅钎料的焊接工艺、润湿性以及显微组织的影响.结果表明,不同的纳米颗粒掺杂对钎料的焊接工艺有不同的要求.纳米颗粒掺杂能够改善焊料的润湿性,其加入也能明显细化焊点的显微组织.纳米颗粒粒径越小,对焊料的性能改善作用越大.     

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.     

Comparative study of microstructures and properties of three valuable SnAgCuRE lead-free solder alloys

Lead-free solders with excellent material properties and low cost are essential for the electronics industry. It has been proved that mechanical properties of SnAgCu alloys can be remarkably improved with a minute addition of rare earth (RE) elements. For comparison and optimization, three valuable solder candidates, Sn3.8Ag0.7Cu0.05RE, Sn3Ag0.5Cu0.05RE, and Sn2.9Ag1.2Cu0.05RE, were chosen due to the excellent properties of their own SnAgCu basic alloys. Wetting properties, melting temperature, bulk tensile properties, and joint tensile and shear properties were investigated. In addition, the microstructures of solder joints were observed and the effects of microstructure on mechanical properties were analyzed. Experimental results indicated that the tensile and shear strengths of solder joints were decreased from Sn3.8Ag0.7Cu0.05RE, Sn2.9Ag1.2Cu0.05RE, to Sn3Ag0.5Cu0.05RE, in order. Such difference in mechanical properties could be attributed to the influence of slightly coarse or strong Cu₆Sn₅ scallops in the reaction layer as well as superior eutectic network and large volume percentage of large primary intermetallic compounds (IMCs) inside the solder joints. It is also suggested that the size and volume percentage of large primary IMCs inside the solder be controlled. In addition, serration morphology was observed at the edge of large primary and eutectic IMCs in the three solder joints, which could be related to the content of Ag, Cu, and RE. The serration morphology was proved to be beneficial to mechanical properties theoretically. Furthermore, the three alloys investigated possessed similar wetting properties, melting temperatures, and bulk tensile properties.