Formation of bulk Ag<Subscript>3</Subscript>Sn intermetallic compounds in Sn-Ag lead-free solders in solidification

A series of Sn-Ag solders were prepared by arc melting and their phase evolution was investigated as a function of cooling rates. It was found that bulk Ag₃Sn intermetallic compounds (IMCs) separated out only in the slowly cooled Sn-4.0Ag solder. This would be explained by the strong kinetic undercooling, arising from the rapid cooling conditions, which leads to the actual eutectic point shifts in the direction of higher Ag concentration. Thus, the eutectic and hypereutectic alloys experience a metastable hypoeutectic solidification route instead. All formed fractions of bulk Ag₃Sn IMCs in solders, measured by quantitative microstructural analysis and thermal analysis, are larger than those predicted by the equilibrium phase diagram. The reasoning for this could be attributed to fine Ag₃Sn phases, which cling to the primary Ag₃Sn crystal during the eutectic reaction for their matching crystalline orientation relationship. Furthermore, the fraction of bulk Ag₃Sn IMCs increases gradually with increasing the cooling rate in the slowly cooled Sn-4.0Ag alloy, which fits with the prediction of eutectic solidification theory: the increase of cooling rate would decrease the surface energy of fine Ag₃Sn particles and primary Ag₃Sn crystal, and make fine Ag₃Sn particles cling to primary Ag₃Sn crystal easily to form bulk Ag₃Sn IMCs.     

Lead-free solder reinforced with multiwalled carbon nanotubes

In this study, varying weight percentages of multiwalled carbon nanotubes were successfully incorporated into 95.8Sn-3.5Ag-0.7Cu solder to synthesize novel lead-free composite solders. The composite solders were synthesized using a powder metallurgy route consisting of blending, compaction, sintering, and extrusion. The extruded materials were then characterized for their physical, thermal, and mechanical properties. With the addition of increasing weight percentage of carbon nanotubes, the composite solders experienced a corresponding decrease in density values and an improvement in wetting properties. The melting temperatures of the composite solders were found to be unchanged with additions of carbon nanotubes. However, improvements in the mechanical properties, in terms of microhardness and tensile properties, were observed with increasing weight percentages of carbon nanotubes.     

Nanoparticles of the Lead-free Solder Alloy Sn-3.0Ag-0.5Cu with Large Melting Temperature Depression

Due to the toxicity of lead (Pb), Pb-containing solder alloys are being phased out from the electronics industry. This has lead to the development and implementation of lead-free solders. Being an environmentally compatible material, the lead-free Sn-3.0Ag-0.5Cu (wt.%) solder alloy is considered to be one of the most promising alternatives to replace the traditionally used Sn-Pb solders. This alloy composition possesses, however, some weaknesses, mainly as a result of its higher melting temperature compared with the Sn-Pb solders. A possible way to decrease the melting temperature of a solder alloy is to decrease the alloy particle size down to the nanometer range. The melting temperature of Sn-3.0Ag-0.5Cu lead-free solder alloy, both as bulk and nanoparticles, was investigated. The nanoparticles were manufactured using the self-developed consumable-electrode direct current arc (CDCA) technique. The melting temperature of the nanoparticles, with an average size of 30 nm, was found to be 213.9°C, which is approximately 10°C lower than that of the bulk alloy. The developed CDCA technique is therefore a promising method to manufacture nanometer-sized solder alloy particles with lower melting temperature compared with the bulk alloy.     

电子组装中助焊剂的选择 (续一)

助焊剂是电子组装技术中最重要的材料之一,其通过去除材料表面氧化膜及污染物等,保证钎料和母材之间很好的润湿,形成良好焊点。介绍了助焊剂的作用与组成、分类与性能要求以及涂覆方式和工艺评定,有助于实际生产中助焊剂的选择与使用。     

钠过量对铌酸钾钠基陶瓷结构及性能的影响

用常规氧化物固溶法制备了(Na0.520+xK0.480)0.915Li0.085NbO3(x=0～0.025)无铅压电陶瓷,研究了过量Na的掺杂量对其烧结温度、微观结构及压电性能的影响。结果表明,过量Na掺杂在烧结过程中促进液相的产生,导致样品烧结温度显著降低。室温下样品均为四方结构。随x的增加,样品的斜方-四方相变温度tO-T趋向低温。当x从0.010增大到0.025时,Qm从131提高到238,tanδ和d33分别从0.034和125pC/N下降至0.012和105pC/N,表明过量Na实际上起到了"硬性掺杂"的作用。     

电子组装中助焊剂的选择(待续)

助焊剂是电子组装技术中最重要的材料之一,通过其去除材料表面氧化膜及污染物等,保证钎料和母材之间很好地润湿,形成良好焊点.介绍了助焊剂的作用与组成、分类与性能要求以及涂覆方式和工艺评定,有助于实际生产中助焊剂的选择与使用.     

Evaluation of Cu-bumps with lead-free solders for flip-chip package applications

Low cost electroplated Cu-bump with environmental friendly Sn solder was developed for flip-chip applications. The seed layer used was Ti/WN_x/Ti/Cu where WN_x was used as the Cu diffusion barrier and Ti was used to enhance the adhesion between bump and the chip pad. Thick negative photoresist (THB JSR-151N) with a high aspect ratio of 2.4 was used for electroplating of copper bump and Sn solder. The Sn solder cap was reflowed at 225° for 6 min at N₂ atmosphere. No wetting phenomenon was observed for the Sn solder as evaluated by energy-dispersed spectroscopy (EDS). The Cu-bump with Ti/WN_x/Ti/Cu seed layer not only have higher shear force than the Cu-bump with Ti/Cu seed layer but also has higher resistance to fatigue failure than the Au, SnCu, SnAg bumps.     

Lead-Free Bumping Using an Alternating Electromagnetic Field

A novel lead-free bumping technique using an alternating electromagnetic field (AEF) was investigated. Lead-free solder bumps reflowed onto copper pads through AEF have been achieved. A comparison was conducted between the microstructures of the lead-free solder joints formed by the conventional thermal reflow and AEF reflow. Keeping the substrate temperature lower than that of the solder bumps, AEF reflow successfully created metallurgical bonding between the lead-free solders and metallizations through an interfacial intermetallic compound (IMC). The AEF reflow could be finished in several seconds, much faster than the conventional hot-air reflow. Considering the morphology of the interfacial Cu₆Sn₅ IMC, a shorter heating time above the melting point would be a better choice for solder joint reliability. The results show that AEF reflow is a promising localized heating soldering technique in electronic packaging.     

基于交互作用的无铅波峰焊接正交试验分析

通过开展无铅波峰焊正交试验,以大量的试验数据为依据确定了无铅电子组装工艺参数,建立了无铅波峰焊工艺参数间的关系模型,考察了不同试验因素的组合对焊点缺陷率的影响,对工艺参数进行了优化,并得到了各因素对实验结果重要性的影响顺序及优化的参数结果。     

无铅波峰焊设备的特点

相对于传统的Sn-Pb焊料,无铅焊料需要较高的焊接温度,而且润湿性差,另外免清洗助焊剂和水溶性助焊剂的固体含量低,活性温度高和活性区间窄。无铅焊料和助焊剂的特性决定了无铅波峰焊设备在结构和材料选用上有很大不同。通过对无铅波峰焊设备的各个部分的特点进行分析,为传统的旧波峰焊设备的改造提供参考。