Effect of Stress State on Growth of Interfacial Intermetallic Compounds Between Sn-Ag-Cu Solder and Cu Substrates Coated with Electroless Ni Immersion Au

Excessive intermetallic compound (IMC) growth in solder joints will significantly decrease the reliability of the joints. IMC growth is known to be influenced by numerous factors during the component fabrication process and in service. It is reported that, other than temperature and holding time, stress can also influence the IMC growth behavior. However, no existing method can be used to study the effect of stress state on IMC growth in a controlled manner. This paper presents a novel method to study the effect of stress on interfacial IMC growth between Sn-Ag-Cu solder and a Cu substrate coated with electroless Ni immersion Au (ENIG). A C-ring was used and in-plane bending induced tensile and compressive stresses were applied by tightening the C-ring. Results revealed that in-plane compressive stress led to faster IMC growth as compared with in-plane tensile stress.     

Microstructures and properties of Sn-Ag-Cu lead-flee solder alloys containing La

Trace amounts of La were utilized to improve the melting behaviors, microstructures, tensile properties and microhardness of Sn-3.0Ag-0.5Cu lead-free solder alloy. La has little effect on the melting behavior of Sn-3.0Ag-0.5Cu alloy according to the differential thermal analysis （DTA） tests. The X-ray diffraction （XRD） patterns show that β-Sn, Ag3Sn and Cu6Sn5 coexist in the as-cast solder alloys and LaSn3 phases emerge when adding 0.4% La. The microstructures modified by La are more uniform and much finer than that of baseline alloy, and the coarse LaSn3 particles with complex dendrites are observed by optical microscopy （OM） and scanning electron microscopy （SEM） when the addition of La is up to 0.4%. The composition of the LaSn3 phases is identified by energy-dispersive spectroscopy （EDS）. There are considerable improvements in mechanical properties with 0.05% and 0.1% addition, but degenerations by adding 0.4%La. The Vickers microhardness of β-Sn and eutectic area are both enhanced with the addition of La and the microhardness of LaSn3 is much larger than those of β-Sn and eutectic area.     

不同剪切速率下锡银铜系/镍金焊点的断裂行为研究

对锡银铜系高可靠性焊料合金的研究虽然已经较为广泛,但是缺乏对其焊点在不同应变速率下的剪切强度和断裂模式的研究。本文将牌号为SAC305和Innolot不同成分的锡银铜系焊料合金锡膏印刷在镍金镀层(Ni(P)/Au)上回流成BGA焊点,采用DAGE 4000 HS焊接强度测试仪进行不同剪切速率下的剪切性能测试,并对其剪切曲线、剪切强度及断裂能进行计算和分析,再采用金相显微镜和扫描电子显微镜(SEM)对焊点界面微观结构及断口进行表征分析。结果表明：合金本身成分的差异导致焊点界面金属间化合物层(IMC层)厚度和分布存在差异,随着剪切速率的增加,SAC305和Innolot合金焊点的强度总体上都随之增加,且焊点的断裂模式由焊点基体内部的韧性断裂向界面金属间化合物脆性断裂发生转变,Innolot合金由于其他金属元素添加导致的强化作用使得其剪切强度得到较大提升而塑性损伤。     

雾化介质对无铅焊锡粉末形貌及粒度分布的影响

利用自行设计的超音速雾化制粉装置,研究不同雾化介质对SnAgCu系无铅焊锡粉末有效雾化率、粒度分布及球形度的影响。结果表明:在一定雾化条件下,氦气雾化粉末具有最高的有效雾化率,最佳的球形度、表面光滑度及粒度分布;氮气雾化的粉末具有较好的综合性能;与氦气、氮气相比,氩气雾化粉末综合性能较差;空气雾化粉末雾化率较高,但粉末较粗、表面粗糙。     

Sn-07Cu-xAg-yBi无铅钎料润湿性试验

为降低Sn-Ag-Cu系无铅钎料中Ag的含量以减少钎料的成本,对Sn-xAg-0.7Cu(x=0.5,1.0,1.5,3.0)及Sn-0.5Ag-0.7Cu--yBi(y=1,3,5)无铅钎料在不同钎焊温度和不同钎剂活性条件下,进行了钎焊润湿性试验,分析了不同Ag含量、Bi含量对润湿性的影响.试验结果表明:在Sn-Ag-Cu系无铅钎料中,随着Ag含量的降低,钎抖的润湿性降低;随着Bi含量的增加,钎料的润湿性提高.     

纳米压痕法测量Sn-Ag-Cu无铅钎料BGA焊点的力学性能参数

对Sn-3．0Ag-0．5Cu无铅体钎料和Sn-4．0Ag-0．5Cu无铅钎料BGA(ball grid array)焊点进行了Berkovich纳米压痕法实验,通过改变不同的加载速率研究了钎料的蠕变特征．钎料压痕载荷位移曲线蠕变部分表现出了明显的加载速率相关性．基于Oliver-Pharrr法确定的体钎料和BGA焊点的Young’S模量分别为9．3和20GPa．基于压痕做功概念确定的体钎料和BGA焊点的应变速率敏感指数分别为0．1111和0．0574．钎料的力学性能有着明显的尺寸效应。     

Interfacial reactions of Sn-Ag-Cu solders modified by minor Zn alloying addition

The near-ternary eutectic Sn-Ag-Cu alloys have been identified as leading Pb-free solder candidates to replace Pb-bearing solders in microelectronic applications. However, recent investigations on the processing behavior and solder joints reliability assessment have revealed several potential reliability risk factors associated with the alloy system. The formation of large Ag₃Sn plates in Sn-Ag-Cu joints, especially when solidified in a relatively slow cooling rate, is one issue of concern. The implications of large Ag₃Sn plates on solder joint performance and several methods to control them have been discussed in previous studies. The minor Zn addition was found to be effective in reducing the amount of undercooling required for tin solidification and thereby to suppress the formation of large Ag₃Sn plates. The Zn addition also caused the changes in the bulk microstructure as well as the interfacial reaction. In this paper, an in-depth characterization of the interfacial reaction of Zn-added Sn-Ag-Cu solders on Cu and Au/Ni(P) surface finishes is reported. The effects of a Zn addition on modification of the interfacial IMCs and their growth kinetics are also discussed.     

Effect of silver content on thermal fatigue life of Sn-xAg-0.5Cu flip-chip interconnects

The thermal fatigue properties of Sn-xAg-0.5Cu (x=1, 2, 3, and 4 in mass%) flip-chip interconnects were investigated to study the effect of silver content on thermal fatigue endurance. The solder joints with lower silver context (x=1 and 2) had a greater failure rate compared to those with higher silver content (x=3 and 4) in thermal fatigue testing. Cracks developed in the solders near the solder/chip interface for all joints tested. This crack propagation may be mainly governed by the nature of the solders themselves because the strain-concentrated area was similar for tested alloys independent of the silver content. From the microstructural observation, the fracture was a mixed mode, transgranular and intergranular, independent of the silver content. Higher silver content alloys (x=3 and 4) had finer Sn grains before thermal cycling according to the dispersion of the Ag₃Sn intermetallic compound, and even after the cycling, they suppressed microstructural coarsening, which degrades the fatigue resistance. The fatigue endurance of the solder joints was strongly correlated to the silver content, and solder joints with higher silver content had better fatigue resistance.     

Effect of rare earth element addition on the microstructure of Sn-Ag-Cu solder joint

The effects of minimal rare earth (RE) element additions on the microstructure of Sn-Ag-Cu solder joint, especially the intermetallic compounds (IMCs), were investigated. The range of RE content in Sn-Ag-Cu alloys varied from 0 wt.% to 0.25 wt.%. Experimental results showed that IMCs could be dramatically repressed with the appropriate addition of RE, resulting in a fine microstructure. However, there existed an effective range for the RE addition. The best RE content was found to be 0.1 wt.% in the current study. In addition to the typical morphology of Ag₃Sn and Cu₆Sn₅ IMCs, other types of IMCs that have irregular morphology and uncertain constituents were also observed. The IMCs with large plate shape mainly contained Ag and Sn, but the content of Ag was much lower than that of Ag₃Sn. The cross sections of Cu₆Sn₅ IMCs whiskers showed various morphologies. Furthermore, some eutectic-like structures, including lamellar-, rod-, and needle-like phases, were observed. The morphology of eutectic-like structure was related to the RE content in solder alloys. When the content of RE is 0.1 wt.%, the needle-like phase was dominant, while the lamellar structure prevailed when the RE content was 0.05 wt.% or 0.25 wt.%. It is suggested that the morphology change of the eutectic-like structure directly affects the creep properties of the solder joint.     

Effect of Cu concentration on the reactions between Sn-Ag-Cu solders and Ni

The reaction between the Sn-Ag-Cu solders and Ni at 250°C for 10 min and 25 h was studied. Nine different Sn-Ag-Cu solders, with the Ag concentration fixed at 3.9 wt.% and Cu concentrations varied between 0.0–3.0 wt.%, were used. When the reaction time was 10 min, the reactions strongly depended on the Cu concentration. At low-Cu concentrations (≦0.2 wt.%), only a continuous (Ni_1−xCu_x)₃Sn₄ layer formed at the interface. When the Cu concentration increased to 0.4 wt.%, a continuous (Ni_1−xCu_x)₃Sn₄ layer and a small amount of discontinuous (Cu_1−yNi_y)₆Sn₅ particles formed at the interface. When the Cu concentration increased to 0.5 wt.%, the amount of (Cu_1−yNi_y)₆Sn₅ increased and (Cu_1−yNi₆)₆Sn₅ became a continuous layer. Beneath this (Cu_1−yNi_y)₆Sn₅ layer was a very thin but continuous layer of (Ni_1−xCu_x)₃Sn₄. At higher Cu concentrations (0.6–3.0 wt.%), (Ni_1−xCu_x)₃Sn₄ disappeared, and only (Cu_1−yNi_y)₆Sn₅ was present. The reactions at 25 h also depended strongly on the Cu concentration, proving that the strong concentration dependence was not a transient phenomenon limited to a short reaction time. The findings of this study were rationalized using the Cu-Ni-Sn isotherm. This study shows that precise control over the Cu concentration in solders is needed to produce consistent results.