Cu-Sn互连界面的尺寸效应及微力学特性

发布时间：2018-04-11 14:56

本文选题：体积收缩效应 + IMC垂直生长　；参考：《华中科技大学》2015年博士论文

【摘要】：Sn基焊料在电子器件中被广泛地用于连接器件与基板,以提供所需的机械支撑和电信号通路。可靠互连焊点的性能主要取决于在回流中熔化的焊料与固态基板间的反应。此反应在焊料/基板界面所生成的IMC层会在后续的器件服役期间或者高温老化期间持续生长,导致焊点中的IMC比例不断上升。由于IMC层(如Cu6Sn5和Cu3Sn)的力学性能,例如杨氏模量、硬度等,与焊点的焊料基体和被连接的金属表面(如基板焊盘、引脚)有很大差别。因此,焊点中IMC比例的上升会导致焊点在外载荷作用下内部变形明显不协调,这会引起重要的互连焊点可靠性问题。另外,IMC生长过程中同时伴随着体积收缩。在微小焊点中,由于IMC的比例较高,IMC生长引起的体积收缩会进一步导致焊点的尺寸变化、残余应力、影响焊点的宏观变形行为以及宏观断裂行为。在过去的几十年中,虽然有大量的关于界面IMC层以及其对焊点可靠性影响的研究工作。但是,关于由IMC层生长导致的体积收缩所引起的焊点各种可靠性问题的研究,文献中少有报导。本文研究了Sn基无铅焊点在老化后的IMC生长以及由IMC生长导致的体积收缩和由于体积收缩导致的焊点界面残余应力的演化。另外,还对IMC层生长伴随的体积收缩对焊点整体的压蠕变变形和拉伸断裂行为的影响进行了研究和分析。主要研究成果如下：研究了Sn-1%Cu/Cu焊点中界面处沿扩散方向以及垂直于扩散方向的IMC生长。在175℃C老化之后,Cu6Sn5、Cu3Sn层的水平生长与老化时间的关系为h1MC=0.27t1/2+4.6,hcu6Sn5=0.16t1/2+4.1和hCu3Sn=0.17t1/2,其中t为老化时间,单位为h；h为IMC层厚度,单位为gm。在相同老化温度下,垂直IMC的高度与老化时间呈抛物线规律,其关系式为y=0.11(?)t,其中t为老化时间,单位为h,y为垂直IMC的高度,单位为gm。另外,垂直IMC为两层结构,外层为Cu6Sn5内部为Cu3Sn,但是CU3Sn先于Cu6Sn5形成。因此,IMC的垂直生长中Cu6Sn5和Cu3Sn的形成顺序与水平IMC层中两种IMC的形成顺序相反。老化过程中,IMC沿两个方向的生长与老化时间均呈抛物线关系,这表明两个方向的生长均是由扩散主导的。但是,由于相对较长的扩散距离,IMC垂直生长速率明显低于水平生长速率本文还对由界面IMC层生长引起的焊点体积收缩进行了研究。在不同的老化时间之后,使用纳米压痕系统对经过特殊设计的试样表面的形貌进行测量。通过测量发现,在175℃老化1132.5h后,焊点的塌陷可以达到1.2μm。并且,焊点的塌陷与老化时间在本文的实验条件下符合△h=-0.031×(?)的关系。由此得到的尺寸变化系数(焊点塌陷高度与IMC层的厚度比)为αexperiment=、0.114。假设Cu6Sn5和Cu3Sn均为各向同性并且致密的结构,焊点塌陷的理论值为△htotal=-(0.04-0.004x)(?)t(其中x为由Cu3Sn生成的Cu3Sn的体积占总体积的比例；t为老化时间,单位为h)。因此,尺寸变化系数的理论值为αideal=-0.147+0.0147x。通过比较可以得出,焊点塌陷的理论计算结果与实验测得的结果相符合。焊点中IMC层生长导致的体积收缩极有可能受限于相邻的焊料和基板,这可能导致焊料/Cu界面残余应力的产生。因此,老化后,在Sn-1%Cu焊料、界面Cu6Sn5层、界面Cu3Sn层以及Cu基板进行控制深度的纳米压痕实验,以研究焊点中残余应力随老化时间的演变。实验结果显示焊料/Cu界面不同部分的应力状态与实际位置以及成分有关：(1)老化过程中,Cu3Sn/Cu界面附近的Cu基板中央以及边缘均处于压应力状态,其平均值为560MPa；(2)焊料/Cu6Sn5界面附近的焊料中央部分和边缘的应力为70MPa和90MPa；(3)老化后,界面Cu6Sn5层中的压应力在中央和边缘分别上升至4GPa和3GPa；(4)相对地,在老化过程中,Cu3Sn层内在中央和边缘的拉伸应力均有上升,分别达到了1.7GPa和0.5GPa。从焊点中相邻两部分的应力-老化时间(S-t)曲线的对比结果中,可以得出：(1)界面Cu6Sn5层和相邻的焊料得到的S-t曲线没有明显的关联；(2)在焊点中央,Cu基板和界面CU3Sn层内的应力随着老化时间呈相反的趋势演变,这表明焊点中央的Cu3Sn层主要受相邻的Cu基板的约束；(3)焊点边缘的Cu3Sn层主要由相邻的Cu3Sn层约束。通过搭建蠕变实验装置,对SAC305/Cu焊点的压蠕变行为进行了初步研究。通过研究发现,当压应力为17.8～22.9MPa、温度为433～463K时,SAC305/Cu焊点的稳态蠕变应变为0.6～1.4%,对应的稳态压蠕变变形为2.5～5.8μm。在该条件下Sn-1%Cu/Cu焊点稳态蠕变过程中,界面处IMC层的厚度增加约1μm左右。由于IMC生长引起的高度变化为0.05～0.16μm,占稳态压蠕变过程中焊点高度下降总量的1～-4%。因此,在焊点稳态蠕变过程中,IMC生长伴随的体积收缩所引起的焊点高度变化对实验测量得到的焊点蠕变变形总量的影响可以忽略不计。通过在FIB制备的Cu6Sn5和Cu3Sn微柱上进行微悬臂梁测试,研究了Sn-1%Cu/Cu界面处的Cu6Sn5层和Cu3Sn层的拉伸断裂行为。在微悬臂梁测试中,Cu6Sn和Cu3Sn微柱在外加载荷条件下一直保持弹性变形,直至断裂。测试后,由Cu6Sn5微柱的断口形貌可知Cu6Sn5的主要断裂模式为穿晶断裂和沿晶断裂。而Cu3Sn微柱的断裂仅存在沿晶断裂。IMC微柱断裂模式的不同主要是由界面Cu6Sn5层和Cu3Sn层的微观组织结构决定的。另外,通过Cu/SAC305/Cu焊点的拉伸实验,发现在老化前焊点的断裂模式为韧性断裂,断裂强度为70.8±9.0MPa。当IMC颗粒沿焊料晶界处集中时,由于IMC生长伴随的体积收缩会引起应力和缺陷集中,导致焊点的拉伸强度下降至58.9±6.8MPa。当焊点老化81h后,焊点从IMC层中断裂,并且断裂强度下降明显,为57.8±8.4MPa。Cu/SAC305/Cu焊点的断裂模式和断裂强度的变化是由于IMC颗粒的特殊分布或者IMC生长伴随的体积收缩导致IMC层中应力和孔洞的积累导致的。通过对比,发现IMC微柱断裂强度远高于焊点在IMC层中断裂时的断裂强度。这证明IMC生长过程中伴随的体积收缩所导致的应力集中和孔洞等缺陷可以明显降低焊点整体的可靠性。
[Abstract]:Sn based solders in electronic devices are widely used for connecting devices and substrate to provide the required mechanical support and electrical signal pathway. Reliable performance of solder joints mainly depends on the melting in the reflow solder in solid state reaction between the substrate and the reaction at the solder / IMC interface layer substrate generated will be served in the follow-up period or device during high temperature aging continued growth, resulting in the solder joints of IMC. Due to the rising proportion of the IMC layer (such as Cu6Sn5 and Cu3Sn) on the mechanical properties, such as young's modulus, hardness, and solder matrix of solder joints and the metal surface are connected (such as substrate pads, pins) are very different. Therefore, the rising proportion of IMC in the solder joints will lead solder joints under external load internal deformation is obviously not harmonious, it will cause the solder joint reliability issues important. In addition, the IMC growth process is accompanied by volume contraction in the micro. In small solder joints, due to the higher proportion of IMC, the volume contraction caused by IMC growth will come in.
Step size change caused the solder joint, residual stress, deformation behavior of solder joints and the influence of macro fracture behavior. In the past few decades, although there are a lot of research work on the interface of IMC layer and its influence on the reliability of solder joints. However, the research on the reliability of various solder joint by IMC layer growth volume contraction induced by the cause, the literature reports are very few.
This paper studies the Sn based lead-free solder joints after aging IMC growth and shrinkage caused by IMC growth and the evolution of the interface of solder joint residual shrinkage caused by stress. In addition, effects of IMC layer growth with volume shrinkage on joint overall compressive creep deformation and fracture behavior were studied and analyzed the main research results are as follows:
The interface of Sn-1%Cu/Cu solder joints along the direction perpendicular to the direction of diffusion and diffusion of the growth of IMC. After Cu6Sn5, C 175 degrees of aging, the relationship between Cu3Sn level growth and aging time of h1MC=0.27t1/2+4.6, hcu6Sn5=0.16t1/2+4.1 and hCu3Sn=0.17t1/2, where t is the aging time, the unit is h; h is IMC thickness, unit is GM. at the same aging temperature, aging time and vertical height of IMC a parabolic law and its relationship to y=0.11 (?) t, where t is the aging time, the unit is h, y is the vertical height of IMC, the unit is GM.
In addition, the vertical IMC is divided into two layers, the outer layer is Cu6Sn5 internal Cu3Sn, but CU3Sn prior to Cu6Sn5 formation. Therefore, the formation of two kinds of IMC sequence and the level of IMC layer in the form of Cu6Sn5 and Cu3Sn in reverse order vertical growth of IMC. In the process of aging time, IMC along two directions of growth and aging were a parabolic relation, which means that the two direction of the growth is dominated by diffusion. However, due to the diffusion of a relatively long distance, vertical IMC growth rate was significantly lower than the level of growth rate
We also studied by interfacial IMC layer growth of solder volume contraction induced. After different aging time, the use of nano indentation system through the special design of the surface morphology of the sample were measured. By measuring, at 175 DEG C after aging for 1132.5h, the solder joint can reach 1.2 m. and collapse, collapse and aging time of solder joints under the experimental condition with h=-0.031 * (?). The relationship between the size of the coefficient of variation (joint collapse height and IMC layer thickness ratio) is a experiment=, 0.114. Cu6Sn5 and Cu3Sn are assumed isotropic and compact structure, the solder joint collapse of the theoretical value is htotal=- (delta 0.04-0.004x) (?) t (wherein X is Cu3Sn generated Cu3Sn volume accounted for a proportion of the total volume of T; as the aging time, the unit is h). Therefore, the size change coefficients for a ideal=-0.147 +0.0147x. by comparing It is concluded that the theoretical calculation results of the collapse of the solder joints are in accordance with the experimental results.
Solder volume shrinkage of IMC layer growth is likely to lead to limited to the adjacent solder and substrate, which may lead to residual solder /Cu interfacial stress. Therefore, after aging at Sn-1%Cu solder interface Cu6Sn5 layer, Cu3Sn layer and Cu substrate interface of nano indentation experiment control the depth of the evolution of the residual of the solder joint the stress with the aging time. Experimental results show that the solder /Cu interface in different parts of the stress state and the actual position and relevant components: (1) in the process of aging, Cu3Sn/Cu near the interface of the Cu substrate are central and edge compressive stress state, the average value of 560MPa; (2) the central part of the stress of solder near the solder /Cu6Sn5 interface and the edge of the 70MPa and 90MPa; (3) after aging, the interface of Cu6Sn5 layer in the compressive stress in the central and edge were increased to 4GPa and 3GPa (4); in contrast, in the aging process, the inner layer of Cu3Sn The tensile stresses at both the central and the edge are all rising, reaching 1.7GPa and 0.5GPa., respectively.
From the adjacent solder joint stress and aging time of the two part (S-t) the comparative result of the curves, we can conclude that: (1) S-t Cu6Sn5 curve of the interface layer and the adjacent solder has no obvious correlation; (2) in the central Cu substrate and the solder joint, the stress in the interface of CU3Sn layer with aging time show the opposite trend of evolution, which indicates that the Cu3Sn layer is mainly influenced by the central solder Cu substrate adjacent to the constraint; (3) the edge of the Cu3Sn solder layer is mainly composed of Cu3Sn layer adjacent constraints.
By building the creep experimental device on compressive creep behavior of SAC305/Cu solder joints were studied. Through the study found that, when the pressure is 17.8 ~ 22.9MPa, the temperature is 433 ~ 463K, the steady state creep strain of SAC305/Cu solder joint is 0.6 ~ 1.4%, the corresponding steady-state pressure creep deformation is 2.5 ~ 5.8 M. in the Sn-1%Cu/Cu under the condition of steady state creep process of solder joints at the interface, the thickness of the IMC layer increases about 1 m. Because of the change of height caused by growth of IMC is 0.05 ~ 0.16 m, accounting for the steady pressure during creep of the solder joint height decline of the total 1 ~ -4%. so in solder joint steady creep process, negligible solder solder creep the height change of volume shrinkage of IMC with the growth of the measured total deformation effect.
The preparation of FIB Cu6Sn5 and Cu3Sn micro column on micro cantilever beam test of Sn-1%Cu/Cu at the interface of the Cu6Sn5 layer and Cu3Sn layer. The tensile fracture behavior of micro cantilever beam test, Cu6Sn and Cu3Sn micro column in the external load conditions remain elastic deformation, until the fracture. After the test, the main the fracture mode of fracture morphology by Cu6Sn5 shows that Cu6Sn5 micro column is transgranular fracture and intergranular fracture. The fracture of Cu3Sn micro column only exists intergranular fracture.IMC micro column fracture mode is mainly determined by the different microstructure of interface of Cu6Sn5 layer and Cu3Sn layer.
In addition, the tensile test of Cu/SAC305/Cu solder joints, found in fracture mode before aging solder joint is ductile fracture, the fracture strength was 70.8 + 9.0MPa. when IMC particles along the grain boundaries when the concentration of solder, the volume shrinkage of IMC with the growth will cause concentration and defects, resulting in the tensile strength of the solder joint decreased to 58.9 + 6.8MPa. when the solder joint after aging for 81h, from the IMC layer in the solder joint fracture, and fracture strength decreased significantly, change of fracture mode and fracture strength of 57.8 + 8.4MPa.Cu/SAC305/Cu solder joint is due to volume shrinkage or special distribution of IMC particles with IMC growth leads to stress and leads to the accumulation of holes in the IMC layer. By contrast, IMC micro column the fracture strength is much higher than the fracture strength of the solder joint crack interruption in IMC layer. It is proved that the IMC growth process with the volume contraction caused by stress concentration and holes can be obviously Reduce the reliability of the solder joint as a whole.

【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG407

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