【作者】 王波；

【导师】 吴懿平； 吴丰顺；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2010， 博士

【摘要】 电子封装微互连正向高密度化方向快速发展,微焊点的尺寸及互连高度在不断降低。本文研究了微小互连高度下(小于100μm)焊点的界面反应及可靠性。微焊点互连高度的降低加强了焊点两侧界面反应的交互作用,使焊点界面微观组织变得更加复杂,对焊点的性能有很大的影响,给电子产品的可靠性带来巨大的挑战。本文采用Sn、Sn37Pb和Sn9Zn三种不同类型的互连材料连接Cu和Ni块体,获得互连高度分别为100μm、50μm、20μm和10μm三明治结构的焊点,通过对这些焊点的研究揭示微焊点随着互连高度的降低而产生的微观组织及力学性能变化。主要研究成果如下：研究了回流过程中Cu/Sn/Cu焊点互连高度对焊料层微观组织、界面金属间化合物(IMC)厚度和比例以及焊点力学性能的影响。随着互连高度降低,焊点两侧的IMC厚度逐渐降低,但是IMC在焊点中的体积比例升高,形成焊点所消耗的Cu层厚度降低,但焊料层中Cu的平均浓度却升高。在0.1mm/min的拉伸速率条件下,焊点互连高度越低,其焊料层的应变速率越高,焊点抗拉强度升高。而且,当焊点互连高度降低至10μm后,焊料层在高度方向上将从多晶粒转变为单晶粒,焊点的拉伸断裂模式为Cu6Sn5/Cu界面分层断裂和Cu6Sn5晶粒的解理断裂,这与其他三种高度焊点的断裂发生在焊料层中并呈韧性断裂不同。研究了Cu/Sn/Cu焊点在150℃等温时效过程中的组织结构与性能的变化规律。在互连界面扩散结构(Cu/Cu3Sn/Cu6Sn5/Sn)中Cu与Sn的扩散方向相反,扩散通量不同,并随时效过程中Cu3Sn和Cu6Sn5的生长而变化,由此建立了在Cu3Sn/Cu6Sn5界面处Cu6Sn5与Cu3Sn两相生长理论模型。基于此生长理论模型,在反应初期,Cu3Sn快速生长,并消耗Cu6Sn5层,此时,Cu6Sn5厚度降低,Cu3Sn厚度升高,IMC层总厚度升高。并且初始Cu6Sn5层的厚度越薄,则1MC层增厚速度越快。随着时效时间的延长,Cu3Sn的生长速度降低,Cu6Sn5层的消耗也逐步减少,Cu3Sn/Cu6Sn5界面处IMC的生长依靠扩散至两相界面的Cu和Sn原子,最终各焊点的IMC层总厚度会趋于相同。时效期间在高度方向上具有多晶粒的焊料层组织粗化降低了焊点的抗拉强度,断裂发生在焊料层中；互连高度为10μm的时效焊点拉伸断裂发生在Cu3Sn/Cu6Sn5界面,这与时效处理之前的断裂模式不同。研究了回流与时效过程中Cu/Sn/Ni焊点界面反应交互作用对微焊点的组织、结构与性能的影响规律。回流过程中,由于发生了焊点两侧界面反应的交互作用,在Ni侧形成不致密连接的Ni-Cu-Sn薄层与(Cu,Ni)6Sn5沉积层的双层结构,所有不同互连高度焊点的拉伸断裂都发生在Ni侧IMC层中。时效过程中靠近Ni表面的薄层Ni-Cu-Sn会转变为(Ni,Cu)3Sn4,它与(Cu,Ni)6Sn5的结合强度较弱,因此,时效焊点的拉伸断裂大多发生在(Ni,Cu)3Sn4/(Cu,Ni)6Sn5界面。对比研究了经回流工艺得到的互连高度均为100μm,同质基材互连焊点和异质基材互连焊点在微观组织和力学性能上的变化。在Cu/Sn/Cu、Ni/Sn/Ni和Cu/Sn/Ni三种焊点,发现Ni/Sn/Ni焊点抗拉强度最大,Cu/Sn/Ni焊点抗拉强度次之,Cu/Sn/Cu焊点抗拉强度最低；Cu/Sn/Cu和Ni/Sn/Ni焊点的拉伸断裂发生在焊料层内部,而Cu/Sn/Ni焊点的拉伸断裂发生在Ni侧IMC层中,这是因为Cu/Sn/Cu和Ni/Sn/Ni焊点两侧的IMC层生长致密,而Cu/Sn/Ni焊点Ni侧IMC为连接不致密的双层结构。研究了回流和时效过程中互连高度对Cu/Sn-37Pb/Cu焊点对焊料层微观组织、界面金属间化合物(IMC)厚度和比例以及焊点力学性能的影响。在回流过程中,互连高度越低,两侧的IMC厚度越薄,而焊料层中的Pb浓度越高；在接下来的时效过程中,互连高度越低,IMC层增厚速度越快,焊料层中Pb浓度升高越迅速。焊点的抗拉强度随着焊料层应变速率的增大而升高,随着Pb浓度的增大而降低。采用0.1mm/min的拉伸速率对焊点进行拉伸试验,当互连高度从100μm降低至20μm时,焊点抗拉强度升高,焊料层应变速率为主要影响因素；对于互连高度为10μm焊点,由于150℃时效500h前后焊料层中Pb的浓度可从50wt.%升高至90wt.%左右,焊点抗拉强度为最低。所有高度焊点的拉伸断裂都发生在焊料层中,并呈韧性断裂模式。研究了回流与时效过程中互连高度对Cu/Sn-9Zn/Cu焊点中焊料层成分、IMC生长及结构的影响。回流过程中,随着焊点互连高度的降低,焊料层中Cu浓度升高,而Zn浓度降低,焊料层中形成的游离态Cu5Zn8颗粒密度升高,这使互连高度越低的焊点抗拉强度越低,在0.1mm/min的拉伸速率下,焊点的拉伸断裂发生在焊料层内。时效过程中,随着互连高度的降低,焊点中焊料层内部形成的Cu6(Sn,Zn)5越早。更多还原

【Abstract】 The rapid development of the high-density solder interconnection in the electronic packaging industry brings about the continiuous decrease in the size and stand-off height (SOH) of solder joints. Therefore, the present paper focuses on the interfacial reactions and reliability of solder joints influenced by the SOH lower than100μm. The interfacial reactions in the solder joints with same or different base materials at the two sides become more complicated with the decreasing SOH, which significantly influences the microstructure and mechanical property. And it is a big challenge for the reliability of electronic products.In the present paper, three different kinds of solder alloys, Sn, Sn37Pb and Sn9Zn, were used to join Cu bulk and Ni bulk for obtaining the sandwich-structure solder joints with SOH of 100μm,50μm,20μm and 10μm. These researches revealed the influence of SOH on the microstructure and the mechanical property of solder joints. The research results are listed as follows:The study on the as-reflowed Cu/Sn/Cu revealed the influence of SOH on the solder bulk microstructure, IMC thickness, IMC proportion and the mechanical property of solder joints. With the reducing SOH, the IMC thickness decreases, while the volume ratio of IMC increases. And also, the consumed Cu thickness decreases, while, the Cu concentration increases in the solder bulk with the reducing SOH. Tensile tests with a constant crosshead speed of 0.1mm/min were carried out to assess the mechanical property. It was found that the strain ratio of solder bulk increases with the reducing SOH, which improves the ultimate tensile stress (UTS) of solder bulk. When the SOH is reduced to be 10μm, the solder bulk consists of one Sn grain in height. The tensile fracture occurs in the IMC layer with the delamination of Cu6Sn5/Cu interface and the cleavage of Cu6Sn5 grain. And this is different from the fracture modes of solder joints with the other three SOHs, which all fractures in the solder bulk in a plastic mode.The study on the Cu/Sn/Cu solder joints aged at 150℃revealed the changes of microstructure and mechanical property. In the Cu/Cu3Sn/Cu6Sn5/Sn diffusion couple at the one side of solder joint, Cu and Sn diffuse in opposite direction, while, the their diffusion fluxes change with the growth of Cu3Sn and Cu6Sn5 during the thermal aging process. Based on the ratio of Cu to Sn diffusion fluxes at the Cu3Sn/Cu6Sn5 interface, the growth model of Cu6Sn5 layer and Cu3Sn layer has been established. According to the growth model, the Cu3Sn grows fast at the cost of Cu6Sn5 layer at the early stage, while, the thickness of the total IMC layers increases with the aging process. Moreover, the thinner original Cu6Sn5 layer, the faster the IMC growth. With the aging process, the growth rate of Cu3Sn slows down, as well as the consumption rate of the Cu6Sn5 layer, at this time, the IMC growth depends on the reaction between diffusion fluxes of Cu and Sn at the Cu3Sn/Cu6Sn5 interface. Finally, the IMC thickness would grow similarly. Because of the coarsened Sn grains in the solder joints with multiple grains in height during the aging process, the UTS decreases, and the fractures occur in the solder bulk; the fracture of the aged solder joint with 10μm SOH occurs with the delamination of Cu3Sn/Cu6Sn5 interface, which is different from the fracture mode before aging.The study of the as-reflowed and aged Cu/Sn/Ni solder joints reveals the change rules of the micrstructure and mechanical property of the solder joints with different base materials at the two sides. Due to the coupling effect during the reflow process, the incompact duplex IMC layers of the thin Ni-Cu-Sn layer and the deposited (Cu,Ni)6Sn5 layer at Ni side are formed, therefore, fractures occur in the IMC layer at Ni side for all Cu/Sn/Ni solder joints in the tensile test with a constant crosshead speed of 0.1mm/min. In the aging process, the thin Ni-Cu-Sn layer would transform into (Ni,Cu)3Sn4, which also have a weak bonding with the deposited (Cu,Ni)6Sn5 layer, as a result, the fracture of the aged solder joints mostly occurs at the (Ni,Cu)3Sn4/(Cu,Ni)6Sn5 interface.The study compared the microstructure and mechanical property of the as-reflowed sandwich solder joints with the same or different materials at the two sides. All of the solder joints were prepared with a SOH value of 100μm. Among the Cu/Sn/Cu, Ni/Sn/Ni and Cu/Sn/Ni solder joints, it is found that the Ni/Sn/Ni has the highest UTS, then the Cu/Sn/Ni, finally the Cu/Sn/Cu; The fractures occur in the bulk of the solder joints with the same materials at the two sides, while in the IMC layer at Ni side of the Cu/Sn/Ni solder joints, and this difference results from the compact IMC layer in the Cu/Sn/Cu and Ni/Sn/Ni solder joints and the incompact duplex IMC layers in the Cu/Sn/Ni solder joints.The study on the as-reflowed and aged Cu/Sn-37Pb/Cu revealed the influence of SOH on the solder bulk microstructure, IMC thickness, IMC proportion and the mechanical property of solder joints. With the reducing SOH of the as-reflowed solder joints, the IMC thickness decreases, while the Pb concentration increases in the solder bulk. During the subsequent thermal aging process, the IMC thickness and the Pb concentration in the bulk increase faster in the solder joints with lower SOH. The UTS increases with the higher strain rate, while it decreases with the higher Pb concentration in the solder bulk. The tensile tests with a constant crosshead speed of 0.1mm/min were used to assess the mechanical property of solder joints, and it is found that the UTS increases with the SOH reduced from 100μm to 20μm, when the strain rate is the major influencing factor. The solder joint with 10μm SOH has the lowest UTS value, because the Pb concentration in the bulk could range from 50wt.% to 90wt.% before and after the thermal aging at 150℃for 500h. All of the fractures occur in the solder bulk in a plastic mode.The study of as-reflowed and aged Cu/Sn-9Zn/Cu reveals the influential mechanism of SOH on the solder bulk composition, IMC growth and mechanical property. For the as-reflowed solder joints, the Cu concentration increases, while the Zn concentration decreases in the solder bulk with the reducing SOH, therefore, the density of dissocative Cu5Zn8 particles increases in the solder bulk. The overmany dissocative Cu5Zn8 particles destroy the integrity of solder bulk, decreasing the UTS. All of the fractures occur in the solder bulk in the tensile tests with a constant crosshead speed of 0.1mm/min. During the aging process, the Cu6(Sn,Zn)5 forms earlier in the solder bulk with the reducing SOH.更多还原