【作者】 张金松；

【导师】 吴懿平；

【作者基本信息】 华中科技大学 ， 材料学， 2008， 博士

【摘要】 本文以电子封装的无铅材料为对象,分别采用试验、理论分析、数值模拟和数学推导的方法系统研究了纯Sn覆层在温度循环条件下的晶须生长机理,高温（160°C）和高电流密度（2.2×10⁴A/cm²）、室温（25°C）和较低电流密度（0.4×10⁴A/cm²）条件下无铅焊点的电迁移机理,得到以下主要研究结论:在温度循环条件下,锡须从镀层表面裂纹处生长,其密度与表面裂纹的数量有关,其长度与循环次数有关。不同电镀酸液配方通过影响镀层微观结构来促进锡须生长,Mix酸液的镀层锡须生长密度高于MSA酸液的2³个数量级,前者的锡须长度大于后者的锡须长度;表面裂纹越多,锡须生长的密度速率越高。随着循环次数的增加,生长快的锡须长度速率下降、生长慢的锡须长度速率上升,所有锡须的长度有逐渐趋于一致的态势。Cu₆Sn₅的楔形生长及迁移包含了三个阶段:Cu₆Sn₅在Cu-Sn界面处的生长,Sn大晶粒的分割及小晶粒的再结晶,Cu₆Sn₅沿Sn晶粒的晶界迁移。镀层中的晶粒层数越多、晶粒尺寸越小、其晶界也越多,越有利于松弛内部压应力、抑制锡须生长。晶粒的取向为低指数（101）、（112）、（220）和（211）晶面时,镀层不容易生长锡须;晶粒的取向为高指数（321）和（312）晶面时,镀层容易生长锡须。低指数晶面的晶界是大角度晶界（>15°）,能提供更多的空位以松弛内部压应力;锡须生长使镀层的晶粒取向由高指数晶面和较小角度晶界转变为低指数晶面和大角度晶界。不同材料的热膨胀系数和杨氏模量差异导致在氧化膜内产生热应力;水平的热应力会产生一个垂直于氧化膜的切应力分量形成对氧化膜晶界的剪切,加剧应力集中、削弱晶界强度、产生沿晶裂纹。镀层表面越粗糙,垂直于氧化膜的切应力分量越大,表面裂纹和锡须生长也越多。镀层的致密度可以用于评估镀层表面裂纹生长的趋势,致密度越高的镀层表面裂纹越少、锡须生长越少。在电迁移过程中,焊点阴极界面的IMC长大、剥落和迁移,形成IMC的极性生长和演化。阴极UBM耗尽后Cu层开始溶解,Cu层附近区域IMC为Cu₆Sn₅,焊料中阴极附近区域IMC为Ni₃Sn₄;迁移到阳极IMC为(Cu_x,Ni_1-x)₆Sn₅。高温/高电流密度条件下的电迁移包含了初始、空洞扩展和快速失效三个阶段;室温和较低平均电流密度条件下的电迁移只发现了初始和快速失效两个阶段,空洞扩展阶段不显著。初始和空洞扩展阶段经历的时间很长,快速失效阶段经历的时间很短。在高温/高电流密度条件下,大空洞在阴极界面形成和扩展,与IMC迁移的方向垂直;其驱动力为IMC形成时体积缩小产生的拉应力和转角处电流聚集产生的由内侧向外侧的应力梯度。在室温/较低电流密度条件下,焊点阴极界面没有发现大块IMC和空洞,焊点内部的IMC数量和尺寸都很小。在电迁移初始阶段,界面反应是准平衡态的可逆过程;在电迁移快速失效阶段,界面反应是非平衡态的不可逆过程。当0.4T_m<T_solder<0.7T_m ,界面反应主要为固/固界面反应中晶界扩散模式;当0.7T_m<T_solder<T_m,界面反应主要为固/固界面反应中体扩散模式;当T_solder>T_m,界面反应主要为固/液界面反应,浓度梯度引起的原子迁移和温度梯度引起的热迁移是原子迁移的主要方式。数值模拟表明,电流密度聚集是产生电迁移失效的根本原因,焊点的实际温度低于100°C,避免了高温下原子热扩散对电迁移的干扰。不均匀的温度分布产生较大的热应力,由于Ni/Cu层在XY平面受到约束,因此变形主要在Z方向;热应力越大的焊点其变形量也越大。电迁移条件下,焊点的Ni-Sn扩散偶中存在两种扩散组元:Sn（A组元）和Ni（B组元）。根据唯象方程,推导出试验条件下阴极和阳极扩散反应区的不同原子净流量和界面IMC的生长速率表达式。焊点中阴极界面IMC生长速率比阳极更快;不同焊点的温度差异引起了阴极界面的原子净流量和IMC的生长速率差异,导致不同焊点的电迁移程度差异。根据焊点中阴极和阳极温度、不同组元的浓度C和扩散系数D差别不大的假设,建立了阴极和阳极IMC厚度之差及厚度之和与不同原子迁移力的表达式。Sn的热迁移力比其电迁移力小两个数量级、比Ni的电迁移力小一个数量级;Ni的应力迁移力约为其电迁移力的几分之一;Sn的电迁移力比Ni的电迁移力大,二者都在同一个数量级(10¹⁷N)水平。IMC中Ni的扩散系数与Sn和Ni的浓度梯度迁移力相关,是一个取决于温度和试验条件的参数;此外,二者的浓度梯度迁移力的大小约为10^-16N数量级,略高于电迁移力。因此;电迁移力和浓度梯度迁移力是IMC生长的主要驱动力。更多还原

【Abstract】 This thesis studied the mechanisms of Sn whiskers growth on the pure Sn coating in Thermal Cycling（TC） condition and electromigration of lead-free solder joints in two onditions: High temperature（160°C）/High current density （2.2×10⁴A/cm²） （HH） and Low temperature（25°C）/Low current density （0.4×10⁴A/cm²） （LL）.Under the TC condition, whiskers grew from the surface cracks on the coating. Their density and length were relative to the quantities of cracks and the times of thermal cycling, respectively. The chemical composition of electroplating solution changed the coating microstructures to form surface cracks and promote whiskers growth. The coating applied Mix acid chemical had the much higher density and larger length in whiskers growth than that applied MSA acid chemical.The length of all whiskers has a trendency to be uniform with cycles increasing. Cu₆Sn₅ wedge has three stages in growth and migration. Cu₆Sn₅ grows at the Cu-Sn interface to wedge in a large Sn grain at first. And then, Cu₆Sn₅ growth divides the large Sn grain into two grains and changes their crystal orientations to re-crystallize them. Finally Cu₆Sn₅ migrates along the boundaries of Sn grains. More layers and smaller sizes of Sn grains will provide more grain boundaries to relax the inner compressive stress and suppressing whiskers growth. Sn grains with low-index lattice planes, such as （101）, （112）, （220） and （211） can retard whiskers growth. Inversely, Sn grains with the high-index lattice lanes, such as （321） and （312） can promote whiskers growth. Grains with the low-index lattice planes have the large-angle boundaries （>15°） to absorb the inner compressive stress. The high-index lattice planes and small-angle boundaries will reverse their values with whiskers growing.The high temperature generated a high thermal stress due to the mismatch of Young’s modules and coefficients of thermal expansion （CTE） among different materials. The shear component of the thermal stress vertically acted on the boundaries of oxide film to degrade the boundary strength and produce surface cracks. This shear force increased with the roughness of the surface. A novel compact density test was introduced to evaluate the susceptibility of cracks to the surface. The surface having a high compact density was so robust to resist cracks and whiskers growth.During electromigration, IMCs formation and evolution appeared polar effects at the cathode and anode. After the Ni layer was consumed completely, IMCs at the Cu-Sn interface, in the solder and at the anode interface were identified as Cu₆Sn₅, Ni₃Sn₄ and (Cu_x,Ni_1-x)₆Sn₅, respectively. In HH condition, electromigration possessed three stages, the initial stage, the void propagation stage and the failure stage. And in LL condition, electromigration possessed only two stages, the initial stage and the failure stage. In HH condition, the current crowding produced a driving force to promote large voids formation and propagation along the carhode interface and vertically the direction of electrons. In LL condition, a few IMCs and few voids were found both in the solder joint and at the cathode interface.To thermodynamic principles, the interfacial reaction is a quasi-equilibrium state in the initial stage and is a non-equilibrium state in the failure stage. When 0.4T_m<T_solder<0.7T_m, the interstitial diffusion is dominant in the solid/solid reaction. When 0.7T_m<T_solder<T_m, bulk diffusion is dominant in the solid/solid reaction. When T_solder>T_m, the atomic migration driven by concentration and temperature gradients are dominant. The numerical simulation reveals that the current crowding is the most reason to electromigration. In this study, the actual temperatures in different solder joints were below 100°C, which did not induce magnificent atoms thermodiffusion to submerge electromigration. However, different temperature distributions caused the thermal stress and strain according to Young’s modules and CTE mismatch among different materials. The large strain appeared in Z direction because the deformations were restricted in XY plane as the thermal stress was high.In electromigration, the expressions to atomic flux and Ni-Sn IMC growth rate have been derived from the phenomenological equations. IMCs have a faster rate to grow at the cathode than that at the anode. The different temperature generates the difference of atomic fluxes and IMCs growth rates at the cathode, which also causes the nonuniform electromigration degree in different solder joints. If concentrations and diffusivities of both elements are same in IMCs at the cathode and anode, respectively, the expressions of different atomic driving forces can be established correlative to the IMCs thickness sum and difference. The electromigrstion force of Sn atom is not only larger than that of Ni atom, but also two order magnifications than the thermomigration force of itslef. The electromigration force of Ni atom is multi-times than the stressmigration force of itslef. Although the electromigration forces of Sn and Ni atom reach 10¹⁷N, the concentration-migration forces of them seem to be 10^-16N. It is sure that the electromigration and concentration-migration force are the main driving forces for IMCs formation and growth.更多还原