| Sn3.8Ag0.7Cu在静态载荷下的力学行为及其计算模拟 | |
| Alternative Title | Mechanical Behavior and it’s computional simulation of Sn3.8Ag0.7Cu under static loading |
| 王小京 | |
| Subtype | 博士 |
| Thesis Advisor | 尚建库 |
| 2009-05-26 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料学 |
| Keyword | 剪切 共晶sn-ag-cu 组织粗化 电迁移 各向异性 |
| Abstract | 在电子行业无铅化的要求下，经过近10年的研究，Sn-Ag-Cu共晶无铅焊料，以其优越的机械、物理性能而成为使用最广泛的无铅焊料合金。同时，电子产品朝着多功能化和微型化不断发展，元器件封装密度大为提高。一些先进封装形式，如球栅阵列（BGA）、倒装焊（flip-chip）等得到广泛应用。在BGA焊点结构中，晶粒个数越来越少，甚至仅包含几个晶粒。所以，研究焊点(包括焊料和界面) 在这些先进封装中的力学行为对提高封装体的可靠性具有重要意义。本文主要包括四方面内容：(1)焊点的速率相关性；(2)电流弱化效应；（3）电流作用下的组织粗化；(4)晶体塑性理论模拟。 对于Sn-Ag-Cu无铅焊料，室温已经超过其熔点的0.6，因此焊料合金的力学性能表现出很大的速率相关性。关于焊点的速率相关性能的研究，采用单焊点搭接接头，在速率范围6×10−3 s-1~0.9×10−1s-1内进行剪切试验。结果显示Cu/Sn3.8Ag0.7Cu/Cu焊点的剪切强度会随着剪切速率的增大而提高，而断裂方式由较低应变速率下的焊料内部转变为较高应变速率下的焊料/IMC界面断裂。这种转变可用焊料、焊料界面区、以及脆性化合物层不同的速率敏感性得到解释。 电流引起的焊点强度的弱化表现为：通电后焊点的强度降低，延伸率在通电初期（0~1000h）内没有变化，可能原因是电迁移促使焊料内部产生的空位所致；在通电时间加长的情况下，焊点强度和延伸率均有下降，可能的原因是电迁移引起空位在阴极界面附近聚集，形成空洞所致。 焊点在剪切载荷下的断裂方式，随着通电时间的延长发生转变，即，由焊点内部的延性断裂，转变为焊料/界面附近的断裂，最后为化合物内部的脆性断裂。 同时，在电流作用下，焊点内部组织Ag3Sn粒子会发生粗化，我们将其粗化过程嵌入位错蠕变模型中，定量描述了电流作用时间对粒子粗化，以及粒子粗化对于蠕变速率的影响。从而得出分布在焊点中的Ag3Sn粒子会随着通电时间的延长尺寸增加，导致蠕变速率提高的结果。模型描述结果和实验所得统计结果吻合很好。 由于flip-chip封装结构中焊点的不断小型化，一个微小焊点中所包含的晶粒个数不断减少。在这种情况下，用基于各向同性的传统模型评估焊点的可靠性已经不太适用。本文采用了各向异性晶体塑性本构关系对锡基焊料的形变进行了描述，得到适合模拟的一系列参数。 |
| Other Abstract | The eutectic Sn-Ag-Cu ( SAC) alloy has become the most prevalent lead-free solder after a decade of intense research owing to it’s excellent mechanical and physical properties. At the same time, the density for electronic package assembly, such as BGA (Ball Grid Array) structure in Flip Chip, is getting higher and higher to achieve multifunctions and miniaturization of electronic products. In the BGA solder structure, the grain number in one single joint is becoming smaller, even down to several grains. Therefore, mechanical behavior of the solder directly determines the reliability of joint. Under the advanced package techniques it is meaningful to improve the reliabilities of package assemblies. This study has investigated the following aspects of SAC solder reliability: 1) rate dependent behavior of the joint; 2) current induced weakening in the joint; (3) microstructural coarsening induced by electric current; (4) the anisotropic behavior of the joint. Because the room temperature is higher than 0.6Tm (melt point) of the SAC solder alloy. It’s mechanical property was highly rate dependent. The rate dependence was examined by conducting single lap joint tests on Sn3.8Ag0.7Cu (SAC)/Cu solder joints. It was found that strain rate had a strong influence on the shear strength and fracture mode. With increasing strain rates, the failure mode transited from cohesive manner to solder/IMC interface. The reason for the fracture mode transition was explained by a strain-rate hardening effect. In finite element analysis, the maximum equivalent plastic zone was shifted to the interface, which rationalized the experimentally observed failure path. Coarsening of the microstructure in SAC solder joints under current stressing was observed experimentally and modeled by a dislocation-creep model which incorporates the coarsening of second phase particles in lead-free solder alloys. Both the effects of electric current and strain-enhanced coarsening were considered in the model. The straining effect took into account of both the inelastic-strain history and hydrostatic constraint. The model described well the evolution of the eutectic microstructure and the predictions of the model agreed reasonably well with experimentally observed trends. Due to miniaturization of solder joints in flip chip assembly, a micro-joint of SAC solder may contain very few grains. In such a case, the traditional approach based on the isotropic constitutive description for bulk materials is inadequate in describing the mechanical behavior of SAC solder joint. An anisotropic crystal plasticity model was adopted for the micro-joint. The analysis found: 1) stress strain distributions were highly non-uniform in the beta-tin grain aggregate; 2) the shape of solder joint was still an important factor which determines the distribution of stresses in a micro-joint in cases of the multi-crystal; 3) the stress strain state was sensitive to grain orientation |
| Pages | 146 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17154 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 王小京. Sn3.8Ag0.7Cu在静态载荷下的力学行为及其计算模拟[D]. 金属研究所. 中国科学院金属研究所,2009. |
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