| 低温共晶SnBi和SnIn无铅焊料与Cu基体的界面反应及化合物生长行为 | |
| Alternative Title | INTERFACIAL REACTIONS AND INTERMETALLIC COMPOUND GROWTH BETWEEN LOW TEMPERATURE EUTECTIC SNBI AND SNIN LEAD-FREE SOLDERS AND CU SUBSTRATE |
| 尚攀举 | |
| Subtype | 博士 |
| Thesis Advisor | 尚建库 |
| 2010-01-07 | |
| 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 | 无铅焊料 42sn58bi 48sn52in 元素扩散 界面反应 界面化合物(Imc) Bi偏聚 Kirkendall孔洞 透射电子显微镜(Tem) |
| Abstract | 随着电子元器件朝着高性能、小型化和多功能化的发展，电子封装的尺寸越来越小。如在倒装芯片封装技术中，随着芯片表面输入/输出端口 (I/O) 数量的增加，焊点的直径越来越小，焊球的尺寸已接近50m。在如此小的焊球中，焊点与焊点下金属化层反应所形成的化合物层，成为影响焊点可靠性的重要因素。尤其是近年来，由于Pb具有毒性，世界范围内对微电子焊料无Pb化的要求日益高涨，而目前开发的无铅焊料均以Sn为主要成份，与传统的共晶SnPb焊料相比，Sn基无铅焊料与基体金属化层反应后，形成化合物的速度更快。因此， 研究Sn基无铅焊料与基体金属的界面化合物演化行为对认识和提高电子产品可靠性具有重要的意义。 共晶42Sn58Bi和48Sn52In合金作为低温无铅焊料的代表，在工业中具有重要的应用价值。本论文采用透射电镜为主要手段，分别研究了Cu/42Sn58Bi/Cu和Cu/ 48Sn52In/Cu互连体在回流焊和固态时效过程中界面化合物的生长机制和相类型，并对界面缺陷如Bi偏聚、Kirkendall孔洞等进行了分析，提出了界面化合物在多晶和单晶Cu基体上的不同生长机制，主要结果如下： Cu3Sn在多晶、单晶Cu基体上的生长机制不同。回流焊后，在多晶Cu基体上形成的Cu3Sn颗粒呈等轴晶，厚度不均匀，Cu3Sn/Cu6Sn5界面呈波浪形；而在单晶Cu基体上形成的Cu3Sn颗粒呈柱状晶，且界面平直，说明Cu3Sn层较为均匀。在随后的固态时效过程中，多晶Cu基体上的Cu3Sn同时在Cu/Cu3Sn和Cu3Sn/Cu6Sn5界面上形核。Cu/Cu3Sn界面通过新Cu3Sn晶粒的生成而往Cu侧扩展，而在Cu3Sn/Cu6Sn5界面上，Cu3Sn以消耗Cu6Sn5的方式生长。在单晶Cu基体上，固态时效过程中，仅在Cu/Cu3Sn的界面形成新的Cu3Sn晶核，位置优先选择在Cu基体与柱状Cu3Sn晶粒的三叉晶界处。经过7天的长时间时效后，在基体Cu与Cu6Sn5之间形成两层Cu3Sn，靠近基体Cu侧为等轴晶层；而靠近Cu6Sn5侧为原始柱状晶层。在Cu3Sn/Cu6Sn5界面上几乎没有新的Cu3Sn晶粒生成，回流焊中形成的柱状Cu3Sn晶粒继续朝着Cu6Sn5侧生长。对Cu3Sn在单晶、多晶Cu基体上的动力学研究表明，Cu3Sn在多晶Cu基体上的生长速度比在单晶Cu基体上快。其主要原因为，在多晶Cu基体上形成的Cu3Sn晶粒尺寸较小，晶界数量较多，元素沿晶界的扩散对化合物生长起主导作用。 对Cu/42Sn58Bi/Cu互连界面各相间取向关系的研究表明，界面Sn/Cu6Sn5/Cu3Sn/Cu相邻相间均存在特殊的取向关系。实验观察发现，在Sn/Cu6Sn5界面上存在[-100]Sn//[177]Cu6Sn5,(02-2)Sn//(71-2)Cu6Sn5；Cu6Sn5/Cu3Sn界面上存在[100]Cu3Sn//[-251]Cu6Sn5,(032)Cu3Sn//(-11-7)Cu6Sn5；在回流焊的初期Cu6Sn5与基体Cu直接接触，因此Cu6Sn5与基体Cu之间也存在特殊的取向关系:[-1-10]Cu//[1-5-3]Cu6Sn5,(2-2-2)Cu//(-9-32)Cu6Sn5及[211]Cu//[102]Cu6Sn5,(1-31)Cu//(44-2)Cu6Sn5；在单晶Cu基体上，回流焊后Cu3Sn形成均匀的柱状晶层，此柱状Cu3Sn晶粒与Cu基体存在特殊的取向关系，其中Cu3Sn的[122]、[211]、[100]、[102]和[131]晶带轴分别与Cu的[100]、[110]和[211]晶带轴平行。 对Cu/48Sn52In /Cu互连界面的研究表明，界面化合物的相结构和种类取决于回流焊温度。200°C以下液态反应及固态时效过程中，界面形成两层晶粒尺寸不同的Cu2(In,Sn)化合物， 靠近Cu基体侧的化合物晶粒尺寸较小，约为50nm，而靠近焊料侧晶粒尺寸较大，约为细晶层尺寸的10倍。在200℃液态反应的过程中，界面只有一层Cu6(In,Sn)5化合物。当液态反应温度升高到250℃时，界面形成明显的两层化合物，靠近Cu基体侧为Cu9(In,Sn)4，而靠近焊料侧为Cu6(In,Sn)5。对界面化合物生长动力学研究表明，低温固态时效的过程中，界面化合物的生长由体扩散和沿晶界的扩散共同控制，计算所得到的化合物生长的时间指数 约为0.25，远远小于体扩散控制下的标准值0.5。而在较高的温度下，界面化合物的生长由体扩散所控制，其化合物的生长激活能为33.49kJ/mol。 对Cu/42Sn58Bi/Cu和Cu/48Sn52In/Cu互连体界面缺陷的形成机制进行了系统地研究。TEM观察发现，在Cu/42Sn58Bi/Cu互连体界面的化合物生长过程中，固溶在Cu3Sn中的Bi原子首先析出，在界面能量降低的驱动下于Cu/Cu3Sn界面偏聚。随着时效时间的增长，Bi在Cu/Cu3Sn界面上长大呈颗粒状，孔洞出现在Bi颗粒与Cu基体之间。分析认为：Bi颗粒在Cu/Cu3Sn界面的偏聚，占据了界面上空位的位置；同时阻碍Cu原子的扩散，使两个Bi颗粒之间Cu原子的扩散出现局域化，加速了界面上由于Cu与Sn扩散系数不同而造成的空位的产生；当界面上空位数量达到一定值时，空位发生聚合；孔洞优先在Bi颗粒的两端形核，并向Bi颗粒与基体Cu之间扩展。而在48Sn52In/Cu互连体中，100ºC固态时效7天后发现，孔洞出现在两层晶粒尺寸不同的Cu2(In,Sn)化合物之间，其主要原因为In和Sn原子在两层不同形貌的Cu2(In,Sn)化合物层中的扩散速度存在较大的差异所造成。 |
| Other Abstract | As the trend of electronic components moves towards high performance, miniaturization and multifunction, packaging systems continues to scale down. For example, in flip chip package technology, as the number of input-output (I/O) on the chip surface increases, the diameter of solder bumps decreases, and the size of the flip-chip solder balls is approaching 50m. In such a small solder balls, the intermetallic compounds (IMCs) formed between solder bump and under bump metallization (UBM) become a significant part of the joint, key to the reliability of the solder joint. In recent years, lead-free solders have attracted much attention around the world due to the toxicity of Pb. For Sn-containing lead-free solders on Cu UBM, since the growth rate of IMC between Sn and UBM is faster than that in traditional eutectic SnPb solder, the growth mechanism of interfacial IMCs between Tin-containing solder and substrate is a critical issue for understanding and improving the reliability of electronic devices. Eutectic SnBi and SnIn alloys as representative low temperature candidates of lead-free solders have been widely used in electronic industry. In this study, growth mechanism and phase types of interfacial IMC layers between Cu/42Sn58Bi/Cu and Cu/eutectic SnIn /Cu interconnects were investigated by transmission electron microscopy during reflowing and solid-state aging process. The growth mechanisms of Cu3Sn on polycrystalline Cu and single crystal Cu substrates were examined. The interfacial flaws, such as Bi atomic segregation and Kirkendall voids, were observed and analyzed. The growth mechanisms of Cu3Sn layer on polycrystalline and single crystal Cu substrates were different. After reflowing, the Cu3Sn layer on polycrystalline Cu was made up of equiaxed grains. The growth of Cu3Sn was not uniform, and the interface between Cu3Sn/Cu6Sn5 was wavy-like. By comparison, on single crystal Cu substrate, the growth of columnar grains resulted in a thin uniform Cu3Sn layer perpendicular to the interface. During the subsequent solid-state aging, on polycrystalline Cu, new Cu3Sn grains nucleated and grew both at the interfaces of Cu/Cu3Sn and Cu3Sn/Cu6Sn5. At the Cu/Cu3Sn side, the formation of new Cu3Sn grains shifted interface to the Cu side whereas at the Cu3Sn/Cu6Sn5 interface, the Cu3Sn grew by the consumption of Cu6Sn5 phase. On the single crystal Cu, new Cu3Sn grains only nucleated at the Cu/Cu3Sn interface, while the original columnar Cu3Sn grains grew to the Cu6Sn5 side at the Cu3Sn/Cu6Sn5 interface. The new triangular Cu3Sn grains at the Cu/Cu3Sn interface were found to nucleate at the triple junction sites between Cu and two columnar Cu3Sn grains. Finally, two Cu3Sn sublayers, an initial columnar sublayer and a newly-formed equiaxed sublayer formed between Cu and Cu6Sn5 layer. The growth kinetics of Cu3Sn on polycrystalline and single crystal Cu were also investigated. The growth rate of Cu3Sn on the polycrystalline Cu was faster than that on the single crystal Cu. This was attributed to more nucleation sites and smaller grain size of Cu3Sn formed on the polycrystalline Cu, and the larger effects of grain boundary diffusion on the polycrystalline Cu than on single crystalline Cu. The orientation relationships between the neighboring phases of Sn/Cu6Sn5/Cu3Sn/Cu at the Cu/42Sn58Bi/Cu interconnect were investigated. Several special orientation relationships,[-100]Sn//[177]Cu6Sn5,(02-2)Sn//(71-2)Cu6Sn5,[100]Cu3Sn//[-251]Cu6Sn5,(032)Cu3Sn//(-11-7)Cu6Sn5 were found at the Sn/Cu6Sn5 and Cu6Sn5/Cu3Sn interfaces, respectively. At the early stage of reflowing, Cu6Sn5 phase was in direct contact with Cu substrate, and therefore, the orientation relationships,:[-1-10]Cu//[1-5-3]Cu6Sn5,(2-2-2)Cu//(-9-32)Cu6Sn5 and [211]Cu//[102]Cu6Sn5,(1-31)Cu//(44-2)Cu6Sn5, developed between Cu6Sn5 and Cu. Moreover, there are twelve groups of orientation relationships observed between Cu3Sn and Cu. Although the parallel planes between Cu3Sn and Cu were not necessarily the low-index planes, the lattice mismatches were small in all of the observed orientation relationships. Interfacial reactions and growth kinetics of Cu/eutectic SnIn /Cu interconnect at the soldering temperature range from 80°C to 250°C were investigated. The experimental results indicated that the crystal structure of IMC formed at the interface depended on soldering temperature. Below 200°C, only one type of IMC was observed at the eutectic SnIn/Cu interface, which is Cu2(In,Sn). However, it had two different morphologies, coarse-grained IMC at the solder side and fine-grained IMC at the Cu side. When the soldering temperature was increased to 200°C, Cu6(In,Sn)5 was the only IMC formed at the eutectic SnIn/Cu interface. At 250°C, two kinds of IMCs formed at the eutectic SnIn/Cu interface: Cu6(In,Sn)5 on the solder side and Cu9(In,Sn)4 on the Cu side. The growth kinetic analyses indicated that during solid-state aging at the temperature range of 80 to 100°C, the calculated value of time exponent was about 0.25, which is smaller than the standard value of volume diffusion (0.5), indicating that the growth of IMCs was controlled by both the volume diffusion and the grain boundary diffusion. When the sample was liquid-state aged above 200°C, the volume diffusion controlled the growth of interfacial IMCs with an the activation energy of IMC’s growth of 33.49kJ/mol. The interfacial flaws formed at the eutectic SnBi/Cu and eutectic SnIn/Cu interconnects were investigated systematically. At the interface of the eutectic SnBi/Cu interconnect, as the IMC grew, Bi atoms dissolved in the Cu3Sn layer segregated to the Cu/Cu3Sn interface. After prolonged thermal aging, Bi formed fine particles at the Cu/Cu3Sn interface in SnBi/Cu solder joints. The interfacial voids were found near the Bi particles but on the Cu side. It was suggested that the Bi atoms took up the vacancy sinks and acted as barrier for Cu diffusion. As a result, the Cu flux became highly concentrated between the Bi particles, which accelerated the coalesce process of vacancy due to the different diffusing velocity of Cu and Sn in Cu3Sn layer. Further condensation of the vacancies would lead to void formation at the interface. However, at the interface of the eutectic SnIn/Cu interconnect, the Kirkendall voids were observed between two different morphological Cu2(In,Sn) layers after solid-state aging at 100ºC for 7 days, but not between IMC and Cu. This should be due to the different diffusing rates of In and Sn atoms in fine-grained Cu2(In,Sn) layer and coarse-grained Cu2(In,Sn) layer. |
| Pages | 145 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17236 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 尚攀举. 低温共晶SnBi和SnIn无铅焊料与Cu基体的界面反应及化合物生长行为[D]. 金属研究所. 中国科学院金属研究所,2010. |
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