【作者】 杨明；

【导师】 翁履谦； 李明雨；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2012， 博士

【摘要】 随着封装体积的减小，界面金属间化合物（IMCs, Intermetallic compounds）在焊点内所占的比重越来越大，其形貌、尺寸、晶体取向以及厚度等将会对电子器件互连点的可靠性造成严重的影响。虽然界面IMCs生长行为已受到了广泛的研究，但是由于尚缺少一些关于界面IMCs生长的重要信息，并且一些研究中的实验方法也不太恰当，因此到目前为止界面IMCs的生长行为还没有被清晰的揭示。本课题就针对这些研究中的不足之处，对Sn基钎料与Cu焊盘之间的界面反应进行研究，揭示了界面IMCs在焊点形成以及随后高温服役过程中的形貌、织构演变、粗化行为、生长动力学以及这几种生长行为之间的相互影响。本课题的研究结果对无铅钎料的研发及应用，以及提高焊点可靠性都具有重要的意义。本文设计了可以获取任意时刻下形成于液态钎料与Cu焊盘之间的界面反应物的实验方法，通过对这些反应物进行分析，揭示了界面IMCs在液相反应过程中的生长行为。研究结果澄清了焊点形成过程中界面Cu₆Sn₅晶粒的形貌演变机制：在液相反应过程中界面Cu₆Sn₅都保持着圆滑的扇贝状形貌，前期生长主要受晶界扩散控制而后期主要受体扩散控制。在整个实验过程中，Cu3Sn的生长一直受体扩散控制。在钎料凝固过程中，如果钎料基体内Cu含量足够高，Cu将会以Cu₆Sn₅的形式在界面处析出并以界面处已存在的Cu₆Sn₅晶粒为核，沿着其[0001]方向生长，长成长条棱镜状晶粒，棱镜状晶粒平直表面的晶面指数为（10-10）。研究了焊点形成过程中界面Cu₆Sn₅的晶体取向。研究结果表明，与单晶Cu的情况相似，生长于多晶Cu界面处的Cu₆Sn₅晶粒也表现出明显的织构行为，但是两种织构的形成机制不一样。多晶Cu上的织构形成于界面Cu₆Sn₅的粗化过程中，主要原因是不同反应体系内，不同取向的Cu₆Sn₅晶粒的稳定性不同。界面Cu₆Sn₅晶粒的织构型生长行为影响其形貌演变和粗化行为。发现了Sn3.5Ag钎料与多晶Cu焊盘液相反应过程中界面IMCs的生长厚度与一些研究学者的预期不同，Ag并没有抑制界面处IMCs的生长和Cu焊盘的消耗。相反地，通过改变钎料与界面IMCs之间的界面能，Ag的添加会影响界面Cu₆Sn₅的生长取向和粗化行为，使得在相同反应条件下出现在Sn3.5Ag/Cu界面处的Cu₆Sn₅晶界多于出现在Sn/Cu界面处的，进而导致在Sn3.5Ag/Cu反应中更大的Cu焊盘的消耗和界面IMCs的生成。研究了固相反应过程中形成于Sn基钎料与Cu焊盘之间界面IMCs的晶体取向，结果表明Sn37Pb/Cu和Sn3.5Ag/Cu界面处的Cu₆Sn₅晶粒呈现出织构型生长，并且织构的形貌主要决定于初始焊点界面Cu₆Sn₅晶粒的晶体取向。在固态钎料中，[0001]晶向垂直于界面的Cu₆Sn₅晶粒比其他取向的晶粒更为稳定，造成了200oC形成的Sn37Pb/Cu焊点和240oC形成的Sn3.5Ag/Cu焊点在固相老化过程中界面Cu₆Sn₅晶粒形成[0001]晶向垂直于焊盘的织构；而对于280oC形成的焊点，由于在重熔过程中绝大部分[0001]晶向垂直于界面的Cu₆Sn₅晶粒已消失，因此在界面处未能出现[0001]晶向垂直于焊盘的织构。通过对两种不同温度下形成于Sn37Pb/Cu界面IMCs层厚度的测量，发现Sn沿Cu₆Sn₅的[0001]晶向扩散较快，使得在相同的固相老化条件下，200oC形成于Sn37Pb/Cu焊点界面处的IMCs多于280oC形成于Sn37Pb/Cu焊点界面处的IMCs。讨论了不同形貌的界面IMCs对焊点剪切性能的影响。根据前几部分的研究结果，制备出了一些具有特定形貌界面IMCs的焊点，并对其进行了剪切性能测试。实验结果表明焊点的剪切强度主要受钎料基体微观组织以及界面Cu₆Sn₅形貌的影响，与界面IMCs厚度关系不大。在剪切高度比较低的情况下，焊点主要在界面处发生断裂。不同形貌的界面Cu₆Sn₅将会对焊点剪切强度产生重要的影响：深深嵌入进钎料基体的棱镜状Cu₆Sn₅可以有效地抑制焊点剪切过程中钎料基体内的塑性变形和裂纹扩展，因此具有棱镜状界面Cu₆Sn₅的焊点剪切强度要明显高于具有扇贝状和层状界面Cu₆Sn₅的焊点的剪切强度。在焊点固相老化过程中，随着钎料基体的粗化以及层状界面Cu₆Sn₅层的出现，焊点强度逐渐降低到一个稳定值。更多还原

【Abstract】 The trend towards the miniaturization of electronic products leads to the needfor a shrinkage of joint size, resulting in a high volume fraction of intermetalliccompounds （IMCs） generated at the Sn-based solders/Cu interface. The morphology,grain size, growth orientation, and thickness of interfacial IMCs will greatly affectthe joint reliabilities and therefore, have attracted extensively studied. However, dueto the lack of some available information and the inappropriate experimentalmethods used in most studies, the growth behaviors of interfacial IMCs have notbeen clearly revealed. In this study, the growth behaviors （e.g., morphologicalevolution, growth orientations, coarsening behaviors, and growth kinetics） of theIMCs formed at Sn-based solders/Cu interface were investigated. The results fromthis study should contribute to the research and development of lead-free solders andthe improvement of the reliabilities of solder joints.In this dissertation, the growth behaviors of the IMCs formed duringliquid-state soldering are investigated based on an experiment in which the liquidsolder is removed prior to the end of soldering. This approach allows for the captureand monitoring of the interfacial IMCs formed during liquid-state soldering andavoid the influence of Cu₆Sn₅precipitated from the solder matrix during cooling.The results reveal that round, scallop-type Cu₆Sn₅grains with a strong texture format the molten solder/Cu interface and that their growth is controlled more by grainboundary diffusion at the beginning of the reaction followed by volume diffusion,whereas the growth of Cu3Sn is only volume-diffusion-controlled. During cooling,many Cu atoms are rapidly precipitated in the form of Cu₆Sn₅, and to reduce thenucleation energy, the precipitation usually takes place on the existing IMC interfaceand form long prism-type grains. The elongation direction of the prism-type grainsis [0001] direction of the existing interfacial Cu₆Sn₅grains.The orientation evolution of interfacial Cu₆Sn₅formed during the jointformation is investigated. Similar as reported on Cu single crystals, strong texturesin Cu₆Sn₅layers can also form on polycrystalline Cu, but the texture formationmechanisms differ. The texture formation on polycrystalline Cu occurs during theripening growth and results from the differences in stability of the interfacial grainswith various orientations at different temperatures. This textured growth ofinterfacial Cu₆Sn₅grains affect their morphological evolution and coarseningbehaviors.The thickness of IMC layers formed at Sn/Cu and Sn3.5Ag/Cu interfacesduring liquid-state soldering is determinated and compared. In contrast to the predictions of some studies, Ag does not inhibit interfacial IMC growth. Instead, bychanging the interfacial energy between the molten solder and the interfacial IMC,the addition of Ag affects the growth orientation and coarsening behavior ofinterfacial Cu₆Sn₅grains. These changes lead to more Cu₆Sn₅grain boundaries atthe interface and therefore greater IMC formation and Cu consumption in theSn3.5Ag/Cu reaction than in the Sn/Cu reaction under the same reflow conditions.The orientation evolution of interfacial Cu₆Sn₅formed during solid-state agingis investigated. The results show that the interfacial Cu₆Sn₅grains exhibit texturedgrowth under solid-state condition and their preferred orientations are affected bythe as-soldered joints. Cu₆Sn₅grains with [0001] direction normal to the interfaceare stable in solid and molten Sn37Pb solder at200oC and molten Sn3.5Ag solder at240oC, but will be rapidly consumed at280oC, which leads to the formation ofdifferent textures in Cu₆Sn₅layer during the solid-state aging treatment to the jointsformed at different temperatures. Also, the effects of the texture evolution on thegrowth of interfacial IMCs are evaluated. The results show that Sn diffusion along[0001] direction of Cu₆Sn₅is faster and therefore more interfacial IMCs aregenerated in the Sn37Pb/Cu joints formed at200oC than those formed at280oCunder the same solid-state reaction conditions.The influence of different interfacial IMCs morphologies on the jointmechanical reliability is discussed. The joints with some certain morphologies ofinterfacial Cu₆Sn₅grains were prepared based on the above results and conductedthe shear test with different shear height. The results show that the joint strength ismainly affected by the microstructures of solder matrix and the morphologies ofinterfacial IMCs, rather than the thickness of interfacial IMC layers. In the case oflower shear height, the interfacial Cu₆Sn₅with various morphologies affects theshear strength of solder joints significantly. As the plastic deformation and crackpropagation during shear test could be effectively inhibited by the prism-type grainsextruded into solder matrix deeply, the solder joints with prism-type interfacialIMCs showed lager strength compared with the joints with scallop-type orlayer-type IMCs. After aging, the shear strength decreased due to coarsened soldermatrix and flattened interfacial Cu₆Sn₅. In the case of higher shear height, as theplastic deformation is far away from the interface, the interfacial IMCs grains haslittle effect on the strength of solder joints. The joint strength is more dependent onthe microstructure of the solder matrix.更多还原