Ni-W-P/Cu双镀层无铅焊点界面微结构及其剪切失效行为

The emergences of brittle P-rich layer, spalling of intermetallic compound and breakdown of plating at the interface of electroless Ni-P plating lead-free solder joint seriously restrict its application as a diffusion barrier. Therefore, in order to solve the series of issues, a new technology is proposed to produce Sn58Bi lead-free solder interconnect with high reliability by adopting a bi-layer of electroless Ni-W-P/electroplating Cu. The project mainly focus on the fundament of scientific issues about interfacial microstructure and shear failure behavior of solder joint. The interfacial reaction, microstructure, bonding mechanism, growth kinetics of intermetallic compound and the mechanism of suppressing IMC to spall will be researched. From the perspective of interfacial fractal, the characterization in fractal of IMC/solder in bi-layer plating solder joint will be primarily investigated, and the relationships between irregular interfacial characterization of fractal and the parameters of soldering process or aging treatment will be set up. The shear strengths of single Ni-W-P plating and bi-layer plating solder joints will be tested, and a comparative analysis will be conducted between their results. Subsequently, the strengthen effect of the bi-layer plating will be clarified. Furthermore, the mechanism of interfacial shear deformation, crack propagation and fracture mechanism of the solder joint will be studied. Eventually, the relationships between the shear strength and the scale, morphology and fractal of interfacial IMC in bi-layer plating solder joint. The study results will contribute to the design and manufacture of solder interconnets with high reliability. It is of scientific significance and application value.

镀Ni-P层无铅焊点界面出现的脆性富P层、界面化合物剥落及镀层被破坏等问题严重制约了其作为阻挡层作用的发挥和应用。因此，为了解决这一系列问题，本项目提出采用"化学镀Ni-W-P/电镀Cu双镀层"技术制备高可靠性Sn58Bi无铅互连焊点，围绕焊点界面微结构及其剪切失效行为的基础科学问题展开研究。探究焊点界面反应、微观结构、结合机理、金属间化合物（IMC）生长动力学及抑制IMC剥落机理；从界面分形理论新视角，重点研究双镀层焊点IMC/焊料界面的分形特征，建立焊点不规则界面分形特征与焊接工艺或时效参数的关系；测试Ni-W-P单、双镀层焊点的剪切强度并进行对比分析，明晰双镀层焊点的强化效果；进一步探讨双镀层焊点界面剪切变形机制、裂纹扩展和断裂机理；定量描述双镀层焊点剪切强度及其失效行为与界面IMC尺度、形态和分形特征之间的关系。项目有助于高可靠性互连焊点的设计与制造，具有较大的科学意义和应用价值。

电子元器件中无铅锡基互连焊点的可靠性问题已经成为电子封装可靠性研究的重点内容之一，特别是关于焊点的界面形貌、微观结构与其可靠性的相互关系，使得这一部分更加受关注。当前SnAgCu系无铅钎料已经在电子封装领域内得到广泛应用，但在应用中依然有焊点可靠性问题值得深入研究。本项目选择应用较广的Sn-3Ag-0.5Cu（SAC305）无铅钎料作为实验使用的焊料。.采用120 oC、180 oC和200 oC三个不同的固态时效温度对SAC305/Cu焊点进行老化实验，实验结果表明，回流焊后焊点界面只有扇贝状Cu6Sn5金属间化合物（IMC）出现，而在随后的时效中才发现Cu3Sn相出现。在中低温度时效条件下，初始的扇贝状Cu6Sn5 IMC转化为平坦层状，但在高温时效条件下，转化为驼峰状。界面IMC的驼峰形貌主要形成原因是IMC表面Cu原子的异质形核的累积生长主宰了界面的生长形式。界面Cu6Sn5和Cu3Sn相的生长与时效时间的平方根成线性关系，即表明在时效过程中IMC的生长遵循体扩散机制。经计算，焊点界面总IMC、Cu6Sn5和Cu3Sn的激活能分别为115.2、122和98 kJ/mol。.对SAC305/Cu-Ni(P)/Cu双镀层焊点在250 oC下回流10分钟后进行不同温度的固态时效且观察其在高温时效过程焊点界面的形貌变化。实验结果表明，在150 oC时效条件下，1.19 µm厚度的镀层依然具有很好的屏障作用。随着时效温度和时间的延长，镀层的厚度逐渐变薄甚至消失。在接近熔点的210 oC固态时效条件下，镀层变薄并消失的速度很快且在之后抑制界面相互扩散的效果不再明显，这与镀层厚度有关。界面形成新的化合物(Cu,Ni)6Sn5。与无镀层焊点的界面比较，双镀层对于界面化合物的生长具有非常明显的抑制效果。.针对纯铜板、镀Ni(P)基板和双镀层基板的焊点性能评估采用了剪切实验。实验结果表明，镀Ni(P)基板焊点的剪切强度则随着时效时间的延长而逐渐减小，双镀层基板焊点的力学性能变化则比较平缓，因其双层阻挡扩散作用使得界面IMC生长缓慢，导致界面IMC并没有过厚增长。焊点界面断裂位置大量的出现在IMC内部，无论时效时间的长短，双镀层界面中的IMC剪切强度数值稳定在21 MPa左右，双镀层焊点体现出良好的热稳定性。