Sn/Cu钎焊冷却阶段液固界面IMC生长机理研究

The growth of intermetallic compound (IMC) at liquid/solid interface during soldering reaction has become a basic and essential issue that greatly affects the reliability of solder joints in electronic packaging. Our recent research shows that the growth of IMC at Sn/Cu interface during soldering followed the classical growth mechanism at the heating preservation stage, whereas deviated from that at the cooling stage. It is quite necessary to study the growth mechanism of interfacial IMC during the cooling stage of a soldering process, basing on which a scientific soldering reaction model can be established. To solve the technical problem that the interfacial reactions during heating preservation stage and cooling stage are hardly studied separately, synchrotron radiation real-time imaging technology combining with high pressure air blowing method will be carried out at the Shanghai synchrotron radiation facility to in situ investigate the growth behavior of the IMC at Sn/Cu interface at the cooling stage of soldering process. And with the help of numerical modeling of temperature field, the growth mechanism of interfacial IMC at the cooling stage will be analyzed. An in-depth exploration of the growth and evolution of interfacial IMC at the cooling stage can reveal the diffusion behavior of atoms and its controlling factors. The expected outcome of this project is proposing a suitable growth model of IMC at solid/liquid interface during cooling stage of soldering process. Thus the theory of IMC growth during the whole solder process can be perfected. Moreover, this growth model can provide theoretical and technical supports for effectively controlling the size and morphology of IMC at Sn/Cu soldering interface and improving the reliability of solder joints.

钎焊过程生成金属间化合物（IMC）的液固界面反应是电子封装钎料连接的基础和核心科学问题。申请者近期研究表明：Sn/Cu钎焊液固界面IMC生长在保温和冷却阶段遵循不同机制，而经典IMC生长理论仅适用于保温阶段，有必要深入研究钎焊冷却阶段界面反应机理，这样才能建立起更科学准确的钎焊界面反应模型。为解决钎焊保温与冷却阶段难以独立分开研究的技术难题，申请者利用上海光源同步辐射实时成像装置并结合高压空气吹扫实验手段研究Sn/Cu钎焊冷却阶段液固界面IMC的生长行为，借助冷却阶段焊点温度场数值模拟等方法，对钎焊液固界面冷却阶段IMC反应机理进行研究。深入探讨钎焊冷却过程中IMC生长、演化行为，揭示冷却阶段元素扩散行为及控制因素。研究预期建立起钎焊冷却阶段液固界面IMC生长模型，并以此完善整个钎焊过程界面IMC生长理论，为有效控制Sn/Cu钎焊界面IMC尺寸和形态，提高接头可靠性提供理论和技术支持。

电子器件的微型化对钎焊界面的可靠性提出了更高的要求，深入研究钎焊界面反应机制意义重大。现有研究都将钎焊当作一个连续过程，以此为基础建立的界面反应机制与实际偏差较大。我们研究表明，钎焊冷却阶段对最终界面IMC生长及形态有显著的影响。利用上海光源同步辐射实时成像技术原位观测Sn/Cu钎焊界面IMC(Cu6Sn5)生长行为，结合高压空气吹扫手段来阻断后续反应，获得保温阶段IMC生长数据，并对比完整钎焊获得冷却阶段IMC生长数据；再结合焊点中Cu浓度分布和温度场的模拟，揭示冷却阶段元素的扩散行为及控制因素，从而建立钎焊冷却阶段液固界面IMC生长模型。同时也研究了典型Cu6Sn5晶粒形貌的生长过程及合金元素添加对冷却阶段IMC生长行为的影响。主要获得的结果如下：.1. Sn/Cu钎焊冷却阶段界面IMC生长机制与保温阶段遵循不同的生长机制，冷却阶段界面IMC厚度生长遵循h=ktn (n=1)规律，为界面反应控制机制，即Cu的沉淀通量起控制作用。建立了冷却阶段界面Cu6Sn5层生长模型—沉淀通量驱动(PFC)界面反应理论模型。.2.首次利用同步辐射成像在Sn-3.5Ag/Cu钎焊液固界面原位观察到Cu6Sn5晶粒之间存在液态通道，提高了Cu的晶界扩散通量，同时降低界面Cu6Sn5生长激活能，因此Ag的添加阻碍了保温阶段Cu6Sn5晶界的横向吞并，促进Cu6Sn5晶粒纵向生长，使得Sn-3.5Ag/Cu界面具有比Sn/Cu界面更小的宽高比。.3.含Ag锡基钎料/Cu钎焊冷却阶段界面Cu6Sn5生长受Ag3Sn纳米颗粒对界面Cu6Sn5晶粒的钉扎效应与Cu沉淀通量共同作用，低Ag浓度钎料/Cu界面IMC生长的主要控制因素是Cu沉淀通量；高Ag浓度钎料/Cu界面IMC生长的主要控制因素是Ag3Sn纳米颗粒的钉扎效应。.4. Sn-xCu/Cu冷却阶段界面IMC的形貌受初始Cu浓度的影响不大，都遵循粗糙界面连续生长、二维形核和长大及螺旋位错三种晶粒生长机制。钎焊长时间保温后，钎料中初始Cu浓度对冷却阶段界面Cu6Sn5晶粒生长方式有很大影响，在低Cu浓度下，Cu6Sn5遵循粗糙界面连续生长，二维形核和长大及螺旋位错三种晶粒生长机制；在高Cu浓度下， Cu6Sn5仅遵循粗糙界面连续生长机制，并且在垂直于Cu基板方向上长大速度更快。