【作者】 罗忠兵；

【导师】 王来； 赵杰；

【作者基本信息】 大连理工大学 ， 材料学， 2011， 博士

【摘要】 随着电子产品向便携式、高可靠性及无铅环保方向发展,电子封装及其钎焊接头的可靠性也面临着前所未有的挑战。剪应力作用下的钎焊凸点力学性能问题已经成为电子器件电路失效的重要原因。其中,钎焊界面金属间化合物(IMC,通常是Cu6Sn5)的结构形态是钎焊接头质量的关键因素。焊点尺寸的减小和Sn基钎料的应用,在增加界面IMC层比例的同时,也凸显了钎料/IMC界面本征行为的不连续性。在实际服役过程中,界面IMC层的演变是复杂的,晶粒会发生粗化,钎料/IMC界面由钎焊状态下的波浪起伏变得更加平直,IMC层内及层间的缺陷数量也会相应增加。尤其是Cu6Sn5在服役过程中会发生晶体结构转变,转变前后的体积膨胀有可能在界面处形成应力从而恶化接头的力学可靠性。然而到目前为止,界面Cu6Sn5结构转变的基本规律还不清楚,界面形态和缺陷的研究还不够全面,界面IMC层在接头剪切变形和断裂过程中的作用机理尚不十分明确。因此,研究界面结构形态演变及对其接头剪切行为的影响,对高密度封装无铅互联技术的发展是非常有意义的。本文以Sn-0.7Cu/Cu接头为研究对象,通过添加少量的Ni来改变界面结构,分别研究Cn-Sn界面IMC晶体结构及形态(包括界面厚度、晶粒大小、界面粗糙度、界面缺陷)随时效时间和温度的演变规律；同时采用“三明治”结构钎焊接头,辅以剪切应变速率和温度的变化,探讨界面演变与剪切行为的相关性。主要结论如下：(1)利用XRD研究了Sn-0.7Cu/Cu界面Cu6Sn5的固态结构转变。结果表明：钎焊后空冷的接头经等温时效后,界面η’-Cu6Sn5的时间-温度-形成曲线呈“C”状,时效温度在135-150℃左右时η’相形成所需时间最少,约为3h。当钎焊时的冷却速率降低时,η’相的形成速度减慢。研究同时表明Ni的加入显著抑制了η’相的形成。(2)观察界面IMC的形貌发现,时效后的钎焊接头中界面Cu6Sn5三叉晶界处有圆形孔洞存在。孔洞数量在钎焊后空冷的Sn-0.7Cu/Cu中最多,在钎焊后炉冷及含Ni的接头中较少。主要原因是：钎焊时助焊剂等造成的气泡易吸附在界面IMC晶界处,从而在时效过程中抑制了Cu6Sn5生长；降低钎焊接头凝固的冷却速率时,气泡有充分的时间逸出：添加少量的Ni后,提高了界面Cu6Sn5晶粒的长径比并加快了其在钎焊过程中的生长,不利于气泡的吸附。(3)研究Cu-Sn界面化合物厚度、形貌、钎料/IMC界面粗糙度等形态演变发现,IMC厚度、晶粒直径随时效时间延长、时效温度升高、冷却速率降低而不断增加。添加少量的Ni以后,Cu3Sn生长受到显著抑制,钎焊及高温时效条件下的Cu6Sn5晶粒得以细化。(4)钎焊接头剪切强度及断裂机制的研究结果表明：在钎焊、低应变速率、高剪切实验温度条件下,因钎料/IMC界面力学性能失配,裂纹易在界面IMC附近的钎料基体中形成并扩展,发生韧性断裂,接头剪切强度由钎料基体控制；试样经时效后,界面IMC厚度和晶粒直径的增加降低了IMC自身的断裂韧性,接头倾向于沿界面IMC发生脆性断裂,剪切强度较相同条件下钎料基体强度降低。界面的影响机制为：界面IMC厚度和晶粒大小仅影响断裂类型；界面Cu6Sn5中的缺陷及高弹性模量的界面IMC都会促进脆断的发生；界面Cu6Sn5的固态结构转变和钎料/IMC界面粗糙度影响很小。(5)剪切测试参数对接头剪切行为的影响体现在：随着剪切实验温度由室温降低到-40℃,钎料与IMC之间的弹性模量差值减小,钎料/IMC界面力学失配得到改善,接头更倾向于表现为钎料基体的行为；随着应变速率的提高,钎料与IMC之间的弹性模量差值增加,界面失配严重,倾向于表现为界面IMC的脆断。更多还原

【Abstract】 With the development of portable, reliable and lead-free electronic products, there are great challenges for the reliabilities of electronic packaging and solder interconnects. The mechanical failure under shear stress of solder bump has become one main reason for the open circuit of electronic devices. The evolution of interfacial intermetallic compound (IMC), mainly Cu6Sn5 in soldering with Cu substrate, is regarded as a critical factor. With the miniaturization of bump in size and the wild acceptation of Sn-based solders, the increasing proportion of interfacial IMC layer also aggravates the discontinuity of the intrinsic behaviors at the solder/IMC interface. However, the evolution of the interfacial IMC in service is complex, such as coarsening of grains, flattening of solder/IMC interface, increasing of defects. Moreover, the crystal structure of Cu6Sn5 would transform from one to another. During this procedure, the volume expansion might result in stress at the solder/IMC interface which is harmful to the integrity of interconnects. So far, it is not very clear that how interfacial IMC layer influences the shear deformation and fracture of solder joints. The effects of the transformation of the crystal structure of Cu6Sn5 have not been verified. Therefore, studies on the interfacial evolutions and their effects on the shear behavior of solder joints are very helpful for the development of high-density lead-free packaging technology.In this study, Sn-0.7Cu/Cu joint was selected. Minor addition of Ni was conducted to modify the interfacial structures. The evolutions of structure and morphology (including the thickness, grain size, interfacial roughness and defects) of interfacial Cn-Sn IMC with aging time and temperature were studied, respectively. Solder joint with a sandwich structure was used in shear test. The correlations between the interfacial evolutions and the shear behavior were investigated at different shear strain rates and experimental temperatures. The conclusions are as follows:(1) The solid-state transformation of interfacial Cu6Sn5 in Sn-0.7Cu/Cu was studied by XRD. Results show that after isothermal aging, the time-temperature-formation curve of the interfacialη’-Cu6Sn5 under air cooling condition presented as a "C" shape. A minimum formation time was observed at aging temperature 135-150℃. When the cooling rate of soldering decreased, the time needed for the formation ofη’phase was increased. The results also show that the addition of Ni inhibited the formation ofη’phase significantly. (2) Voids were observed at the triple grain boundary of aged interfacial Cu6Sn5 from the top morphology. The amount of the voids were the most in Sn-0.7Cu/Cu joint under air cooling condition, but fewer in those cooling with furnace and containing minor Ni. The main reason is that the growth of interfacial Cu6Sn5 during aging procedure was inhibited by the pores adsorbed on the boundaries of Cu6Sn5 grains. With the decreasing of cooling rate in soldering, there was enough time for pores to escape from the triple grain boundary. With minor addition of Ni, the ratio of length to radius and the growth rate of interfacial IMC were enhanced, and thus the pores became metastable.(3) The morphology evolutions of interfacial Cn-Sn IMC, including the thickness, grain size and interfacial roughness, were studied, respectively. With the elongation of aging time, elevation of temperature and decrease of cooling rate, the thickness and grain size were continuously increased. After minor addition of Ni, the growth of Cu3Sn was significantly inhibited. Moreover, the Cu6Sn5 grains in soldering and those aged at high temperatures were refined.(4) The shear strength and fracture mechanism of solder joints were analyzed. Results indicate that under as-soldered state, low strain rate or high testing temperature condition, ductile fracture was observed of which the crack propagated in the solder matrix near the interface due to the mismatch of the mechanical properties between solder and interfacial IMC. The shear strength of the solder joint was controlled by the solder matrix. After aging, the fracture toughness was decreased due to the coarsening of interfacial IMC. Brittle fracture along IMC was discovered, of which the shear strength was reduced compared with that of solder matrix under the same aging condition. The influencing mechanisms of the interface are listed as follows. The thickness and grain size of interfacial IMC only affected the type of fracture. The defects in interfacial Cu6Sn5 and the higher elastic modulus of interfacial IMC would promote the brittle fracture. The influence of the crystal structure transformation of interfacial Cu6Sn5 and the roughness of solder/IMC interface was small.(5) When the deformation temperature in shear test decreased from room temperature to-40℃, the difference of elastic modulus between the solder matrix and IMC was reduced and the mechanical mismatch was alleviated. The joint inclined to be controlled by the behavior of solder matrix. When the strain rate increased, the difference of the elastic modulus was increased and the mismatch became serious. Under this condition, the brittle behavior of interfacial IMC was more dominant.更多还原