【作者】 杨帆；

【导师】 李明雨；

【作者基本信息】 哈尔滨工业大学 ， 材料科学与工程， 2021， 博士

【摘要】 随着微电子技术的不断更迭与产业发展的迫切要求,电力电子器件正在朝着高频化、高度集成化和高度智能化的方向发展。尤其在部分应用领域,电力电子器件需要面临高温、强辐射、高电流密度等苛刻的服役环境。这对器件中功率芯片的封装技术带来了巨大挑战。目前高温功率芯片封装的解决方案中,低温烧结银技术是最具前景的封装方式。由于尺寸效应,纳米或微米银颗粒可以在远低于块体银熔点的温度下实现冶金接合与组织致密化。银颗粒烧结成形后的烧结体具有着与致密块体银相似的物理特性,如高熔点、高导热、高导电以及高机械强度等。虽然低温烧结银膏的制备以及互连焊点工艺探索已经取得了很大的进展,但是要想将烧结银技术进行广泛地推广与应用,还需要深入理解银烧结体及其互连焊点的物理特性,特别是不同服役状态下的力学性能。因此,本文以低温烧结纳米银的方法为例,阐述纳米银互连焊点在不同服役状态下的组织结构与力学性能,揭示组织结构与力学性能间的内在关系并改善焊点的服役可靠性。本文提出了一种梯度加压、一次成形的热压烧结工艺,成功制备了无裂纹缺陷的纳米银烧结体样品,并研究了不同烧结工艺参数下银烧结体的组织结构与力学性能。组织结构方面,银烧结体的致密度主要取决于烧结压力,孔洞尺寸分布和形状则受到烧结压力与烧结温度的共同影响,组织的粗化行为主要受烧结温度调控。力学性能方面,银烧结体的杨氏模量取决于密度,屈服强度和延伸率受到孔隙率与晶间组织结构的共同影响。孔洞尺寸分布与形状、晶粒尺寸和韧带尺寸只能作为影响银烧结体力学性能的次要因素。此外,对比了纳米银烧结体、微纳米银烧结体和微米银烧结体的组织结构和力学性能,其韧带屈服强度、延伸率和应变速率敏感性均与晶粒尺寸相关。目前焊点烧结工艺升温时间长,不利于纳米银的致密化扩散与工业化应用。本文采用预烘干后直接热压烧结的两步法进行了烧结焊点制备,分别研究了焊点组织结构、剪切强度、纳米银/铜焊盘界面连接机制与冷热冲击性能。结果表明,纳米银焊点在经过225℃、5 MPa和10 min的热压烧结后,焊点剪切强度达到161.7 MPa,优异的剪切强度主要归因于低孔隙率以及细晶强化。烧结过程中,纳米银和铜焊盘间根据局部压力的差异分别形成Ag/Cu₂O/Cu和Ag/Cu两种连接界面。此外,微纳米银焊点由于具有更低的热膨胀系数和杨氏模量,在冷热冲击过程中组织稳定性优于纳米银焊点。铜的氧化问题严重威胁着铜基板上烧结银焊点的高温服役可靠性。本文首先对铜基板上热压烧结纳米银焊点的高温服役可靠性进行了评估。在此基础上,提出了引入局部瞬时液相扩散焊的方法来提高纳米银焊点在高温服役过程中的组织稳定性。结果表明,热压烧结纳米银焊点中的烧结银为气密性组织,保证了焊点在200℃下长期服役可靠性以及300℃下短期服役可靠性。老化过程中,焊点的组织退化主要来自于铜氧化物的生长以及银铜界面元素互扩散形成的疏松组织结构。经过改良后的复合焊点展现了优异的高温稳定性,焊点两侧的Ag-Sn化合物可以有效阻碍氧气的扩散与烧结银组织粗化,下界面的Cu-Sn和Ag-Sn化合物可以降低铜原子的扩散速率进而抑制铜氧化物生成。经过300℃老化1000 h,复合焊点依然可以保持60 MPa以上的剪切强度。更多还原

【Abstract】 With the continuous innovation of microelectronic technology and urgent requirements for industrial development,power electronic devices are developing towards the trend of high freaquency,high integration and high intelligence.Especially in some specific areas,the power electronic devices are supposed to serve under the environment of high temperature,strong radiation and high current density.Great challenges are imposed on the packaging technology of power chips in the devices.Low temperature sintering Ag technology is the most promising solution of chip packaging for high temperature application.Due to the size effect,metallurgical bonding and densification among nano/micro Ag particles occur at a temperature much lower than the melting point of bulk Ag while the sintered Ag exhibited similar physical properties with its bulk counterpart,such as high melting point,high thermal and electrical conductivity and high m echanical strength.Although the study on the preparation of low temperature sintering Ag paste and the fabrication of sintered Ag joints has achieved great success,it is significant to deeply understand the physical properties of Ag sintered bulk and its interconnection joints,especially the mechanical property at different service periods.Hence,the sintered Ag joints were investigated and the dependence of mechanical property on the microstructure at different service periods was studied in details.In this dissertation,a thermo-compression sintering method with gradient pressure and one-step forming was proposed,by which the Ag nanoparticle（NP）sintered bulk was prepared with no crack and defect.The microstructure and mechanical property of the Ag NP sintered bulk were investigated.In the respect of microstructure,the density of sintered bulk is mainly determined by sintering pressure,the size distribution and shape of pores are affected by sintering pressure and temperature and the coarsening behaviour is controlled by sintering temperature.In the repect of mechanical property,the Young’s modulus is determined by density and the yield strength and elongation are affected by porosity and intergranular microstructure,while the size distribution and shape of pores,grain size and ligament size have a only second order of the importance.In addition,the mechanical property of Ag NP sintered bulk,Ag microflake+nanoparticle（MF+NP）sintered bulk and Ag MF sintered bulk were compared and discussed.The application of sintering Ag technology is restricted by the long processing time.In this paper,the Ag sintered joints were prepared by a two-step method including low temperature drying and isothermal thermo-compression sintering.The microstructure and shear strength of joints,the bonding mechanisms between Ag NPs and Cu substrate and the mechanical reliability during thermal shock test were studied.The results show that the mean shear strength of Ag NP sintered joints is161.7 MPa when the sintering parameters are set as 225℃,5 MPa and 10 min.The superior mechanical strength is contributed to the low porosity and fine grain strengthening.Ag/Cu₂O/Cu and Ag/Cu interfaces formed between Ag NPs and Cu substrate during the sintering process.Besides,the Ag MF+NP sintered joints exhibited excellent structural stability during the thermal shock test,which was attributed to low coefficient of thermal expansion and Young’s modulus.The oxidation issue seriously threatens the reliability of sintered Ag joints on Cu substrate.First,the mechanical reliability of Ag NP sintered joints was evaluated during high temperature storage test.Then,the local transient liquid phase bonding was induced to improve the high temperature stability of Ag NP sintered joints.The results show that the hermetic microstructure forms in the sintered Ag,which guarantees long-term service at 200℃and short-term service at 300℃in air.The main reasons for the strength degradation of the aged joints are the growth of Cu oxide and the formation of loose microstructure due to nonequal interdiffusion of Ag and Cu atoms.The composite joints exhibited superior high temperature resistance.Ag-Sn intermetallic compounds（IMCs）formed at the edges of sintered joints that could inhibit the penetration of oxygen and the microstructural coarsening behaviour of sintered Ag.Cu-Sn and Ag-Sn IMCs formed at the bottom of the joints simultaneously.This IMC barrier hindered the diffusion of Cu atoms towards the sintered Ag layer and the formation of Cu oxides effectively.A high-temperature（300℃）storage test in air demonstrated that the shear strength of this kind of joints maintained above 60 MPa for 1000 h.更多还原