全金属间化合物微焊点的制备及性能表征研究进展

基于微凸点和硅通孔的3D封装是一种通过多层芯片垂直互连堆叠，将芯片技术与封装技术进行结合的新技术，满足了电子产品微型化、高性能的发展需求，是实现下一代超大规模集成微系统的核心互连技术。3D封装技术对互连焊点有一个特殊的要求，即在向上堆叠芯片时，低级封装的焊点不应被重新熔化。低温下制备全金属间化合物(Intermetallic compound, IMC)微焊点为实现叠层芯片互连提供了新的解决方案。本文对近年来全IMC微焊点的制备工艺和基本原理进行综合分析；揭示Cu-Sn系统界面反应的扩散机制及IMC生长动力学规律；对比不同键合参数下全IMC微焊点的服役可靠性。最后，指出各种全IMC微焊点制备工艺的优缺点，并对其未来研究发展趋势进行展望。     

磁控溅射Cu-Cr合金薄膜与Sn-Ag-Cu焊料在时效处理时的界面反应

采用磁控溅射的方式沉积不同Cr含量的Cu-Cr合金薄膜,通过与Sn-Ag-Cu(SAC)焊料在240 ℃下回焊形成焊点结构,然后将试样置于180 ℃下进行真空时效处理。研究Cu-Cr合金作为凸点下金属化(UBM)层时与SAC形成焊点的焊接可靠性。使用配备能量色散X射线光谱仪的场发射扫描电镜和多功能推力测试仪等分析界面金属间化合物(IMC)的形貌及焊点的剪切强度。结果表明,SAC/Cu-Cr焊点结构在回焊后形成了不同于传统的SAC/Cu焊点扇贝状IMC的针状IMC。在时效处理后,Cr在晶界处的偏析形成了富铬层,其作为扩散阻挡层阻碍Cu扩散到IMC中,使得Cu₃Sn和柯肯达尔空洞的生长受到抑制。剪切强度测试结果表明,回焊后SAC/Cu-Cr试样比SAC/Cu试样具有更高的剪切强度。Cr靶电流为1.5 A的Cu-Cr合金UBM层形成的焊点结构具有较小的IMC厚度,且拥有最高的焊点剪切强度。证实了Cu-Cr 合金UBM层有利于提高焊接可靠性。     

β-Sn晶粒取向及温度对Cu/SAC305/Cu微焊点时效界面反应的影响

为了探究不同等温时效温度下β-Sn晶粒取向及晶界特征对界面反应的影响,采用准原位观测手段对不同Sn取向的Cu/Sn3.0Ag0.5Cu/Cu(Cu/SAC305/Cu)微焊点进行研究. 结果表明,在不同温度下时效时,微焊点两侧界面IMC(Cu₆Sn₅ + Cu₃Sn两相)自始至终呈现对称性生长,表明时效过程中β-Sn晶粒取向及晶界的存在不会影响界面反应. 但是随着时效温度的升高,界面IMC的形貌和厚度发生明显变化. 在100 ℃时效后,界面处生成扇贝状的Cu₆Sn₅和较薄的不连续的Cu₃Sn层;在125 ℃时效后,界面处生成扇贝状的Cu₆Sn₅和较薄的连续的Cu₃Sn层;而在150 ℃时效后,界面IMC由层状Cu₆Sn₅和层状Cu₃Sn双层结构组成. 时效温度的升高促使Cu和Sn原子扩散加快,促进了扇贝状Cu₆Sn₅向层状转变并造成Cu₃Sn的快速生长. 同时,基于界面IMC厚度随时效时间的演变规律,获得了不同时效温度下微焊点界面IMC生长曲线,可为Sn基微焊点的可靠性评价提供依据.     

电迁移作用下SnPb与Ni(P)界面金属间化合物的极性生长

采用球栅阵列封装(BGA)焊点研究共晶SnPb焊点中的电迁移行为,分析了电迁移作用下SnPb焊点与Ni(P)镀层界面金属间化合物(IMC)的极性生长现象,从原子迁移的角度提出了互连焊点微结构演化的微观机理.焊点在焊接过程中形成厚度约为2 μm的Ni3Sn4 IMC层,随后的120℃热处理并不会导致界面IMC的明显生长.而电迁移作用下,阳极焊点与镀层界面IMC出现异常生长,同时阴极焊点与镀层界面IMC生长受到抑制,最终在同一焊点中形成极性生长的现象.界面IMC的极性生长与电迁移引起的原子通量有关,Sn原子通量方向与电子流方向相同,而Ni原子通量方向相反,导致阳极界面IMC的异常生长,而相同的原子迁移特性导致阴极界面IMC的生长受到抑制.     

热老化与热循环条件下Bi对Sn-1.0Ag-0.5Cu无铅焊点界面组织与性能的影响

研究了在热老化和热循环过程中Bi的添加对低银无铅钎料Sn-1.0Ag-0.5Cu (SAC105)的BGA焊点界面微观组织演变的影响,分析了Bi的添加对SAC105微焊点热老化与热循环剪切性能的影响. 结果表明,Bi元素添加量为2%(质量分数)时,对热循环过程中微焊点界面处金属间化合物(IMC)层整体厚度的增加起到抑制作用,同时阻碍了热循环过程中因焊点热失配导致的IMC层破碎. 但是Bi的添加促进了界面IMC层中Cu₃Sn层的生长,因此在经过20天以上热老化处理后SAC105-2Bi微焊点界面IMC层厚度与SAC105微焊点接近. 此外,Bi的添加可以显著提升热循环处理后SAC105微焊点的抗剪切能力. SAC105-2Bi微焊点的剪切力学性能受到热循环处理的影响较小. 与SAC105微焊点相比,SAC105-2Bi微焊点的断裂模式更早地从韧性断裂向脆性断裂转变,因此Bi的添加降低了SAC105微焊点的热循环可靠性.     

Sn-1.5Ag-2Zn低银无铅焊料与取向铜界面研究

目的 高密度封装技术能减少焊盘尺寸和焊盘里面所含晶粒的数量,当多晶焊盘转为单晶焊盘时,晶粒的取向会对界面处金属间化合物的形成产生重要影响。选取具有发展前景的Sn-1.5Ag-2Zn作为焊料合金,研究焊料与单晶(111)铜基板界面反应,得到金属间化合物的生长动力学规律。方法 将Sn-1.5Ag-2Zn无铅焊料分别与单晶(111)铜基板及多晶紫铜基板在250 ℃下进行回流焊接5 min,将焊接后的样品在160 ℃下分别进行20 h,100 h,300 h,600 h热处理。用扫描电子显微镜背散射电子成像和二次电子成像、X射线衍射仪(XRD)、能谱仪等研究焊接界面处的显微组织、金属间化合物成分以及性能。结果 合金焊料与铜基板接触迅速生长出凹凸不平类似扇贝状Cu6Sn5金属间化合物层,与单晶铜表面形成的Cu6Sn5晶粒尺寸比多晶铜的大,单晶铜无晶界阻挡原子扩散,影响晶粒形核与长大。合金焊料与单晶(111)铜焊点在160 ℃下热处理20 h快速形成的Cu6Sn5生长速度是多晶铜上焊点的2倍左右。而后随热处理时间增加增长缓慢,热处理600 h后厚层Cu6Sn5由于裂纹扩散出现溃断,金属间化合物的厚度维持在3.5 µm。焊料与多晶铜焊点在热处理过程中生成的Cu5Zn8起到阻挡层的作用,阻挡焊料与铜基板接触,抑制Cu6Sn5生成,热处理300 h后Cu5Zn8破碎分解,Cu6Sn5在阻挡层消失后快速生长,厚度约为2.8 µm。结论 单晶铜上焊点金属间化合物的厚度比多晶铜上金属间化合物的厚度多0.7 µm,热处理后合金焊料与单晶铜界面形成的金属间化合物致密性更好,基本没有孔隙,而合金焊料与多晶铜界面上金属间化合物晶粒间存在明显孔隙,可以预测合金焊料与单晶铜界面的焊接质量更好。     

工艺参数对Sn-0.3Ag-0.7Cu/Cu焊点界面组织和力学性能的影响

互连焊点界面反应形成的金属间化合物(IMC)对焊点服役可靠性会产生显著影响。研究了不同工艺参数(回流温度、回流时间和回流次数)条件下,Sn-0.3Ag-0.7Cu/Cu焊点界面金属间化合物的演变及对焊点力学性能的影响,同时对焊点断裂机制进行了分析。结果表明,随着回流温度和回流时间的增加,金属间化合物η-Cu6Sn5相的形貌由贝状向板条状转变,并可观察到η相溶入焊点内部。在265℃回流时,随着回流时间增加,贝状η相不断长大,晶粒数不断减少;界面IMC的生长符合幂指数生长规律,其生长指数为0.339。回流次数对焊点剪切强度的影响为先增后减,断裂模式从纯剪切断裂到微孔聚集型断裂再到局部脆断转变。     

界面耦合作用对Cu(Ni)/Sn-Ag-Cu/Cu(Ni)BGA焊点界面IMC形成与演化的影响

研究了焊盘材料界面耦合作用对Cu(Ni)/Sn-3.0Ag-0.5Cu/Cu(Ni)BGA(Ball Grid Array)结构焊点焊后态和125℃等温时效过程中界面金属间化合物(IMC)的成分、形貌和生长动力学的影响.结果表明.凸点下金属层(UBM)Ni界面IMC的成分与钎料中Cu含量有关,钎料中Cu含量较高时界面IMC为(Cu.Ni)6Sn5.而Cu含量较低时,则生成(Cu,Ni)_3Sn_4;Cu-Ni耦合易导致Cu/Sn-3.0Ag 0.5Cu/Ni焊点中钎料/Ni界面IMC异常生长并产生剥离而进入钎料.125℃等温时效过程中.Sn-3.0Ag-0.5Cu/Cu界面IMC的生长速率常数随钎料中Cu含量增加而提高.Cu Cu耦合降低一次回流侧IMC生长速率常数;Cu Ni耦合和Ni-Ni耦合均导致焊点一次回流Ni侧界面IMC的生长速率常数增大,但Ni对界面IMC生长动力学的影响大于Cu;Ni有利于抑制Cu界面Cu_3Sn生长.降低界面IMC生长速率,但Cu-Ni耦合对Cu界面Cu_3Sn中Kirkendall空洞率无明显影响     

TiO_2纳米颗粒掺杂对Sn-3.0Ag-0.5Cu-xTiO_2焊点界面Cu_6Sn_5 IMC晶粒生长的影响机理

研究了Ti O2纳米颗粒掺杂影响回流焊过程中Sn-3.0Ag-0.5Cu-x Ti O2焊点界面Cu6Sn5金属间化合物(intermetallic compound,IMC)晶粒生长机理.基于Cu原子扩散通量驱动晶粒成熟生长(flux driven ripening,FDR)理论模型分析了Cu6Sn5IMC晶粒生长机理.结果表明,Ti O2纳米颗粒掺杂改变了焊点界面Cu6Sn5IMC晶粒形貌和尺寸.含Ti O2纳米颗粒的焊点Cu6Sn5IMC晶粒尺寸要小于不含Ti O2纳米颗粒的焊点,且晶粒分布要更加均匀.试验数据与FDR理论模型基本吻合.Cu6Sn5IMC晶粒生长指数分别为0.346,0.338,0.332和0.342,这说明Cu6Sn5IMC晶粒生长是由原子互扩散和晶粒成熟共同控制.     

N5单晶高温合金TLP连接工艺与机理研究

文中针对N5单品高温合金的TLP连接问题,研究了N5单品高温合金TLP接头组织演变规律及其与工艺参数的关系,探讨了接头界面形成机理及接头单晶化机理,通过EBSD晶体取向差分析获得了接头单晶化的条件,并分析了工艺参数及晶体取向差对接头力学性能的影响。论文还研究了TLP热过程对N5单晶高温合金母材组织的影响。     

Effect of rare earth metal Ce addition to Sn-Ag solder on interfacial reactions with Cu substrate

The effect of adding a small amount of rare earth cerium (Ce) element to low Ag containing Sn-1wt%Ag Pb-free solder on its interfacial reactions with Cu substrate was investigated. The growth of intermetallic compounds (IMCs) between three Sn-1Ag-xCe solders with different Ce contents and a Cu substrate was studied and the results were compared to those obtained for the Ce-free Sn-1Ag/Cu systems. In the solid-state reactions of the Sn-1Ag(-xCe)/Cu solder joints, the two IMC layers, Cu₆Sn₅ and Cu₃Sn, grew as aging time increased. Compared to the Sn-1Ag/Cu joint, the growth of the Cu₆Sn₅ and Cu₃Sn layers was depressed for the Ce-containing Sn-1Ag-xCe/Cu joint. The addition of Ce to the Sn-Ag solder reduced the growth of the interfacial Cu-Sn IMCs and prevented the IMCs from spalling from the interface. The evenly-distributed Ce elements in the solder region blocked the diffusion of Sn atoms to the interface and retarded the growth of the interfacial IMC layer.     

Electromigration performance of Pb-free solder joints in terms of solder composition and joining path

The electromigration (EM) performance of Pb-free solder joints is investigated in terms of solder composition (Sn-0.5Cu vs. Sn-1.8Ag) and joining path to Cu vs. Ni(P) under bump metallization (UBM). In the double-sided joints of Ni(P)/solder/Cu, the micro-structure of solder joints and the interfacial intermetallic compound (IMC) layers are significantly affected by solder composition and joining path. The as-reflowed microstructure of Sn-0.5Cu joints consists of small columnar grains with thin IMC layers at both UBM interfaces, independent of the joining path, while Sn-1.8Ag joints have a few large grains of low-angle grain boundaries with thick IMC layers when joined first to Ni(P) UBM, but a few 60° twins with thin IMC layers when joined first to Cu UBM. The EM stressing under high current densities at 150°C reveals that Sn-1.8Ag joints have a superior lifetime compared to Sn-0.5Cu joints. In addition, the EM lifetime of Sn-1.8Ag joints reflowed first on Ni(P) UBM is the longest among four groups of the solder joints examined.     

电迁移对Ni/Sn3.0Ag0.5Cu/Cu焊点界面反应的影响

研究了温度为150℃,电流密度为5.0×10³A/cm²的条件下电迁移对Ni/Sn3.0Ag0.5Cu/Cu焊点界面反应的影响.回流焊后在Sn3.0Ag0.5Cu/Ni和Sn3.0Ag0.5Cu/Cu的界面上均形成了（Cu,Ni）₆Sn₅型化合物.时效过程中界面化合物随时效时间增加而增厚,时效800 h后两端的化合物并没有发生转变,仍为（Cu,Ni）₆Sn₅型.电流方向对Cu基板的消耗起着决定作用.当电子从基板端流向芯片端时,电流导致基板端Cu焊盘发生局部快速溶解,并导致裂纹在Sn3.0Ag0.5Cu/（Cu,Ni）₆Sn₅界面产生,溶解到钎料中的Cu原子在钎料中沿着电子运动的方向向阳极扩散,并与钎料中的Sn原子发生反应生成大量的Cu₆Sn₅化合物颗粒.当电子从芯片端流向基板端时,芯片端Ni UBM层没有发生明显的溶解,在靠近阳极界面处的钎料中有少量的Cu₆Sn₅化合物颗粒生成,电迁移800 h后焊点仍保持完好.电迁移过程中无论电子的运动方向如何,均促进了阳极界面处（Cu,Ni）₆Sn₅的生长,阳极界面IMC厚度明显大于阴极界面IMC的厚度.与Ni相比,当Cu作为阴极时焊点更容易在电迁移作用下失效.     

老化对Sn-Ag-Cu焊料/Ni-P镀层界面结构和剪切强度的影响

对Sn-3．5Ag-0．7Cu／Ni—P界面上的焊点进行了150℃固相老化和250℃液相回流老化实验．两种条件下焊料体内和界面处金属间化合物的成分、长大速率及形貌均有较大差异．在液相回流条件下金属间化合物长大更快,对焊点的可靠性有较大的影响．延长固相老化时间,焊点内生成大尺寸的Ag3Sn相;高温液相回流有Ni3P层生成,降低焊点的焊接强度．     

α-CoSn3 IMC弹性模量各向异性的第一性原理

焊点微型化的同时，界面IMC在整个焊点中所占比例显著增大，微焊点在服役过程中对界面IMC的取向变得更加敏感. 文中基于密度泛函理论计算了α-CoSn₃的晶格常数和弹性模量各向异性，并比较了常见IMC的各向异性指数. 结果表明，[100]方向的抗线性压缩能力小于其它两个主晶轴；(100)剪切面抵抗剪切变形的能力大于其它两个主晶面. 根据差分电子密度图分析了成键轨道的具体贡献. α-CoSn₃三维弹性模量随晶体取向变化的三维图及投影图直观的展现它们各向异性的性质. 计算了α-CoSn₃各向异性指数，并与常见IMC比较了各向异性程度，得到α-CoSn₃各向异性程度居中的结论.     

EVOLUTION OF MICROSTRUCTURE OF Sn-Ag-Cu LEAD-FREE FLIP CHIP SOLDER JOINTS DURING AGING PROCESS

Flip chip bonding has become a primary technology that has found application in the chip interconnection process in the electronic manufacturing industry in recent years. The solder joints of the flip chip bonding are small and consist of complicated microstructures such as Sn solution, eutectic mixture, and intermetallic compounds （IMCs）, whose mechanical performance is quite different from the original solder bulk. The evolution of microstructure of the flip chip solder joints under thermal aging was analyzed. The results show that with an increase in aging time, coarsening of solder bulk matrix and AuSn4 IMCs occurred within the solder. The IMCs that are formed at the bottom side of the flip chip bond were different from those on the top side during the aging process. （Cu, Ni, Au）0Sn5 were formed at the interfaces of both sides, and large complicated （Au,Ni, Cu）Sn4 IMCs appeared for some time near the bottom interface after aging, but they disappeared again and thus （Cu,Ni, Au ）0Sn5 IMC thickness increased considerably. The influence of reflow times during the flip chip bonding （as-bonded condition） on the characteristics of interfacial IMCs was weakened when subjected to the aging process.     

Effect of bismuth on the growth kinetics of intermetallic compounds in Sn-3.5Ag solder joints: A growth kinetic model

The effect on the growth kinetics of the intermetallic compounds (IMCs) in solder/Cu joints, caused by adding Bi to eutectic Sn-3.5Ag solder alloy, was examined at the aging temperatures of 150°C and 180°C. The Cu₆Sn₅ layer growth was significantly enhanced, but the Cu₃Sn layer growth was slightly retarded by the addition of Bi, resulting in significant growth enhancement of the total (Cu₆Sn₅+Cu₃Sn) IMC layer with increasing Bi addition. The IMC layer growth in the Bi-containing solder joints was accompanied by the accumulation of Bi ahead of the Cu₆Sn₅ layer that resulted in the formation of a liquid layer at the Cu₆Sn₅/solder interface. A kinetic model was developed for the planar growth of the Cu₆Sn₅ and Cu₃Sn layers in the solder joints, accounting for the existence of interfacial reaction barriers. Predictions from the kinetic model showed that the experimental results could be well explained by the hypothesis that the formation of a Bi-rich liquid layer at the Cu₆Sn₅/solder interface reduces the interfacial reaction barrier at the interface.