【作者】 杨帆；

【导师】 黄明亮； 姚全斌；

【作者基本信息】 大连理工大学 ， 材料学， 2016， 博士

【摘要】 微型化趋势下无铅焊点尺寸持续减小至微米尺度(微焊点),无铅界面反应受到焊点尺寸、钎焊工艺过程、基体晶体取向与交互作用等因素的影响更加显著,微焊点的可靠性面临严峻挑战。但是,目前经典的界面反应“熟化理论”模型无法解释微型化趋势下无铅界面反应面临的新问题。本论文研究了尺寸效应、钎焊工艺过程、基体晶体取向与交互作用等因素对纯Sn及Sn-Ag-Cu无铅焊点与Cu、Ni基体界面反应的影响,基于原子扩散通量控制的动力学理论及团簇形核理论,分别阐明了不同因素对界面微观组织的影响机制,建立了浓度梯度控制(CGC)的液／固界面反应理论模型,从理论和实验上诠释了无铅焊点界面反应尺寸效应的规律性；利用同步辐射实时成像技术原位表征了无铅钎焊工艺过程对微焊点界面反应的影响规律；揭示了微焊点中基体晶体取向与交互作用对界面金属间化合物(IMC)演变的影响机制；初步评价了微焊点界面组织对其拉伸性能的影响规律。论文主要研究结果总结如下：1.尺寸效应对焊点界面反应影响的研究表明：焊点尺寸的减小改变了钎焊界面反应中焊点界面处的溶质原子(Cu)通量,从而导致了界面反应尺寸效应的产生。具体表现为随着焊点尺寸减小,Sn-xAg-yCu/Cu焊点内平均Cu浓度增加,界面Cu6Sn5晶粒直径增加,界面IMC厚度增加,Cu基体消耗减少；Sn-3.0Ag-0.5Cu/Ni-P焊点内平均Cu浓度下降速率增加,进而加速了界面IMC由针状和短棒状(Cu,Ni)6Sn5转变为多面体状(Cu,Ni)6Sn5,再转变为针状和块状(Ni,Cu)3Sn4的进程；Cu/Sn-3.0Ag-0.5Cu/Ni-P焊点Cu侧(Cu,Ni)6Sn5晶粒直径增加,Cu基体消耗减少,Ni侧界面IMC保持针状的(Cu,Ni)6Sn5且厚度增加。基于原子通量建立了浓度梯度控制(CGC)的液／固界面反应理论模型,其计算结果与实验结果符合良好,揭示了界面溶质原子(Cu)浓度梯度是产生尺寸效应的根本原因；浓度梯度控制(CGC)的界面反应理论模型同样适用于Ni基体上的界面反应。2.钎焊工艺过程对焊点界面反应影响的研究表明：界面Cu6Sn5晶粒在升温阶段以0.11 μm/s的速率向钎料内溶解,在保温阶段以扇贝状晶粒形貌缓慢生长,而在降温阶段快速向钎料内生长使得高宽比增加,生长速率达到0.04μm/s;界面片状Ag3Sn相在升温阶段以0.1-0.2删s的速率完全溶入钎料,在降温阶段过冷(过冷度为31-55℃)条件下以超过5μm/s的速率在界面处随机析出；在温度梯度作用下片状Ag3Sn相倾向于在冷端析出,热端析出的片状Ag3Sn相倾向于再溶解进入钎料。阐明了液态团簇形核对片状Ag3Sn相随机析出以及在温度梯度下倾向于在冷端析出的作用机理,揭示了Ag原子热迁移通量与化学势通量相互作用导致Ag3Sn相热端溶解与冷端生长的机制。3.基体晶体取向与交互作用对焊点界面反应影响的研究表明：对于多晶基体微焊点,Cu/Sn/Cu微焊点界面形成扇贝状Cu6Sn5晶粒,并随反应时间熟化与合并长大,最终形成全IMC焊点；Cu/Sn/Ni微焊点中,交互作用促进了Ni原子参与Cu侧界面反应生成细小的(Cu,Ni)6Sn5晶粒,而Cu原子扩散至Ni侧界面加速了针状的(Cu,Ni)6Sn5晶粒的生长,最终形成的全IMC焊点内有孔洞残留。对于单晶基体微焊点,300℃下(001)Cu基体界面生成相互垂直规则排列的棱晶状Cu6Sn5晶粒,但随反应时间的延长逐渐转变为无择优取向的扇贝状形貌；250℃下(001)Ni基体界面能够形成平行排列的针状(Cu,Ni)6Sn5晶粒。阐明了界面两侧Cu6Sn5晶粒生长接近后加速生长的驱动力是界面两侧Cu原子熟化通量的交互作用,揭示了晶界迁移是两侧相对晶粒合并形成单一晶粒的机制；基于Cu原子通量控制的动力学与团簇理论,阐明了Cu-Ni交互作用与基体晶体取向影响下界面IMC形核生长与演变(形貌、取向)的机理。4.微焊点拉伸性能的研究表明：随界面IMC厚度增加,微焊点拉伸强度增加、脆性增大,断裂位置由钎料内部向IMC界面转变；与钎料焊点相比,全IMC焊点具有更高的拉伸强度(Cu/IMC/Cu焊点达到93MPa,Cu/IMC/Ni焊点达到110 MPa); Cu/IMC/Cu焊点的断裂发生在Cu3Sn/Cu6Sn5界面上,而Cu/IMC/Ni焊点的断裂发生在两侧(Cu,Ni)6Sn5生长融合相接触的界面上；原位表征了裂纹在界面缺陷及应力集中位置萌生与扩展并导致微焊点断裂的过程；阐明了微焊点界面IMC对钎料产生约束应力导致焊点强度增加的机理。更多还原

【Abstract】 The developing trend toward miniaturization leads to the downsizing of solder joints to micro-scale (micro solder joints), which resulted in a more serious effect on interfacial reaction from factors such as solder size, soldering process, and crystal orientation and cross-solder interaction of substrates. The service reliabilites of the micro solder joints were challenged. However, the classic "ripening theory" model of interfacial reaction cannot explain these new issues existing in the lead-free interfacial reaction under miniaturization. The present work researched the effect of solder size, soldering process, crystal orientation and cross-solder interaction of substrates on interfacial reaction of pure Sn and Sn-Ag-Cu lead-free solder joints on Cu and Ni substrates, based on atomic diffusion flux controlled kinetics and the cluster nucleation theory, the evolution mechanism of interfacial intermetallic compounds (IMCs) under the effcet of different factors were clarified, respectively; the theoretical model of concentration gradient controlled (CGC) liquid/solid interfacial reaction was established; the size effcet of interfacial reaction was interpreted from the theoretical and experimental perspective; the evolution of interfacial microstructure during the soldering process was characterized using synchrotron radiation real-time imaging technology; the function mechnisms of crystal orientation and cross-solder interaction of substrates on interfacial IMC evolution were revealed; the effect of interfacial microstructure on tensile properties of the micro solder joints was evaluated. The main conclusions are drawn as follows:1. For the size effect on interfacial reaction:the downsizing of solder joints changed the solute (Cu) atomic fluxes at the joint interfaces, and thus induced the size effect phenomenon. In detail, with the downsizing of solder joints, for Sn-xAg-yCu/Cu solder joints, the average Cu concentration in the solder increased, the Cu6Sns grain diameter increased, the thickness of interfacial IMC layer increased, and the consumption of Cu substrate decreased; for Sn-3.0Ag-0.5Cu/Ni-P solder joints, the decrease rate of average Cu concentration in the solder was accelerated, and consequently the interfacial IMC transformation process from needle and rod type (Cu, Ni)6Sns to polyhedral (Cu,Ni)6Sns, and finally to needle and bulk type (Ni,Cu)3Sn4 was accelerated; for Cu/Sn-3.0Ag-0.5Cu/Ni-P solder joints, (Cu,Ni)6Sns grain diameter increased and Cu consumption decreased on Cu side, and the interfacial IMC maintained needle type (Cu,Ni)6Sns with thickness increased on Ni side. Based on atomic flux, a concentration gradient controlled (CGC) interfacial reaction theoretical model was established, and the calculated results matched well with the experimental ones; interfacial Cu concentration gradient was revealed to be the root cause of the size effect. The concentration gradient controlled (CGC) interfacial reaction theoretical model can also be used to explain the size effect on Ni substrate.2. For the effect of soldering process on interfacial reaction:the top of Cu6Sn5 grains dissolved into the solder at a rate of 0.11μm/s at the heating stage, kept scallop shape and grew slowly at the temperature holding stage, and rapidly grew into solder at a rate of 0.04 μm/s to form high aspect ratio grains at the cooling stage; the interfacial Ag3Sn plates dissolved into the solder at a rate of 0.2μm/s at the heating stage and randomly precipitated at the interface at a rate of more than 5μm/s at the cooling stage with an undercooling of 31-55 ℃; Ag3Sn plates tended to precipitate on the cold end under temperature gradient, and those precipitated on the hot end tended to dissolve into the solder. The functional mechanisms of liquid clusters on random precipitation of Ag3Sn plates as well as cold end preciptiation of Ag3Sn plates under temperature gradient were clarified; interactions between thermomigration flux and the chemical potential flux of Ag atoms leading to Ag3Sn phase dissolution on the hot end and growth on the cold end were revealed.3. For the effect of crystal orientation and cross-solder interaction of substrates on interfacial reaction:in micro solder joints with polycrystalline substrates, scallop interfacial Cu6Sn5 grains formed in Cu/Sn/Cu micro joints, and grew ripened and combined larger until the full IMC joints formed after long reaction time; in Cu/Sn/Ni joints, cross-solder interaction induced that Ni atoms participated in the interfacial reaction on Cu side forming fine (Cu,Ni)6Sn5 grains, and Cu atoms diffused to the Ni side and promoted the grow of needle type (Cu,Ni)6Sns, finally, full IMC joints formed with holes inner the joint. In micro solder joints with single crystal substrates, the orthogonal and regularly arranged prism Cu6Sn5 grains formed at the interface of (001) Cu at 300℃, but transformed to the randomly orientated scallop ones with longer reaction time; parallel needle type (Cu,Ni)6Sns grains formed at the interface of (001) Ni at 250℃. The driven force of the accelerated growth of Cu6Sn5 grains from the opposite two interfaces after growing closed was clarified to be the cross-solder interaction of Cu ripening fluxes; grain boundary migration was revealed to be the mechanism of single grain formation from the opposite two grains; based on atomic diffusion flux controled kinetics and the cluster nucleation theory, the growth and evolution of morphology and orientation of IMCs under Cu-Ni cross interaction and the crystal orientation of substrates were clarified.4. For the tensile properties of micro solder joints:the tensile strength and brittleness of micro solder joints increased and the fracture position transformed from inner solder to IMC interface with the increasing of IMC thickness. The full IMC joint had the highest tensile strength, where Cu/IMC/Cu joint reached 93 MPa and Cu/IMC/Ni joint reached 110 MPa.The fracture position of Cu/IMC/Cu solder joint was at Cu3Sn/Cu6Sns interface, while that of Cu/IMC/Ni joint was at the contacting interface of (Cu,Ni)6Sns from the opposite two sides. The failure processes induced by the interfacial defects and the stress concentrated positions were in situ characterized. The mechanism that the constraining effect of IMC interface induced the increase of solder joint strength was clarified.更多还原