【作者】 沈骏；

【导师】 高后秀；

【作者基本信息】 天津大学 ， 材料学， 2005， 博士

【摘要】 微电子封装工艺中,起到热、电和机械连接作用的无铅焊料合金组织中金属间化合物的形态和分布直接影响着该合金的连接性能。本文以共晶配比附近的Sn-Ag合金为研究对象,通过改变成分配比和凝固速率系统研究了其凝固过程中金属间化合物相（Ag₃Sn）的析出规律。结合显微组织观察、热分析和热力学计算,从凝固过程两相竞争生长的角度揭示了块状金属间化合物Ag₃Sn的形成机理。采用高温时效模拟焊点的高温服役过程,阐明了在持续高温环境下合金组织中金属间化合物相的演化规律。最后探讨了第三组元及异相纳米ZrO2微粒的掺入对合金组织中金属间化合物的析出控制及强化机理。上述研究包括的主要内容和获得的结论是:通过改变冷却介质,系统研究了亚共晶、共晶和过共晶Sn-Ag合金在不同凝固速率下其组织中金属间化合物的形成规律。结果表明:只在缓冷过共晶合金凝固组织中有块状金属间化合物Ag₃Sn的析出。在较快凝固速率下,三种合金组织均呈现亚共晶组织特征,即由初生β-Sn枝晶和由Ag₃Sn相与β-Sn相共晶体所构成。这归因于非平衡条件下的动力学过冷使合金凝固过程按亚稳伪共晶反应进行。提高凝固速率对合金组织的影响为:一方面,符合经典共晶合金枝晶生长规律,其β-Sn枝晶得到细化,即: d = 3. 7t_f^0.43(其中为β-Sn枝晶二次枝晶间距,凝固速率在0.08～10⁴ Ks^-1的范围内)。另一方面,符合弥散强化原理,在共晶体区域中析出纳米Ag₃Sn相提高了其显微硬度。在低速凝固速率下,发展了一种通过合金凝固时的名义热容曲线来确定固相体积转变过程,进而确定组织中块状金属间化合物体积分数的有效方法,结合定量金相分析和热力学计算,揭示了过共晶合金组织中块状金属间化合物Ag₃Sn的形成机理,即:凝固时,合金熔体中的共晶Ag₃Sn相因与初生Ag₃Sn相有共同的晶体结构,会在小过冷度下依附于后者生长并成为块状金属间化合物Ag₃Sn,并且该块状相的体积分数值随着合金凝固速率的提高而增大。采用高温时效处理模拟焊点高温服役过程研究了Sn-3.5Ag合金在持续高温环境下的组织稳定性。结果表明:合金组织中金属间化合物Ag₃Sn相的演化符合系统自由能最小原理。平衡凝固合金组织中Ag₃Sn相趋于破裂和表面球化;而非平衡凝固合金组织中Ag₃Sn相在初生β-Sn枝晶晶界的扩散推移作用下合并成为块状金属间化合物Ag₃Sn。通过精确的热焓计算和精细的组织分析,揭示了非平衡凝固合金组织中纳米Ag₃Sn相的生长驱动力源于其较高的表面能,使其处于热力学亚稳状态。但由于该纳米Ag₃Sn相仅局部分布于共晶组织中,因此该合金在室温下仍然结构稳定。针对缓冷凝固Sn-Ag合金组织中块状金属间化合物Ag₃Sn对焊接性能的不利影响,探讨了在该合金中加入少量第三组元Cu、In和Zn以及异相纳米ZrO₂微粒来控制其形成的可行性。结果表明:在上述组元中只有Zn能影响合金凝固后组织中块状金属间化合物Ag₃Sn的形成,这归因于Zn与Ag在低过冷度下形成的高温β′-AgZn相改变了焊料中的相析出次序,从而有效降低了其熔体过冷度。因此,在选择能控制Sn-Ag合金组织中块状Ag₃Sn相形成的第三组元时,应考虑在凝固时能与Ag结合形成金属间化合物并在Ag₃Sn相之前析出的组元。而异相纳米ZrO₂微粒,因其具有较高的表面活性,可通过表面吸附效应来降低合金组织中金属间化合物Ag₃Sn的表面能,并抑制其进一步生长,从而可有效避免块状金属间化合物Ag₃Sn的形成,并使合金组织细化、力学性能提高。更多还原

【Abstract】 The formation and distribution of Intermetallic Compounds （IMCs） in the microstructure of lead-free solders which acts as the thermal, electronic and mechanical connections, directly affect the soldering performance during micro- electronic packaging process. In the present paper, near eutectic composition Sn-Ag alloys were selected and the separation of IMCs in the solidification processes of them was systematically explored by changing the content of ingredient and the subsequent solidification rate of the alloy melt. The formation mechanism of bulk Ag₃Sn IMCs in solidification was clarified according to the competitive growth of two eutectic phases by means of metallographic analysis, calorimetric measurements and thermodynamic calculation. The evolution of IMCs in the microstructure of the solders working under high-temperature environment was investigated by high- temperature aging treatment. As last, the governing of IMCs and the corresponding strengthening effect was discussed in view of the addition of the third component and ZrO₂ nanoparticles in the solder. More details were given as follows.Firstly, the formation mechanism of IMCs in hypoeutectic, eutectic and hypereutectic Sn-Ag alloys under different solidification rates was systematically investigated by changing the cooling medium of the solder. The results indicate that the bulk Ag₃Sn IMCs only formed in slowly-cooled hypereutectic alloy. All solidified alloys exhibit typical hypoeutectic features, that is, primaryβ-Sn dendrites and lamellar eutectic structure of （β-Sn + Ag₃Sn）, which could be explained by the actual metastable pseudoeutectic reaction resulting from the kinetic undercooling in the condition of the non-equilibrium solidification. The influence of applied solidification rate on the alloy structure fits well to the prediction of classical theory of dendrite growth in eutectic system: the Secondary Dendrite Arm Spacing （d ） of theβ-Sn dendrites decreases with increasing the solidification rate and can be described by d = 3. 7t0f.43. It is also found out that those Ag₃Sn nanoparticles distributing in eutectic zone is benefit to improve the microhardness of alloy, which agrees well with the prediction of Dispersion Strengthening Principle.A new valid method, to determine the volume fractions of bulk Ag₃Sn IMCs in the solidified microstructure of the solder, was firstly developed from the formation rate of solid phase from the measured apparent heat capacity data during the slow cooling process. The thus obtained fractions of bulk Ag₃Sn IMCs in the hypereutecticalloy were compared to that from the metallographic analysis and thermodynamic calculation. During solidification, the leading Ag₃Sn phase could nucleate adhering to the primary Ag₃Sn phase due to their matching crystalline orientation relationships, leading to the formation of bulk Ag₃Sn IMCs at small melt undercooling, then the fraction of bulk Ag₃Sn IMCs increased gradually with increasing the applied cooling rate.Next, the structural stability of Sn-3.5Ag solder during serving in high- temperature environment was explored by means of high-temperature aging treatment. It is observed that the Ag₃Sn phase had broken up and spheroidized in the equilibrium specimen, while the Ag₃Sn phase combined to bulk Ag₃Sn IMCs after the boundary migration of primaryβ-Sn dendrites in non-equilibrium specimen during high-temperature aging. The evolution of the Ag₃Sn IMCs during annealing fits with the minimization of total Gibbs energy. The precise determination of heat enthalpy difference and structure analysis show that the driving force for the growth of Ag₃Sn nanoparticles is coming from the high surface energy when the alloy is in a metastable state. However, the non-equilibrium structure of the solder maintains to be stable for the Ag₃Sn nanoparticles distributing only in the eutectic zone.At last, the possible governing of the formation of bulk IMCs was proposed by adding a small amount of third component （Cu, In and Zn） or ZrO₂ nanoparticles in the solder to avoid the formation of the bulk Ag₃Sn IMCs in slowly-cooled Sn-Ag alloy which may bring serious problems in solder joint. The results indicate that only the addition of Zn can suppress the formation of bulk Ag₃Sn IMCs. This observation could be attributed to the formation and separation ofβ′-AgZn phase, which will decrease the melt undercooling significantly at small undercooling in Sn-3.5Ag-2.0Zn alloy. Hence, the powerful method in governing the formation of bulk Ag₃Sn IMCs by adding third element in Sn-Ag alloy is to select a possible element which can react with Ag element and form other IMCs before the separation of Ag₃Sn phase during solidification. As an effective surface-active material, the introduction of ZrO₂ nanoparticles can suppress the formation of bulk Ag₃Sn IMCs by reducing the surface energy of Ag₃Sn according to the Surface Adsorption Effect. Hence, they can greatly refine the microstructure and improve the mechanical property.更多还原