小尺寸Sn/Cu与Sn/FeNi焊点组织

IMR OpenIR

	小尺寸Sn/Cu与Sn/FeNi焊点组织
	陈材
学位类型	博士
导师	尚建库 ; 张磊
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	焊点厚度微观组织剪切性能冷却速度应变速率敏感性 Gap Size Microstructure Shear Property Cooling Rate Strain Rate Sensitivity
其他摘要	" 电子产品高密度封装的发展促使焊点持续小型化，尺寸下降将使焊点负载时变形受限，且影响焊点接合时的冶金行为，因此研究小尺寸焊点的冶金组织特点与其力学性能之间的相关性具有重要的意义。小型化中，作为可替代Cu的互连材料，FeNi与Sn的互连结构成为关注的焦点。本文以焊点厚度下降为主线，着重比较了与液态Sn冶金反应差异明显的FeNi和Cu所形成焊点组织的差异，并考察小尺寸焊点的剪切性能和主要失效方式，阐明了组织结构、应力分布、应变速率等因素随焊点厚度下降影响剪切性能的基本规律。对Sn/FeNi界面固态反应研究中发现了未曾报道的两种界面化合物次序生长的现象，且揭示出润湿性变化与化合物的生长次序有很好的相关性。该结果证明镀锡FeNi可保持长期良好可焊性，为FeNi在微电子小型化中的应用前景提供了依据。 20 μm到300 μm之间焊点厚度的FeNi和Cu基焊点在不同冷却速度和焊料条件下的微观组织研究发现，焊点厚度对焊点组织的影响与体系的冶金反应活性密切相关。反应活性强的Sn/Cu体系中，在快速冷却时，薄焊点为b-Sn组织，厚焊点中为锡枝晶+网络状（Sn+ Cu6Sn5）共晶的组织；而在冷速较慢时，不同焊点厚度的焊点组织均为b-Sn，Cu6Sn5化合物以颗粒状分布在b-Sn晶粒内部或者晶界上。反应活性差的Sn/FeNi体系，快冷时焊点组织为单一的b-Sn相；慢冷时形成（Sn+(Ni, Cu)3Sn4）的共晶组织，焊点厚度不影响焊点组织。焊点厚度影响焊点组织的原因在于，焊缝小时溶质扩散距离短，在焊点中能更快达到饱和溶解度，而在焊点凝固时所承受的实际冷却速度也较快。通过Sn/Cu和Sn/FeNi搭接焊点的剪切性能研究，结合有限元的应力分析，澄清了焊点剪切的尺寸效应来源。发现焊点厚度对焊点内部的Von Mises等效应力分布影响较小，焊点外形尺寸对剪切强度并无明显强化或弱化作用，Sn/Cu水冷焊点剪切强度的尺寸效应来源于不同厚度焊点的组织差异。回流态的Sn/Cu、Sn/FeNi、Sn1.5Cu/Cu搭接焊点在1×10-4/s到1×101/s应变速率范围内的剪切性能表明，焊点（焊点厚度20 μm~300 μm）均为焊料的失效，没有发现断裂模式的转变，说明界面化合物层的断裂强度以及焊料/化合物的界面强度很高，焊点的性能主要由焊料的性能决定。并得到纯锡焊点在1×10-3/s应变速率下的剪切强度为15 MPa。 Sn/Cu和Sn/FeNi焊点在150oC时效后的组织粗化和界面化合物生长对剪切性能的影响研究表明，界面化合物厚度并不是影响焊点强度的重要因素，焊点的性能仍然由焊料控制。只有焊点尺寸降低到10 μm以下时，界面化合物的粗糙度相对于焊料的变形路径宽度不可忽视，焊料的变形受到突出的界面化合物的阻碍，剪切强度有所提高。本论文对理解小尺寸焊点的剪切尺寸效应以及FeNi合金在小型化焊点中的应用具有指导意义。; " The development of high density packaging leads to the miniaturization of solder joints. As gap size decreases, solder joint is subjected to confined deformation and the metallurgical reaction behavior between the substrate and solder is also changed. Consequently, studying the correlation between microstructure and mechanical behavior of miniaturized solder joint is of vital importance. As a potential under bump metallization (UBM) substitute for Cu, the interconnection between FeNi and Sn attracts much attention. By systematically reducing the gap size, this study has focused on the formation of joint microstructure, the shear properties as well as the failure modes for miniaturized solder joints. The influences of microstructure, stress distribution and strain rate on shear properties were investigated. The solid-state reaction between FeNi and Sn was investigated, and two kinds of interfacial intermetallic compounds were found. The wettability of Sn/FeNi coupon after aging was found to be correlated well with the growth of intermetallic compounds. This result indicated that tinned FeNi can maintain good solderability after long-term aging. The microstructures of FeNi and Cu solder joints with gap sizes ranging form 20 μm to 300 μm were investigated as a function of cooling rate and solder composition. The microstructure was influenced by the gap size and the metallurgical reactivity of the solder/substrate system. For quenched Sn/Cu solder joint (reactivity is strong), only b-Sn grains were formed in the 20 μm joint, while Sn dendrite surrounded by (Sn+ Cu6Sn5) eutectic was formed in the 300 μm joint. Under furnace cooling condition, Cu6Sn5 particles were dispersed in Sn grains or grain boundaries no matter what the gap size was. For Sn/FeNi solder joint (reactivity is week), solder joint was solidified into b-Sn mono-phase under quenching conditions and (Sn+(Ni, Cu)3Sn4) eutectic under furnace cooling condition. When the gap size was reduced, the diffusion length of solute atom became shorter so that the solder joint was saturated with solute more quickly. The origin of gap-size effect on shear strength was examined through the shearing of Sn/Cu and Sn/FeNi lap-shear joint assisted with FEA stress analysis. Gap size had little influence on the distribution of Von Mises equivalent stress in the solder joint. As a result, gap size did not show obvious geometrical strengthening or deteriorating effect on shear strength. The size effect on shear strength of Sn/Cu quenched solder joint was caused by microstructural differences for solder joints with different gap sizes. The strain rate sensitivity of as-reflowed Sn/Cu, Sn/FeNi and Sn1.5Cu/Cu solder joint was investigated under strain rates ranging from 1×10-4/s to 1×101/s. Solder joints all failed in the solder and no change of the failure mode was found, which indicated that the failure strengthes of IMC and solder/IMC interface were extremely high, and the joint strength was mainly depended on solder properties. The shear strength for pure Sn solder joint under 1×10-3/s was 15 MPa. The influence of microstructural coarsening and interfacial intermetallic compounds (i-IMCs) growth on shear properties of Sn/Cu and Sn/FeNi solder joints after aging at 150oC was investigated. The thickness of i-IMCs was not of vital importance on joint strength, which was mainly determined by the solder. However, when the gap size was reduced to less than 10 μm, the roughness of the i-IMCs cannot be omitted compared to the width of Sn deformation path, and the shear strength was thus enhanced by the block effect of the extrusive i-IMCs on the deformation of solder."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64441
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	陈材. 小尺寸Sn/Cu与Sn/FeNi焊点组织[D]. 北京. 中国科学院金属研究所,2012.