Effect of crystal anisotropy and IMCs on electromigration resistivity of low temperature flip chip interconnect

K. Murayama, Mitsuhiro Aizawa, K. Oi
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引用次数: 1

Abstract

In recent years, global warming prevention and reduction of energy consumption are increasingly required. And technical point of view, low stress bonding is required. A low temperature bonding using Sn-Bi solder is candidate technique to solve these problems. In this report, improving the reliability of electro-migration at flip chip interconnect in Sn-Bi solder, which is a low-temperature solder, are focused. A combination of six types of Sn-Bi solder and surface finish of interposer pad were studied, these are, combination of Sn30wt.%Bi (Sn30Bi) solder, Sn57wt.%Bi (Sn57Bi) solder, electroless Ni/Au on Cu pad and Ni/Pd/ Au on Cu pad. The effects of Bi concentration, trace element and inter-metallic compounds formation on electromigration resistivity were evaluated. The current density of 40 kA/cm2 was applied at 125 degrees C. Analysis of the microstructure and crystal orientation of the interconnected bumps were performed in all combinations by electron backscatter diffraction and electron probe microanalyzer. Mean time to failure of Sn57Bi/NiAu(thick), that of Sn30Bi/NiAu(thick) and that of Sn30Bi/NiPdAu were19, 141 and 600 hours, respectively. And maximum resistance increases from initial of Sn57Bi/NiAu(thick), that of Sn30Bi/NiAu(thick), Sn30Bi/NiPdAu were 60.6%, 19.5%, 13.2% after 2500 hours. Reducing the total amount of Au and Pd atoms in the solder can delay increase of resistance after current stressing. In cases of NiAu(thick) and NiAu(thin) pads, two types of failure mode were observed after current stressing. One was growth of large (Cu, Ni)6Sn5 with thick Ni(P rich) layer. The other was the mode in which (Cu, Ni) 6Sn5 hardly grow and unreacted Sn layer remains. In case of c-axis of the beta-Sn grain aligned with the electron flow, Ni diffusion from Ni pad to the solder was accelerated. On the other hand, in the case of NiPdAu pad, only one failure mode was observed after current stressing. It is a mode in which the unreacted Sn layer remains. Even if c-axis of the beta-Sn grain aligned with the electron flow, Ni diffusion from Ni pad was not observed. Focusing on the intermetallic compounds (IMCs) formed on the interposer pad after 10% resistance change, in cases of NiAu(thick) and NiAu(thin) pads, un-uniform and needle-type (Cu, Ni) 6Sn5 were formed. Large Ni diffusion from their grain boundary into IMCs were observed after current stressing. On the other hand, in the case of NiPdAu pad, thin (Ni, Cu)3Sn4 and uniform and dense scallop-type (Cu, Ni)6Sn5 were formed after 10% resistance change. They were still stabled after current stressing. The significant increase in resistance in the early stage is influenced by Bi concentration in solder. And Au atoms play a role of accelerating diffusion and increase in resistance at early stage on electro-migration test. Dense scallop-type (Cu, Ni)6Sn5 layer on Ni pad showed as an effective barrier diffusion of Cu or Ni. It was found that if a uniform and dense IMCs were formed on the interposer pad before current stressing, it would show high electro-migration resistivity even with low temperature solder.
晶体各向异性和IMCs对低温倒装互连电迁移电阻率的影响
近年来,预防全球变暖和降低能源消耗的要求越来越高。从技术角度来看,低应力粘合是必需的。利用锡铋焊料进行低温键合是解决这些问题的备选技术。本文主要研究了如何提高低温锡铋焊料倒装互连电迁移的可靠性。研究了六种Sn-Bi焊料的组合和中间焊盘的表面光洁度,即Sn30wt的组合。%Bi (Sn30Bi)焊料,Sn57wt。%Bi (Sn57Bi)焊料,Cu焊盘化学镀Ni/Au和Cu焊盘化学镀Ni/Pd/ Au。评价了铋浓度、微量元素和金属间化合物形成对电迁移电阻率的影响。在125℃下施加电流密度为40 kA/cm2,通过电子背散射衍射和电子探针微量分析仪分析了所有组合下互连凸起的微观结构和晶体取向。Sn57Bi/NiAu(厚)、Sn30Bi/NiAu(厚)和Sn30Bi/NiPdAu的平均失效时间分别为19、141和600小时。2500 h后,Sn57Bi/NiAu(厚)、Sn30Bi/NiAu(厚)、Sn30Bi/NiPdAu的最大电阻增幅分别为60.6%、19.5%、13.2%。减少焊料中Au和Pd原子的总量可以延缓电流应力后电阻的增加。在NiAu(厚)和NiAu(薄)衬垫的情况下,电流应力作用后观察到两种类型的破坏模式。一是生长出较大的(Cu, Ni)6Sn5,并具有较厚的富Ni(P)层。另一种模式是(Cu, Ni) 6Sn5几乎不生长,未反应的Sn层仍然存在。当β - sn晶粒c轴与电子流对齐时,Ni从Ni焊盘向焊料的扩散速度加快。另一方面,NiPdAu衬垫在电流应力作用下只观察到一种破坏模式。这是一种未反应的锡层仍然存在的模式。即使β - sn晶粒的c轴与电子流对齐,也没有观察到Ni从Ni垫中扩散。在NiAu(厚)衬垫和NiAu(薄)衬垫中,形成了不均匀的针状(Cu, Ni) 6Sn5;在电流应力作用下,Ni从晶界扩散到IMCs中。另一方面,在NiPdAu衬垫中,电阻变化10%后形成薄的(Ni, Cu)3Sn4和均匀致密的扇贝型(Cu, Ni)6Sn5。它们在当前的压力下仍然稳定。早期电阻的显著增加受焊料中铋浓度的影响。在电迁移试验中,金原子在早期具有加速扩散和增加电阻的作用。Ni衬垫上致密的扇贝型(Cu, Ni)6Sn5层是Cu或Ni的有效屏障扩散层。结果表明,在电流应力作用前,如果在中间焊盘上形成均匀致密的imc,即使在低温焊料中也会表现出较高的电迁移电阻率。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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