The Effect of Grain Orientation of β-Sn on Copper Pillar Solder Joints during Electromigration

Yun Huang,Kexin Xu,Xing Fu,Si Chen,Xiaofeng Yang,Xinjie Wang,Zhiwei Fu,Yijun Shi,Hongtao Chen

doi:10.3390/ma15010108

Abstract

The grain orientation of Sn-based solder joints on copper pillars under the combined action of electron wind force and temperature gradient greatly affects their electromigration damage. The copper pillars with Sn-1.8Ag lead-free solder on the top was subjected to a current density of 1.5 × 104 A/cm2 at 125 °C to study the electromigration behaviors. The grain orientation was characterized by scanning electron microscopy (SEM) equipped with electron backscattered diffraction (EBSD) detector. Metal dissolution and voids formation in the cathode as well as massive intermetallic compounds(IMC) accumulation in the anode were observed after electromigration. Closer examination of solder joints revealed that the Sn grain whose c-axis perpendicular to electric current may have retarded Cu diffusion to anode and IMC accumulation. In addition, the newly formed Cu6Sn5 exhibited preferred orientation related to the electric current direction.

Highlights

As the miniaturization and high-performance requirement of microelectronic devices continue to increase, micro-bumps become the mainstream trend in high-density packaging [1,2]
Shen [18] reported that the sample with α angle lower than 25◦ (α was defined as the angle between c axis of Sn grain and current flow) exhibited fast accumulation of interfacial IMC by drawing a relationship between the average α in the solder joints with the thickness of the interfacial IMC in electromigration experiments on a few solder joints composed of several grains with different α
In the unconsumed Sn, c axis of Sn grain was almost perpendicular to the electron flow. It can be been in the solder joint with the grain orientation of large α, there were many Sn matrix remained due to their stronger electromigration resistance

Summary

Introduction

As the miniaturization and high-performance requirement of microelectronic devices continue to increase, micro-bumps become the mainstream trend in high-density packaging [1,2]. Many reliability issues emerge owing to the shrinking solder volume [8,9] One such issue is that the increasing current density through each copper pillar may exceed the electromigration threshold (104 A/cm2), which causes the electromigration damage [10]. Shen [18] reported that the sample with α angle lower than 25◦ (α was defined as the angle between c axis of Sn grain and current flow) exhibited fast accumulation of interfacial IMC by drawing a relationship between the average α in the solder joints with the thickness of the interfacial IMC in electromigration experiments on a few solder joints composed of several grains with different α. Copper pillars are a popular interconnection method in 3-D packaging, there are few studies on the electromigration damage in this kind of solder joint. SEM and EBSD images were utilized to reveal the relationship between electromigration damage and tin grain orientation

Experiment

Microstructure Evolution of Cu Pillar after Electromigration Test

The Effect of Grain Orientation on Electromigration Damage

Conclusions