IMCs Microstructure Evolution Dependence of Mechanical Properties for Ni/Sn/Ni Micro Solder-Joints.

Ning Ren,Heng Fang,Chenyi Hou,Ye Tian,Dong Wang,Yatao Zhao,Fan Chen,Yiping Wu,Kyung-Wook Paik

doi:10.3390/ma13010252

Abstract

The current miniaturization trend of microelectronic devices drives the size of solder joints to continually scale down. The miniaturized joints considerably increase intermetallic compounds (IMCs) volume fraction to trigger mechanical reliability issues. This study investigated precise relationships between varying IMC volumes and mechanical properties of Ni/Sn(20μm)/Ni micro-joints. A designed method that followed the IMC volume as the only variable was used to prepare micro-joint samples with different IMC volumes. The continuously thickened Ni3Sn4 IMCs exhibited a noticeable morphology evolution from rod-like to chunky shape. The subsequent tensile tests showed unexpected tensile strength responses as increasing Ni3Sn4 volume, which was strongly associated with the Ni3Sn4 morphological evolutions. Fractographic analysis displayed that the ductile fracture dominates the 20%–40% IMC micro-joints, whereas the brittle fracture governs the 40%–80% IMC micro-joints. For the ductile fracture-dominated joints, an abnormal reduction in strength occurred as increasing IMCs volume from 20% to 40%. This is primarily due to severe stress concentrations caused by the transformed long rod-typed morphology of the Ni3Sn4. For the brittle fracture-dominated joints, the strength appeared a monotonous increase as the Ni3Sn4 volume increased. This may be attributed to the increased crack resistance resulting from continuous coarsening of the chunky Ni3Sn4 without any voids. Moreover, the finite element analysis was provided to further understand the joint failure mechanisms.

Highlights

To date, micro solder-joints are regarded as critical components to realize vertical interconnections for chips stacking in three-dimensional (3D) through-silicon via (TSV) packaging [1,2]
At 7 min soldering time, a thin intermetallic compounds (IMCs) layer with a short-rod shape was observed to form at the Ni/Sn interface from Figure 4a, and several small chunky IMCs emerged at the interface
The IMC volume occupied around 20% in the joint

Summary

Introduction

Micro solder-joints are regarded as critical components to realize vertical interconnections for chips stacking in three-dimensional (3D) through-silicon via (TSV) packaging [1,2]. With the joints continually downsized, the Ni/Sn/Ni micro-joint should be favorable to be adopted as interconnection structure in 3D TSV packaging because of slow interfacial reactions and a unique reaction product (i.e., Ni3 Sn4 intermetallic) [17,18] This intermetallic possesses better mechanical performance and higher fracture toughness compared to typical IMCs (i.e., Cu6 Sn5 and Cu3 Sn) [12,19,20]. A practicable method that extends the soldering time at a constant cooling rate enables the IMCs growth without changing bulk solder microstructural characteristics. This method was utilized in this study to prepare the Ni/Sn(20 μm)/Ni micro-joint samples with varying IMC volumes. The emphasis of this study was placed on a precise dependence of the IMCs volume on tensile properties and failure mechanisms of Ni/Sn/Ni micro-joints with a constant stand-off height of 20 μm using a tensile test

Experimental Procedures

Numerical Procedures

Microstructure Characterization of the IMCs

IMC Growth Kinetics

Conclusions