Evolution mechanism of interconnect interface and shear properties of 64.8Sn35.2Pb microbump during flip chip bonding

Abstract

Effect of flip chip bonding parameters on microstructure at the interconnect interface and shear properties of 64.8Sn35.2Pb microbumps were investigated in this work. Results show that the main intermetallic compound (IMC) at the interconnect interface is (Ni, Cu)3Sn4 phase, and meanwhile a small amount of (Cu, Ni)6Sn5 phase with a size of 50−100 nm is formed around (Ni, Cu)3Sn4 phase. The orientation relationship of [-1 -5 6](Ni, Cu)3Sn4//[152](Cu, Ni)6Sn5 and (601)(Ni, Cu)3Sn4//(-2 01)(Cu, Ni)6Sn5 is found between these two phases, and the atomic matching at the interface of the two phases is low. The highest shear force of 77.3 gf is achieved in the 64.8Sn35.2Pb microbump at the peak temperature of 250 ℃ and parameter V1 because dense IMCs and no cracks form at the interconnect interface. Two typical fracture modes of microbumps are determined as solder fracture and mixed fracture. The high thermal stress presenting in the thick IMCs layer induces crack initiation, and cracks propagate along the α/β phase boundaries in the Sn-Pb solder under shear force, leading to a mixed fracture mode in the microbumps.

1. Introduction

Flip chip bonding, as a critical technology in the electronic packaging field, is used to achieve the vertical interconnection between the chip and the package through microbump in order to realize high-density integrated packaging of the chip [1−3]. Sn-Pb solder has a low melting point, excellent electrical and thermal conductivity, good wettability and chemical properties, and shows a considerable reliability in the service process of flip chip [4−6]. Therefore, Sn-Pb solder is widely used as a microbump interconnect material in the aerospace field, especially in the field of deep-space electronics, where extremely high reliability of electronic components is required [7]. A variety of intermetallic compounds (IMCs) are easily generated between the Sn-Pb solder and the pad, which contains multi-layer metals (Au, Ni, Cu) during flip chip bonding [8, 9]. In general, the formation of IMCs is beneficial to improve the strength of interface, but a large number of hard and brittle IMCs along the interface would deteriorate the bonding quality. Main reason is that this complicated and multi-layer microstructure characteristics at the interconnect interface easily induce the thermal stress after flip chip bonding due to the difference between the coefficient of thermal expansion (CTE) among heterogeneous materials, finally resulting in the formation of defects such as cracks at the interconnect interface and affecting the reliability of electronic packaging products [10].

It is widely recognized that the evolution of IMCs at the interconnect interface was closely relevant to the flip chip bonding parameters. For instance, MOONKAM et al [11] investigated the effect of reflow temperature on type and grain morphology of IMCs at the Sn-0.3Ag-0.7Cu/Cu interface and found that the Cu6Sn5 at the interface transformed from scalloped to prismatic grains with increasing reflow temperature from 250 to 300 ℃. HUANG et al [12] investigated the effect of reflow time on morphology of Cu6Sn5 grains at the Sn-3.5Ag/Cu interface, revealing that the initial scalloped Cu6Sn5 grains evolved into prismatic grains when the reflux time was increased from 30 s to 60 s. Moreover, the relationship between interfacial reactions, the evolution of interfacial IMCs and the mechanical properties of the solder joints was investigated. WANG et al [13] introduced varying amounts of Ni-modified multi-walled carbon nanotubes (Ni-CNTs) into Sn-3.0Ag-0.5Cu solder, and discovered that Ni-CNTs effectively inhibited the growth of the IMC layer and the shear strength of the solder joint increased with a higher concentration of Ni-CNTs. BI et al [14] found that incorporating Co element into the Sn-37Pb/Ni/Cu system led to the formation of a Ni-Co solid solution and (Ni, Co)3Sn4 IMC at the solder/Ni-Co interface after reflowing, which resulted in an increase of shear strength for the solder joints. YANG et al [15] investigated the effect of Cu6Sn5 grain morphology on the shear strength of Sn-3.5Ag/Cu solder joints and found that joints with prism-type interfacial Cu6Sn5 grains exhibited higher shear strength compared to those with scallop-type or layer-type Cu6Sn5 grains.