High temperature reliability and interfacial reaction of eutectic Sn–0.7Cu/Ni solder joints during isothermal aging

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Abstract

The interfacial reactions and growth kinetics of intermetallic compound (IMC) layers formed between Sn–0.7Cu (wt.%) solder and Au/Ni/Cu substrate were investigated at aging temperatures of 185 and 200 °C for aging times of up to 60 days. After reflow, the IMC formed at the interface was (Cu, Ni)6Sn5. After aging at 185 °C for 3 days and at 200 °C for 1 day, two IMCs of (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 were observed. The growth of the (Ni, Cu)3Sn4 IMC consumed the (Cu, Ni)6Sn5 IMC at an aging temperature of 200 °C due to the restriction of supply of Cu atoms from the solder to interface. After aging at 200 °C for 60 days, the Ni layer of the substrate was completely consumed in many parts of the sample, at which point a Cu3Sn IMC was formed. In the ball shear test, the shear strength decreased with increasing aging temperature and time. Until the aging at 185 °C for 15 days and at 200 °C for 3 days, fractures occurred in the bulk solder. After prolonged aging treatment, fractures partially occurred at the (Cu, Ni)6Sn5 + Au/solder interface for aging at 185 °C and at the (Ni, Cu)3Sn4/Ni interface for aging at 200 °C, respectively. Consequently, thick IMC layer and thermal loading history significantly affected the integrity of the Sn–0.7Cu/Ni BGA joints.

Introduction

The electronics industry faces new challenges brought about by new environmental requirements. One of the first steps towards producing more environmentally friendly products and assemblies is to make them lead (Pb)-free. Generally, in the first phase, Pb will be eliminated from the solder. Later, it will be phased out from components and substrates [1], [2]. Among many Pb-free solder alloys, a eutectic Sn–Cu solder (Sn–0.7 wt.%Cu) is considered the most promising candidate alloy to replace the eutectic Sn–Pb solder for wave soldering [3], [4], [5]. In 2000, National Electronics Manufacturing Initiative (NEMI) recommended to replace eutectic Sn–Pb solder by eutectic Sn–Ag–Cu solder in reflow processing and eutectic Sn–Cu solder in wave soldering [5]. In addition, the Sn–0.7Cu solder was also identified as a better alloy for flip-chip soldering compared to Sn–3.5Ag and Sn–3.8Ag–0.7Cu [6].

In addition to solders, printed circuit boards (PCBs) and component surface finishes also have to be Pb-free. Selection of appropriate substrate metallization plays an important role in developing a reliable packaging technology. This is especially so with the adoption of Pb-free solders. Copper is widely used in the under bump metallurgy and substrate metallization for flip-chip and ball-grid-array (BGA) applications. It is known that at the solder/Cu interface, Sn reacts rapidly with Cu to form Cu–Sn intermetallic compounds (IMCs) [7]. High ambient temperatures on the solder joint result in the growth of unwanted IMCs, which weaken the strength of the solder joint due to their brittleness and weakness [8], [9]. Therefore, it is necessary to understand and control the factors that govern the kinetics of the interfacial reaction. Generally, Ni is used as a diffusion barrier layer to prevent the rapid interfacial reaction between solder and Cu substrate [7], [10].

Numerous studies on the Sn–Cu solder alloy have been presented in the literature [3], [11], [12], [13], [14]. Nevertheless, the knowledge of solid-state interfacial reactions and shear strength of Sn–Cu solder/Ni joints remains insufficient. Recently, we have also investigated the interfacial reaction and shear strength of the Sn–0.7Cu solder/Ni BGA joints during isothermal aging at temperature between 70 and 170 °C for up to 100 days [14]. In that study, only an IMC layer of (Cu, Ni)6Sn5 was observed in the samples aged at temperatures between 70 and 150 °C. On the other hand, two IMCs, (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4, were formed after isothermal aging at 170 °C for 50 days. In addition, shear strength decreased after aging for the initial 1 day and then remained nearly unchanged with prolonged aging. And, the fracture always occurred in the bulk solder. In the results of our previous study it was shown through the examination of interfacial reactions and mechanical testing that the Sn–0.7Cu/Ni solder joint had the desired joint reliability. During the study, shear strength could not be directly related to the thickness of the IMC layer formed at the interface. Therefore, the first objective of this study is to investigate, in greater detail, the growth behaviors of (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 products of the reaction between the Sn–0.7Cu solder and Ni substrate, during isothermal aging at higher temperatures. The second objective is to examine the high temperature reliability (shear strength) of the solder joints. The resulting correlations between shear strength and corresponding interfacial reactions are discussed in detail.

Section snippets

Experimental procedures

The BGA solder used in this study was eutectic Sn–0.7Cu (wt.%) with a diameter of 500 μm. This solder alloy has a melting temperature of 227 °C. The substrate was a BT (Bismaleimide Triazine) laminate with subsurface solder bond pads which had a circular opening of 460 μm in diameter. The pad was constructed by electroplating a Au/Ni layer onto the underlying Cu pad with thicknesses of 0.5 and 8.5 μm, respectively. The Sn–Cu solder ball was bonded to the BT substrate in a reflow process employing

Results and discussion

Fig. 1 shows the SEM micrographs of the interface between the Sn–0.7Cu solder and Ni/Cu BGA substrate after having been reflowed at 250 °C for 60 s. The back-scattered electron image mode of the SEM was used to provide more distinguishable boundaries of the interfacial layers. During the reflow process, the Au layer of the substrate dissolved into the molten solder, leaving the Ni layer exposed to the molten solder. The reaction between molten solder and the Ni layer formed a (Cu, Ni)6Sn5 IMC

Conclusion

In this study, the effect of isothermal aging on the interfacial reaction and shear strength of Sn–0.7Cu solder/Ni BGA joints was investigated for samples aged at temperatures of 185 and 200 °C for up to 60 days. After the entire Au layer dissolved into the molten solder, the reaction between the molten solder and Ni layer resulted in the formation of a (Cu, Ni)6Sn5 IMC layer at the interface during the initial reflow. After aging at 185 °C for 3 days, two IMCs of (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 were

Acknowledgment

This work was supported by grant no. RTI04-03-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE).

References (14)

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