Effects of Aging on Interfacial Microstructure and Reliability Between SnAgCu Solder and FeNi/Cu UBM

Abstract

Effects of thermal aging on the interfacial microstructure and reliability of the SnAgCu/FeNi‐Cu joint are investigated. It is found that aging effects depends strongly on the temperature. Aging at low temperature, e.g., at 125 °C, a submicron meter thick FeSn2 IMC layer formed at the SnAgCu/FeNi‐Cu interface during reflowing grows at a rate twenty times slower than the growth rate of the IMC at the SnAgCu/Cu interface. At high temperature, e.g., at 180 °C, the Cu element is found to diffuse through FeNi layer to produce the (Cu, Ni)6Sn5 IMC and this IMC layer grows even faster than the IMC at the SnAgCu/Cu interface. Solder ball shear test results show that the SnAgCu/FeNi‐Cu joint has a comparable strength to the SnAgCu/Cu joint after reflowing, and the strength drop after aging at 125 °C is less than that of the SnAgCu/Cu joint. However, after aging at 180 °C, the strength of the SnAgCu/FeNi‐Cu joint is degraded to a low value, along with a shift in failure mode from the solder fracture to the brittle intermetallics fracture.

Effects of Reflow Time on the Interfacial Microstructure and Shear Behavior of the SAC/FeNi-Cu Joint

“…Based on the results regarding SAC/FeNi-Cu and SAC/FeNi from Guo and Hwang [7,14], this very thin IMC was determined to be FeSn 2 phase with a tetragonal crystal structure determined by Zhu at the same 260˝C reflowing conditions [9]. It is completely in accordance with our previous results on the liquid reaction between Sn based solder and FeNi substrate [18], where Sn reacts with Fe preferentially over Ni.…”

“…It is reported that the FeNi layer has an acceptable wettability for SnAgCu (SAC) solder with or without an adequate pre-treatment in soldering [7], exhibiting a slower interfacial reaction rate compared to the traditional UBMs [8]. Moreover, the shear resistance behavior of an SAC solder joint can be slightly improved by using the FeNi alloy rather than the Cu substrate [9,10]. Moreover, it is found that only FeSn 2 phase with minor Ni solubility formed between the FeNi substrate and Sn solder during liquid reactions at 270˝C [11][12][13][14].…”

“…Early studies on the Fe-Ni alloy are mainly about the brazing or solid bonding processes. Recently, a few publications have reported on the interfacial reactions between Sn based solders and Fe-Ni alloys, discussing its potential application as UBM layer for Sn-based solders [7][8][9][10][11][12][13][14][15][16]. It is reported that the FeNi layer has an acceptable wettability for SnAgCu (SAC) solder with or without an adequate pre-treatment in soldering [7], exhibiting a slower interfacial reaction rate compared to the traditional UBMs [8].…”

Effect of Fe content on the interfacial reliability of SnAgCu/Fe-Ni solder joint

“…As it had been reported before, Fe-Ni alloy exhibits a much slower interfacial reaction rate than that on traditional UBMs using typical Pb-free solders [10], and the shear strength of SnAgCu solder joint could be slightly improved by replacing the Cu/Ni UBM with the Fe-Ni UBM [11]. Moreover, with adequate pre-treatment during soldering, the wettability of Fe-Ni UBM is also quite acceptable [12].…”

Local shear stress-strain response of Sn-3.5Ag/Cu solder joint with high fraction of intermetallic compounds: Experimental analysis

“…The reason was explained to be either the increase in brittle IMC layer thickness or the grain coarsening of Sn-rich phase in solder. Zhu et al [17] investigated the effect of aging on the interfacial microstructure and reliability of SnAgCu solder. They found with the aging at higher temperature (180°C), failure mode transmitted from ductile to brittle due to excessive IMC thickness.…”