Identification of mechanical properties of intermetallic compounds on lead-free solder
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In this paper, we present methods for obtaining mechanical properties of silicon substrates and two intermetallic compounds (IMC) formed at the interfaces between lead-free solders and the copper and between lead-free solders and nickel substrates. To determine the mechanical properties of the silicon substrate, 255 um (100) and 306 um (110) thick silicon wafers were adopted. With proper sensor locations of strain gages and ANSYS finite element models, the in-plane Young's moduli were optimally determined to be E100= 101.6 GPa, E110=140.7 GPa, E-110= 140.5 GPa, which agreed with the trend of theoretical values in each orientation. Some defects made the Young's moduli smaller than the theoretical values. Further, the in-plane Young's moduli of the anisotropic (110) wafer were inversely determined to be E-111= 170.1 GPa, E1-12=157. 9 GPa, which were also smaller but closer to the theoretical values because of its thicker thickness. For IMC material properties, two types of lead free solder were used, namely, SnZn and Sn. For the IMC mechanical property determination using SnZn solder, specimens were prepared by dipping to overcome the poor wetting problem. A uniform layer of Cu33.5Zn66.5 IMC was then formed. For out-of-plane Young's modulus measurement, the specimen was pressed by a nanoindentor. An average value for the Young's modulus was recorded to be 158 GPa. With the whole field slope measurement method, the reflection moir, the CTE of Cu33.4Zn66.5 was found to be approximate to that of copper. As for the Cu-Sn intermetallic compounds, electroplating was used to have an even tin layer on copper substrates. After soldering reaction and annealing, there existed two phases of Cu-Sn IMC, Cu6Sn5 and Cu3Sn. With the nanoindentor, average values for the Young's moduli were determined to be 124 GPa for Cu6Sn5 and 143 GPa for Cu3Sn. The warpage change of Cu3Sn/copper foil structure under thermal load is adopted for the CTE of Cu3Sn, which is determined to be 18.2 ppm/C. Furthermore, nickel substrate electroplated with tin was adopted for Ni3Sn4 IMC, whose Young's modulus was determined to be 152 GPa with nanoindentation. 2005 IEEE.
