Intermetallic Reactions in Reflowed and Aged Sn-58Bi BGA Packages with Au/Ni/Cu Pads
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The reflow of Sn-58Bi solder joints in a BGA package with Au/Ni/Cu pads has been performed by employing various temperature profiles, which results in the formation of (Au0.66Ni0.34)(Sn0.82Bi0.18)4 intermetallic flakes in the solder matrix. The reflow operation performed at a peak temperature of 180 °C for a melting time of 80 s gives a ball shear strength of 9.1 N, which decreases drastically to lower values between 6.4 and 4.6 N after further aging at temperatures from 75 to 125 °C. Double layers of intermetallic compounds with the compositions of (Au0.30Ni0.70)(Sn0.90Bi0.10)4/Ni3Sn4 can be found at the solder/pad interfaces of the aged Sn-58Bi solder joints. Ball shear testing of the reflowed specimens shows ductile fracture through the solder matrix, which changes to brittle cleavage fracture mainly along the (Au0.30Ni0.70)(Sn0.90Bi0.10)4 intermetallic layer after aging at various temperatures. The measurement of ball shear strengths (S) reveals a linear relation with the thicknesses (X) of (Au0.30Ni0.70)(Sn0.90Bi0.10)4 intermetallic layers: S = 7.13 − 0.33X.
Keywordsaging ball shear strength intermetallics reflow Sn-58Bi solder BGA
Sincere thanks go to National Science Council, Taiwan, for sponsoring this research under Grant No. NSC-93-2216-E002-024.
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