Intermetallic reactions in reflowed and aged Sn-58BiBGA packages with Au/Ni/Cu pads

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 (AU(0.66)Ni(0.34))(Sn0.82Bi0.18)(4) intermetallic flakes in the solder matrix. The reflow operation performed at a peak temperature of 180 degrees 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 degrees 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.