Structural Changes of the IMC in Lead Free Solder Joints

The development of Cu-Sn intermetallic compound (IMC) at the solder/Cu joints interface had been studied using five Pb-free solders as SnAg3.0Cu0.5, SnAg3.5Cu0.7, SnAg1.0Cu0.5Bi1.0, SnAg1.5Cu0.7In9.5 and SnCu0.67In2.0 alloys (composition given in mass%). The effects of Bi and In additions on the intermetallic phase formation in the lead-free solder joints with copper substrate were studied. The soldering of the copper plate was conducted at 250 degrees C for 5 s. The solder joint reliability of SnAg3.0Cu0.5 and SnCu0.67In2.0 alloys was assessed with the thermal cycling test in the range from -40 degrees C to -50 degrees C. Altogether 1500 cycles were carried out. The solder joints with SnAg3.5Cu0.7, SnAg1.0Cu0.5Bi1.0 and SnAg1.5Cu0.7In9.5 alloys were subsequently aged at temperatures of 130-170 degrees C for 2-16 days in a convection oven. The joints interface, activation energy, structural integrity were studied on the produced solder joints with optical microscopy, energy dispersive x-ray spectroscopy (EDX) microanalysis. Designed solder alloys, mentioned above, reached the most suitable result for microelectronic. With the increase of the strain rate, the failure mode migrates from the ductile fracture in the bulk solder to the brittle fracture in the IMC layer.