Comparative Study Focused on Mechanical Properties of Solder Joints Considering Solder Pad Surface Finish

The study attends to the evaluation of solder joints from a mechanical point of view. Two lead-free alloys Sn99.7Cu0.3 and Sn96.5Ag3Cu0.5 (SAC305) were selected for samples preparation. To compare the results of current alloys with the most utilized solder in the past, the study comprises eutectic lead solder paste Sn63Pb37, too. Solder joints intended for observation were created by assembling chip resistors. Assembled boards were reflowed employing a hot air convection oven. Mechanical properties were evaluated by performing shear tests and microhardness measurements. Microhardness of bulk solder and intermetallic compounds (IMC) was measured using the micro-indentation Vickers test. For this purpose, a metallographic cross-section of some samples was accomplished. Solder pads on testing boards were coated by Organic Solder Preservative (OSP) and Hot Air Solder Levelling (HASL). Depending on solder paste type, the results show more or less impact of surface finish on the solder hardness. Some difference in microhardness of IMC layers and shear strength was also detected but, in these tests, results are not in favour of one surface treatment.