Void fraction of a Sn-Ag-Cu solder joint underneath a chip resistor and its effect on joint strength and thermomechanical reliability

Abstract

The void fraction in the solder joint of a chip resistor and its effect on the solder joint strength and reliability were investigated. The solder joint of a chip resistor has two regions: solder beneath the component and solder fillet. Although the total void fraction was similar irrespective of the component size, the void fraction of solder beneath the component increased and that of solder fillet decreased as the component size increased. The void fraction decreased considerably under vacuum reflow compared with that under air reflow. Furthermore, the vacuum reflowed samples showed similar void fraction characteristics as the air reflowed samples: the void fraction in the solder beneath the chip resistor increased and that in the solder fillet decreased as the chip resistor size increased. For both air and vacuum reflow, the shear strength of the chip resistor solder joint decreased as the chip size increased. The reliability of the chip resistor joint was evaluated using a thermal shock test. As the number of thermal shock cycles increased, the shear strength of the chip resistor solder joint decreased. Up to 2000 cycles, the shear strength reduction rates were similar irrespective of the component size. However, after 3000 cycles, the shear strength reduction rate of the large components (0805, 1210) was to about 50%, which was twice that of the small components (0402, 0603). Cross-sectional SEM after the thermal shock test revealed that a generated crack merged with a void, forming a long crack and lowering the joint reliability.