Formation, distribution and failure effects of voids in vapor-phase soldered small solder volumes

The formation of voids in small solder volumes, e.g. flip-chip-soldered electronic components, is of vital interest to determine their reliability. The aim of the study was to understand the physical and chemical reasons for void formation in small solder volumes, the determination of void diameters depending on soldering temperature and flux content. A further investigation was made to define the strength of the solder balls depending on void diameter and void distribution. The diameter and the location of the voids was measured by X-ray-detection. The results show an increase of voids with increasing soldering time at constant temperature and a rapid decrease of void diameter at a soldering time nearby 60 sec followed by a slower increase of voids. The distribution of void diameter shows a great amount of small voids and only some big voids. The distribution of the void diameter is a skew distribution. The shear strength of the soldered joints shows only a little influence on the voids amount. These results give hints for the optimized time-temperature-curves for the best soldering profile. Further investigations are necessary to determine the influence of the voids to thermomechanical stresses and vibration behavior.