Study of factors influencing tin whisker growth

In this paper, two factors have been considered which may influence the diameter of tin whisker. One is coating crystal structure around the root of tin whisker; another is crack size in oxide films. For the first factor, four samples have been prepared and uniform conditions to storage are provided. Images from SEM (Scanning Electron Microscope) and EDX (Energy Dispersive X-ray Detector) show that tin whisker growth from surface of Cu/Sn coatings has larger diameter than growth from pure tin coatings, and most of it is tin hillock. It is proven that the existence of Cu/Sn IMC can influence the diameter of tin whisker. In addition, it is concluded that electromigration can enlarge the size of tin hillock. For the second factor, Two experiments have been conducted. In the first experiment, half of coatings has been oxidized and a part of coatings has been covered both bare and oxidized with photoresist. It is proven that photoresist can increase the compressive stress of tin coatings and prevent tin whisker growing from the area covered by the photoresist. In addition, this experiment also confirms that tin whisker tends to grow at weaker spots on the tin surface where the surface oxide films have been broken. In the second experiment, most of tin coatings was covered with photoresist and a series of mini dots are exposed to mimic cracks in surface tin oxide films by means of photoetching. The result shows that tin whisker tends to grow on surface where there is not any photoresist and its diameter is always less than the size of mini dots.