Investigation of Sn-Cu intermetallic compounds by AFM: new aspects of the role of intermetallic compounds in whisker formation

It has been observed that different tin whisker growth properties can result from the same tin deposit on two copper leadframe materials, C19400 and C70250. It was also found that modification of the surface condition of identical substrates, through pretreatment prior to Sn-plating, affects whisker growth behavior. Here, we report an atomic force microscopy (AFM) investigation of intermetallic compounds (IMC) formed on samples with significantly different whisker growth tendencies. AFM data analysis in conjunction with the IMC growth rate, determined using the weight gain method, provides quantitative data of the IMC characteristics. It was found that the whisker propensity is not directly related to the uneven growth front of IMC as detected by AFM. Whisker growth tendency, with respect to whisker density and length, decreases when the unevenness of the IMC layer exceeds a certain level. The results of this work show evidence that the whisker propensity is associated with the copper substrate texture as well as the substrate surface topography.