Abstract
Spontaneous whisker formation from pure tin and tin-based finishes on copper leadframes at ambient temperatures is a major concern before board assembly of electronic devices. In this study, significantly different tendencies of whisker formation were found from the same tin–copper (Sn–Cu) coating electrodeposited on two different copper leadframes, namely, copper–iron (hereafter, CUFE; corresponding to CDA number C19400) and copper–chromium (CUCR; CDA number C18045). After long-term storage at room temperature, no whisker formation occurred from the Sn–Cu coating on the CUCR leadframe, whereas long whiskers, with a maximum length of more than 200μm, were formed from the Sn–Cu coating on the CUFE leadframe. Microstructural FE-STEM/FE-TEM/EBSP characterizations at vertical cross-sections of the Sn–Cu coated leadframes, an XRD stress measurement of the coatings, an FEA analysis of coating stress distributions, a molecular-dynamics simulation of atom diffusion in the coating, and an investigation of the correlation between whisker roots and coating microstructures using a planar slicing method were performed for the two aforementioned samples. The results of these examinations clarified the mechanisms of the formation and suppression of whiskers grown from the Sn–Cu coating, and we established a countermeasure against the spontaneous whisker formation through the selection of the copper leadframe material.
