Effects of Minor Elements in Cu Leadframe on Whisker Initiation From Electrodeposited Sn/Cu Coating

Abstract

Drastically different tendencies of whisker initiation have been observed from the same Sn/Cu coating electrodeposited on two different Cu leadframe materials, CUFE and CUCR. After long-term storage at room temperature, no whisker initiation was observed on the coating on CUCR, whereas long whiskers with a maximum length of more than 200 μm were formed on the coating on CUFE. FE-STEM and FE-TEM microstructural observations of vertical sections of each Cu leadframe with the coating showed that the cross-sectional morphologies of Cu-Sn intermetallic compounds (IMCs) formed at the interface between the coating and the leadframe were as follows: a wedge-shaped structure for the Sn/Cu- CUFE sample and a comb-tooth structure for the Sn/Cu-CUCR sample. Energy dispersive X-ray (EDX) analysis results confirmed that the formation morphologies of the Cu-Sn IMC were affected by minor elements in the Cu leadframes. Fe particles with a large diameter of mainly 50-200 nm precipitated nonuniformly in the CUFE leadframe, which had Fe, Zn, and P as minor elements. Conversely, very small Cr-rich particles, 10-20 nm in diameter, were precipitated in the CUCR leadframe, which had Cr, Sn, and Zn as minor elements. The Fe particles suppressed the growth of the Cu-Sn IMC by acting as obstacles to IMC formation, whereas grain boundaries in the Sn/Cu coating acted as preferential IMC formation sites. The combination of these two effects is thought to produce the wedge-shaped cross-sectional structure of the IMC. On the other hand, the Cr-rich particles did not have such a prominent suppression effect on Cu-Sn IMC growth: only the grain boundaries in the Sn/Cu coating affected the IMC formation site. As a result, IMC with a comb-tooth structure was formed in the Sn/Cu- CUCR sample. The difference in whisker initiation tendency was inferred to be due to the difference in compressive stress on the basis of the Cu-Sn IMC formation morphology in the Sn/Cu coating. Therefore, the stresses were finally measured by X-ray diffraction to determine the correlation between whisker initiation tendency and the morphology of Cu-Sn IMC formation. Based on the results, a model for controlling whisker initiation in Sn/Cu-coated Cu leadframe systems is presented.