New approaches to whisker free tin deposits

In view of their different whisker growth propensities, tin-plated C19400 and C70250 copper lead-frame materials were investigated by atomic force microscopy (AFM) with regard to their intermetallic compound (IMC) characteristics. It was observed that both rate of formation and morphology are distinctively different from each other, whereby approximately twice the amount of IMC formed with C70250 than with C19400. The resultant IMC morphology from both materials consists of a ubiquitous thin IMC layer interspersed with relatively large pyramid-shaped IMC grains. These large grains have formed along the grain boundaries of the tin deposit, whereas the thin part of the IMC layer has grown at the base of the tin grains. The density of the large IMC grains is significantly higher with C70250 than with C19400. This difference is correlated to the tin whisker characteristics of the two tin-plated substrates. A higher density of large IMC grains that results in a reduction of whisker growth is also achieved by tin electrodeposits that show a characteristic crystal texture.