Study On The Barrier Effect Of Copper-tin IMC Interlayer On Copper-tin Interfacial Reaction And Tin Whisker Growth

Copper-tin interfacial reaction is quite common in electronic packaging,such as the reaction between the outer leg of the lead frame and its tin plating,and that between the copper pillar and the tin-based solder cap.A moderate copper-tin reaction enhances the bond strength of the interface,but an excessive one may pose risk to device reliability due to the brittle property of the intermetallic compound?IMC?products.The copper-tin reaction is usually accompanied by the growth of tin whiskers,which are filamentous metal crystals with very high mechanical strength.Tin whiskers grow over time on the tin surface and can be as long as microns or even millimeters.Historically,tin whiskers have caused serious short-circuit faults in electronic devices.In this study,the tin thin films were prepared by electroplating and the copper-tin IMC interlayers with different compositions were prepared by isothermal aging for different periods of time.The effect of the IMC interlayer on the copper-tin interfacial reaction and the tin whisker growth has been investigated.The results of copper-tin interfacial reaction revealed that the interlayer with the composition close to Cu₃Sn can significantly reduce the Cu/Sn diffusion coefficient,and the greater the thickness of the interlayer,the greater the barrier effect.In addition,when the thickness of the IMC interlayer is constant,interlayer with longer aging time?i.e.composition closer to Cu₃Sn?leads to lower Cu/Sn diffusion coefficient,that is,better barrier effect;however,the barrier effect tends to be saturated when the the pre-aging time of the interlayer is above 24h.The study of tin whisker growth found that the multilayer structure of the tin plating can inhibit the growth of tin whiskers,and the greater the thickness of the matt tin interlayer,the better the inhibitation effect.When the thickness of the IMC interlayer is greater than 1?m,it can significantly reduce the number of tin whiskers;and the closer the interlayer's composition to Cu₃Sn,the better the inhibitaion effect.In this paper,the interface diffusion model is used to explain the barrier effect of IMC interlayer on the Cu/Sn interface reaction.The barrier effects of IMC interlayer on the growth of tin whiskers are explained from the perspective of stress and grain structure.