Research On Tin Whisker Growth And Mitigation For Electroplated Pure Tin Films

Sn whiskers are single-crystal metallic filaments with high mechanical strength which nucleate and grow over time in such a way as to lead to device failure in the filed or even a catastrophic loss.During the lead-free process in electronic industry,replacement of SnPb terminations with pure tin has proven to be much more difficult than expected.The main problem is tin whisker formation.In this thesis,the effect of electroplating parameters and structure design of pure tin films on tin whisker formation and growth were investigated.Considering two of the key factors affecting tin whisker gowth,Ni barrier method and non-columnar grain structure formed by alternate electrodeposition of tin films are proposed to prevent Sn whisker growth from pure tin films.Main conclusions are as follows:?1?Sn-Cu intermetallic compound?IMC?formation by interfacial diffusion and columnar grain structure of pure tin film are the key factors affecting tin whisker growth.?2?Experimental results indicate that lower density of Sn whiskers tended to grow from bright Sn films plated on C194 alloys,which were stored at 55°C/85%RH for 8000 hours.And higher density of Sn whiskers tended to grow from bright Sn films plated on FeNi42 alloys.For matte Sn films plated at a higher current density,lower density of Sn whiskers grow from the matte Sn surfaces after thermal and humidity?TH?storage.?3?Analysis of cross-sectional microstructure of pure Sn films indicates that substrate effect on tin whisker growth is mainly due to the formation of IMCs at the interface.The IMC layer of Cu₆Sn₅ formed at the interface between bright Sn film and C194 alloy,and rare IMC of plate-like Ni₃Sn₄formed at the interface between bright Sn film and FeNi42 alloy.After introducing Ni barrier between the Sn film and the substrate,Ni₃Sn₄ formed at all of the interfaces,which restrained the Sn whisker growth.?4?Reliability issue may be caused by the back-end production when the method of Ni barrier plated using nickel sulfaminate bath is adopted to control Sn whisker growth.In order to garentee the mechanical stability of Sn films,the thickness of Ni barrier layer should be controlled between 0.5 and0.1?m for matte Sn plated IC lead-frames to effectively prevent Sn whisker growth.Reflow process can significantly mitigate the growth of tin whiskers.And the effect of both Ni barrier and reflow process on the growth of tin whiskers is the best.?5?Multilayer structure of Sn films could not form by alternately electroplating bright Sn or matte Sn films at different current densities.However,multilayer structure of Sn films could form by alternately electroplating both bright and matte Sn films.Alternately electroplated even matte and bright Sn films with bottom layer of matte Sn,i.e.trilayer or above,could form a non-columnar grain structure.Both test results of temperature and humidity?TH?storage and thermal cycling testing?TCT?indicated that non-columnar grain structure is benefit to prevent tin whisker growth.?6?Increase in the number of alternately electroplated Sn layers is benefit to enhance the blocking effect of non-colunmnar grain structure.When the number of alternately electroplated Sn layers decreases,i.e.bilayer,the blocking effect of grain structure is weakened,but hillock/tin whisker growth is susceptible to the top layer effect.Hillocks/tin whiskers tend to form from the tin film with top layer of bright Sn rather than with top layer of matte Sn.