Study On Growth Behaviors And Suppression Methods Of Tin Whiskers For Solder Layers Of Microbumps In Electronic Packaging

With the development of lead-free solders and miniaturization of electronic devices,solder layers of microbumps have become the main form of lead-free solder in 2.5D and 3D advanced interconnection packaging.Compared with conventional solders,the solder layers of microbump exhibit high density and narrow pitch,and the mechanical,thermal and electrical environment is more severe,leading to the increase of short-circuit risk caused by tin whiskers,which poses a great challenge to the reliability of advanced electronic packaging.Aiming at this problem,an amorphous Co-W barrier layer was prepared by electrodeposition method,and the inhibiting effects of ultra-thin Ni and Co-W barrier layer on tin whisker in traditional high temperature and humidity environment were studied comparatively.Considering the uneven distribution of stress on microbumps,tin whisker growth behavior and mechanism of solder layers on microbumps under continuous compressive stress were systematically studied.Based on the formation mechanism of tin whisker on microbumps,a new idea of using micro-nano cones（MNCs）structures were proposed and the effectiveness of this method was verified by experiments.The main results are as follows:（1）The Ni barrier layer with 450 nm thickness and the amorphous Co-W barrier layer with 200 nm thickness can effectively inhibit the growth of tin whiskers in the conventional high temperature and humidity environment（55°C/85%RH）,and meet the requirements of industrial standards.The amorphous Co-W barrier layer was prepared by electrodeposition,and its surface smoothness,uniformity and stability were better than those of Ni,and the corrosion resistance of Sn layer was improved.Granular Ni₃Sn₄ and flaky Ni Sn₄ intermetallic compounds（IMCs）formed by the reaction of Ni barrier layer with Sn cause volume shrinkage,which results in larger tensile stress in Sn layer,thus suppressing whisker growth.Amorphous Co-W barrier layer primarily inhibits tin whisker growth by significantly reducing the formation of Cu-Sn IMCs.（2）Tin whisker growth behaviors of solder layers on microbumps under continuous compressive stress（16 MPa,5 days）have been studied.The maximum tin whisker length can reach 35.3μm,which overlaps with the adjacent microbumps and causes short circuit,threatening the reliability of advanced electronic packaging.The main morphologies of tin whiskers on microbumps were long columnar,short columnar and hillock at 25°C,55°C and 85°C respectively,and a few whiskers were needle-like and filamentous.With the increase of temperature,the maximum length and average length of tin whiskers decreased,while the maximum diameter,average diameter and linear density of tin whiskers increased.The unreflowed Sn-0.8Ag/Ni/Cu microbumps hardly grew tin whiskers,which was related to the morphology of the solder layer and the formation of Ni Sn₄.After reflow,Sn-3.5Ag/Cu microbumps with the same diameter were easier to grow whiskers than Sn-0.8Ag/Ni/Cu microbumps,which was related to the flaky Ag₃Sn and Ni barrier layers.（3）The tin whiskers grown on Sn-Ag solder layers of microbumps were analyzed by various characterization methods.The results showed that there was trace Ag in the tin whiskers.The grain boundaries of tin whisker and its adjacent grains were twin boundaries,and dislocations existed in the two grains.Based on this,tin whisker formation mechanism for microbump under continuous compressive stress is proposed,that is,solder layers of microbumps produce a lot of dislocations in Sn grains under continuous compressive stress,twin boundaries provide nucleation points for tin whisker formation,and dislocations continue to slip through twin boundaries into whisker grains,resulting in sustained growth of tin whiskers.（4）The tin whisker growth behaviors of the solder layers in different environments have been explained by creep mechanism.The whisker growth mechanism of the Sn layers in high temperature and high humidity environment is dominated by grain boundary diffusion creep.Both the Sn layer tensile stress induced by Ni barrier layers and the inhibition of Cu-Sn IMCs formation by amorphous Co-W barrier layer reduce the driving force of grain boundary diffusion.The whisker growth on solder layers on microbumps under continuous compressive stress mainly follows the dislocation creep mechanism at 25°C,while the effect of grain boundary diffusion creep is significantly improved as the temperature rises to 55°C and 85°C.Different from the dislocations that tend to slip at twin boundaries,the grain boundary diffusion is not limited and therefore Sn atoms can diffuse through grain boundaries into multiple deformable grains which will evolve into whiskers,resulting in the increase of linear density of whiskers.The growth of Sn grains at high temperature leads to an increase in the average diameter of the whisker.The compressive stress is relaxed by creep deformation of solder layers and whisker growth.With the increase of temperature,the creep rate of solder layers increases so the growth rate of tin whisker is slowed down.（5）The MNCs structures significantly suppressed whisker growth for thin Sn layers（2μm）.Under continuous compressive stress,the Ni MNCs prepared by electrodeposition had a significant inhibitory effect on whiskers at 25°C.The Co-W modified Ni MNCs（Co-W/Ni MNCs）structure could effectively suppress whisker growth at 85°C.The inhibiting effects of Ni MNCs and Co-W/Ni MNCs structures on whisker were still effective at high temperature and humidity environment,and Co-W/Ni MNCs exhibited excellent stability.The MNCs structures suppress whisker growth mainly by inhibiting the growth of dynamic recrystallized grains.In addition,a large amount of Sn horizontal grain boundaries induced by MNCs structures may enhance compressive stress relaxation of Sn layers.