Study On The Rapid Growth Behavior And Suppressing Method Of Sn Whiskers In Sn-based Solder Alloys

The spontaneous growth of Sn whiskers in Sn-based solder alloys has been a longstanding problem to threaten the interconnected reliability in the microelectronic packaging.Especially when the packaging density increases and the distance between adjacent interconnected joints decreases continuous ly,the short-circuit problems of the interconnected joints are likely produced by the Sn whiskers with only several microns,therefore,finding effective method to reduce and prevent whisker growth is the important issue that has attracted the widest interest.However,the Sn whiskers are always accidentally generated after a long period of time,which has brought a great deal of inconven ient to study the Sn whiskers.Therefore,to study the formation and suppressing mechanisms of Sn whiskers,a prerequisite is to create a speedup method of the nucleation and growth of Sn whiskers.Currently,the speedup methods for Sn whisker growth are l imited,including using the external stress,electromigration,isothermal aging and thermal cycling.However,the aforesaid methods have some shortcomings,such as the long incubation period for Sn whisker formation and the low growth density of Sn whiskers.Moreover,the sample preparation process can also affect th e growth behavior of Sn whiskers and disturb the experimental results.Based on the aforesaid reasons,this study creatively developed an effective speedup method for Sn whisker growth,deeply analyzed the influence of grain size and elastic modulus of the solder alloys on Sn whisker growth,and further proposed two kinds of suppressing methods of Sn whisker growth.This study may have important guiding significance to clarify the growth behaviors of Sn whiskers and decrease the risk of Sn whisker growth in various Sn-based solder alloys.In this study,we created a kind of Mg/Sn/Mg joints by using a self-developed ultrasonic-assisted soldering equipment,and found that the high-density of Sn whiskers were generated from the surfaces of the Mg/Sn/Mg joints a fter a short time isothermal aging at room temperature.The driving force of Sn whisker growth was demonstrated by the volume expansion in solder due to the formation of Mg 2Sn phase,which was cause by the chemical reaction between the interstitial Mg atom s in solder and Sn atoms.This fabrication process of the Mg/Sn/Mg joints can be used as a speedup method to produce the Sn whiskers in solder and may further be used to study the Sn whisker growth behaviors.Moreover,we fabricated a kind of Sn-Si C composite solder alloys by using mechanical compression and ultrasonic vibration,and realized the Mg/Sn-x Si C/Mg solder joints with different Sn grain sizes by using ultrasonic-assisted-soldering processes and the heterogeneous nucleation of Si C nanoparticles.We also analyzed the effect of the Sn grain boundaries promoting the formation and growth of Sn whiskers,and concluded the influence of the elastic modulus and microstructure of the solder on Sn whisker growth by contrasting growth characteristics of whiskers on surfaces of the Mg/Sn-x Pb/Mg joints.Furthermore,we observed the segregation behavior of Zn atoms at the Sn grain boundaries in Sn-9Zn solder,and found that the steric effect due to the Zn segregation can seriously block the grain boundary diffusion of Sn atoms.We also proposed that the Zn-rich phase with layer structures can destroy the long-range diffusion of Sn atoms,and confirmed the inhibition effect of Zn aomts on the Sn whisker growth.Finally,the Sn/Cu/Sn solder preforms with good boundi ng interfaces were prepared by using cold pressure welding processes,and the IMC/Cu/IMC joints can be obtained by heating the solder performs and Cu pads within a short time.The IMC/Cu/IMC joints can avoid the risk of Sn whisker growth,and also has the high-temperature resistance,which can be used in interconnected packaging for high power and temperature devices.