Study On Growth Behavior And Regulation Of Cu₆Sn₅ In Sn-Cu Solders During Solidification

In recent decades, lead-free solders have been widely studied in the entire world due to the prohibition of lead solders in electronic packaging industries among all the developed countries. Up to now, among all the substitutions of lead solders, the Sn-Cu solder system, which shows good mechanical properties, electrical properties and low cost, is proven to be one of the most promising lead-free solder candidates. However, during the soldering process, the intermetallic compounds (IMCs), which are generally brittle and invalidated easily due to their specific microstructures especially under electric current during service process, are usually formed at the Sn-Cu solder/Cu substrate interface. Therefore, it is of great importance to improve their mechanical and electrical properties by adding trace elements (La, Al and Zn) to refine the microstructures of IMCs in the solders.In this study, trace elements (La, Al and Zn) have been doped into Sn-6.5Cu solder alloys to refine the microstructures of IMCs. Beijing Synchrotron Radiation Facility (BSRF) and Shanghai Synchrotron Radiation Facility (SSRF) have been used as major testing methods to in situ observe the dynamic growth behavior of IMCs during solidification under direct current (DC). The solder alloys are as follows:Sn-6.5Cu、 Sn-6.5Cu-0.06La、 Sn-6.5Cu-0.2Al and Sn-6.5Cu-0.5Zn alloys. Synchrotron radiation imaging experiments have successfully captured the dynamic pictures of growth behavior of IMCs. Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS) has disclosed the morphologies of IMCs and confirmed that the IMCs in the solder alloys are Cu6Sn5.The conclusions are drawn as follows:(1) In Sn-6.5Cu solder alloy, Cu6Sn5 with I-like, Y-like and bird-like shapes are observed. When 100 A/cm2 DC is imposed, the mean size of IMCs is decreased and the growth rate of IMCs is increased. (2) After doping 0.06% La, IMCs are I-like, Y-like and bird-like shapes. Meanwhile, the microstructures of are refined. The growth orientations of Y-like shapes have been changed from 30, 165°,165° to 60°,150°,150°, respectively. (3) After doping 0.2% Al, Cu6Sn5 with X-like, Y-like and bird-like shapes are observed. Moreover, Al element is beneficial to the refinement of Cu6Sn5. When 10 A/cm2 DC is imposed, both the growth rate and the mean size of Cu6Sn5 are increased but relatively decreased when 100 A/cm2 DC is imposed. (4) After doping 0.5% Zn, Cu6Sn5 exhibits I-like and Y-like morphologies. The growth rate of Cu6Sn5 is decreased and the microstructure is refined. In addition, when DC is imposed, the coarsening phenomenon of Cu6Sn5 occurs and the irregular banding-like Cu6Sn5 appears beside them.This article has obviously revealed the morphological evolution of Cu6Sn5 IMCs in Sn-Cu solder alloys during solidification and quantitatively analyzed the effect of trace La, Al and Zn on the growth behavior of CueSn5, which presents an explanation and provides a direct evidence to better understand the growth behavior and formation mechanism of CueSn5 during solidification in lead-free solder alloys, which is of great significance to the improvement of the stability and reliability of solder joints in electronic packaging industries.