Mechanical and interfacial characteristics of Zn-Al solder joints under elevated temperature and vibration conditions

The Zn-Al solder alloy is a promising candidate to replace the conventional high lead solders (Pb-5Sn, Pb-10Sn) in automotive applications, which work in a harsh environment combined with thermal and mechanical loads. In this work, the effects of concurrent vibration and elevated temperature on the Zn-Al solder interconnection on Cu or Ni-P plated Cu substrates were studied. The test samples by forming the Zn-Al solders onto Cu or Ni-P plated Cu substrates were prepared for shear tests followed by IMC analysis to understand the interrelations between their performance and IMCs characteristics. Then, various tests on these samples were carried out under combined conditions of high temperature up to 250°C and vibration frequency between 100Hz and 440Hz, based on the automotive standards. The mechanical analysis are conducted and compared; and final interfacial reactions and fracture analysis and effect of IMCs formations were also examined using Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) which can be correlated with the test results. It has been found that the shear strength of the joints on Ni-P plated Cu samples was generally higher than the Cu samples without metallization under the same testing conditions. In addition, the shear strength of solder joints decreased on both Ni-P coated and uncoated Cu substrates moderately after aging. However, the effect of vibration was not apparent according to the result of shear strength. From the fracture analysis, the modes of fracture for Cu/Zn-Al joints and Cu/Ni-P/Zn-Al joints were primarily characterised as brittle and ductile fracture, respectively.