Determination of melting and solidification temperatures of Sn-Ag-Cu solder spheres by infrared thermography

Abstract

This study introduces the application of infrared (IR) thermography for the determination of melting and solidification temperatures of Sn-Ag-Cu based solder materials commonly used for interconnects in electronic components and devices. The presented setup allows a fast and simultaneous analysis with heating and cooling rates of approx. 0.93 K/s and 0.74 K/s respectively. In order to prove feasibility, measurements were carried out on free standing solder spheres with a diameter of 270 µm which is a typical size for solder interconnects in electronic devices. Four Sn-Ag-Cu alloys (Sn-2.0Ag-0.5Cu, Sn-3.0Ag-0.25Cu, Sn-3.0Ag-0.7Cu, Sn-3.0Ag-0.5Cu) with 19 samples per alloy were investigated per experimental run, resulting in a total of 76 samples per measurement. The reproducible extraction of the respective onset temperatures for melting and solidification is demonstrated by the analysis of representative heating and cooling graphs. The onset temperature of melting, which represents the eutectic temperature of these alloys, was measured between 215.1 °C and 218.1 °C. This is less accurate but in good agreement with additionally conducted Differential Scanning Calorimetry (DSC) measurements (217.2 °C to 218.0 °C), as well as the eutectic temperature from the phase diagram (217.2 °C). The solidification temperature of these samples was statistically evaluated by four subsequent experimental runs (generating up to 76 values per alloy). The measured temperature range combined for all four alloys was between 127.2 °C and 196.4 °C. The DSC results (nine values per alloy; three samples; three runs) are comparable but yield a smaller range from 142.2 °C to 185.2 °C. Furthermore, the melting and solidification temperatures of small Sn-Ag FlipChip solder bumps (approx. ∅ 87 µm) were reproducibly analyzed by the IR thermography method and range from 220.3 °C to 220.5 °C and 176.3 °C to 196.7 °C respectively. The presented IR thermography method has proven a valuable alternative, especially for a statistical analysis of undercooling distributions in small samples, such as Sn-Ag-Cu solder joints.