Wettability Improvement of Solder in Fluxless Soldering under Formic Acid Atmosphere

Soldering is applied to the electronic packaging industry. During soldering, flux is the chemical cleaning agent frequently to remove the oxide layers at the surfaces of the solder and the soldered metallic pad for their sufficient interfacial reactions. However, flux residue is a corrosive issue lowing the reliability of solder joint. Additionally, the flux residue is hardly removed in the micro-bump of three-dimensional integrated circuit packaging. Although some studies have demonstrated the fluxless soldering technique with formic acid (FA) atmosphere, in-situ observation on chemical reduction during the soldering has not been studied well, especially for various metals of the soldered pad. In this study, an Sn-Ag-Cu solder was soldered on various substrates under FA atmosphere. Their wettability of Sn-Ag-Cu (SAC) solder was estimated by the measurements of their contact angle and spreading area. The substrates are Cu, Ni, and electroless nickel immersion gold (ENIG), which was frequently used for electronic packaging. Additionally, a soldering process with commercial rosin mildly activated (RMA) flux with Cu substrate was set as the reference. From the results, it demonstrate that the contact angle of solder on Cu substrate under FA atmosphere was no substantial difference to that of solder using RMA flux. The Pre-heat time affects the wettability, but not obvious, and the wettability was increased with increasing the peak temperature. The effect of the FA atmosphere on the wettability of ENIG was much more significant than those of Cu and Ni because the soldering spreading area of ENIG was ten times larger than those of Cu and Ni. Additionally, the pressure of the FA atmosphere was considered as the effect on the soldering wettabilities. Thus, the soldering was tested at different atmospheric pressures of the FA (1kPa, 10kPa, 80kPa). The findings show the contact angle of each soldering system was slightly affected by the FA pressures.