Investigation Into Chemistry and Performance of No-Clean Flux in Fine Pitch Flip-Chip Package

This study investigates the chemistry and performance of two commercial no-clean fluxes (NCFs) ii.e., NC-1 and NC-2. One water-soluble (WS) flux was also studied for comparison. Chemistry of these fluxes and their residues was studied using the Fourier transmission infrared (FTIR) spectroscopy. Thermal behavior of the flux was evaluated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Hygroscopicity of the residue was determined using water contact angle on residue-contaminated substrates. These fluxes were used to prepare fine pitch (
$150~\mu \text{m}$
) flip-chip package (
$17\times17$
mm
²
) containing solder bump and copper pillar. Performance of each flux was evaluated using wettability of preprinted SAC-305 solder on copper substrate. Die pull strength of solder bump was evaluated and failure mode was analyzed using scanning electron microscopy (SEM). FTIR results showed that carboxylic acids, tertiary amine, and ether were present in NC-1. Carboxylic acid, secondary amide, and ether were observed in NC-2. WS showed the presence of carboxylic acid, secondary amine, ether, and alcohol. Residue showed the presence of carboxylic acid and amine in NC-1 and only amide in NC-2. Evaporation of certain monocarboxylic acids and ether solvent in NCF led to minimal residue compared to WS. Residue of WS was highly hygroscopic compared to N CF. Wetting of solder bump prepared by using NC-1 and NC-2 was better than WS, which is related to flux chemistry. Consequently, higher die pull strength was observed for N CF.