Constituents and performance of no-clean flux for electronic solder

Corrosion reliability is a severe concern related no-clean flux (NCF) residue. It results in a decreased life span of electronic devices, specially fine pitch assemblies. NCFs are formulated from various organic/inorganic components viz. activator, solvent, surfactant, and additive. Each constituent of a flux has specific functions, and its advantages and limitations. The first part of this review paper describes the constituents of NCF, their characteristics and their compositional ranges. The effects of process parameters such as soldering process and soldering temperature on the performance of NCF are then presented in the second part. Finally, the effects of flux residue on the underfill and reliability of flip-chip packaging are discussed. Carboxylic acid containing less number of carboxyl functional group, lower decomposition temperature, lower solubility, higher acid dissociation constant and higher chain length can increase the cleaning efficiency of NCF. Hybrid activators (amine+carboxylic acid) are advantageous in terms of improving the cleaning action and also increasing interactions with underfill. Lower boiling point solvents are preferable as they are the main evaporating constituent. Reflow soldering has been proven as a better choice for NCF at the soldering temperature range of 200-250 degrees C, as it leaves minimum flux residue. Besides, incompatible residue on solder mask or die surface can hinder the flow of underfill and flux residue outgassing during curing generates voids and delamination. Therefore, to formulate an efficient NCF, the characteristics of its constituents need to be understood and the flux composition optimized.

Automated summary

Corrosion reliability related to no-clean flux residue is a severe concern, with the composition and performance of NCF being crucial factors in cleaning efficiency.