Amino acids as activators for wave solder flux systems: Investigation of solderability and humidity effects

The role of flux activators in flux formulation is to remove oxides from metal surface, hence allow the formation of metallurgical bond between metal substrate and solder alloy. However, flux activator in the no-clean flux residue on printed circuit board assemblies (PCBAs) affects the climatic reliability due to its hygroscopic and ionic behavior under humidity exposure.

Four amino acids-based wave flux were investigated using wetting balance test and microscopy inspection, and the humidity effects of four amino acids was analyzed using electrochemical impedance spectroscopy (EIS) and chronoamperometry under various testing conditions. Water absorption and thermal degradation were analyzed using gravimetric method and Fourier-transform infrared spectroscopy (FT-IR).

Combined results indicate glutamine based model flux obtained superior solderability for 96.5Sn-3Ag-0.5Cu (SAC 305) alloy due to suitable melting and activation temperature of amino group for the oxides removal. The robust humidity reliability of amino acids was dependent upon relative high deliquescence point and low moisture absorption under harsh climatic conditions.