Study On Environmental-friendly Water-based No-clean Flux For Lead-free Solder

Due to the requirements of lead-free technology process and green chemistry in electronic industry, the water-based no-clean flux for lead-free solder will possess broad application foreground in the future. Sn0.7Cu and Sn3.0Ag0.5Cu, two kinds of the high temperature lead-free solder which were considered as the most popular used solders for electronic packaging, and no-clean flux matched for these solders are becoming a research focus in domestic and overseas. But most of solvent no-clean fluxes available today for surface mounted technology (SMT) contain a very high percentage of volatile organic compounds (VOCs) which pollute environment easily and cause trouble in safety.The water-based no-clean green flux will emit very low VOCs during welding, give a little residue after welding and thus does not require post-soldering cleaning. Materials and energy associated with cleaning would be conserved and pollution is prevented. Moreover, the water-based flux reduces concerns over worker safety being not flammable. Thus development of water-based no-clean flux is becoming one of the main directions in electronic industry.In this thesis, a series of water-based no-clean fluxes were prepared matched for Sn-0.7Cu and Sn-3.0Ag-0.5Cu lead-free solders. The physical properties such as boiling point and acid value of the selected activator were compared and the wetting properties of the activator were tested. The decomposition of activator and the residual pollution on the plate after welding were tested by thermogravimetry analysis, spreading performance experiment, corrosion testing and scanning electron microscopy. Based on these results, the optimum activator and the optimum dosage were determined. The results showed that using succinic acid and malic acid mixture as activators can improve the activation properties. They were easy to be decomposed at 250℃, and the mass residue is below 0.6% after decomposed. When succinic acid and malic acid were compounded (w=1.6%) as the activator, the spreading area of the flux with low solids content (less than 3%) over Sn-0.7Cu and Sn-3.0Ag-0.5Cu lead-free solder was 59.92 mm2 and 64.16 mm2 respectively, and expansion rate reached 68.94% and 71.36% over Sn-0.7Cu and Sn-3.0Ag-0.5Cu respectively. And the flux showed minimum corrosion properties both on copper and the comb patterns.Based on the spreading performance experiment, glycerol (w=1%) was chosen as film-forming agent, which showed that the copper surface was dry and clean. After adding glycerol(w=1%), the spreading area of Sn-0.7Cu increases from 59.92 mm2 to 62.70 mm2 and expansion rate increases from 68.94% to 70.59%; the spreading area of Sn-3.0Ag-0.5Cu increases from 64.16 mm2 to 67.35 mm2 and expansion rate increases from 71.36% to 73.41 %. As selected 2-butoxy ethanol, ethylene glycol and isopropanol (w=3%) as cosolvent, the flux showed good wetting properties. Based on wetting prosperity test and surface tension test, polyoxyethylene nonyl phenyl ether (50) (w=0.2%) was selected as surfactant. The surface tension of flux system decreased from 51×10-2 N·m-1 to 37.55×10-2N·m-1, the maximum wetting force was 3.44 mN, wetting time was 1.89 s. The flux showed lower surface tension and better wetting properties.The influences of three main components such as activator, surfactant and inhibitor on the corrosion resistance of the environmental-friendly no-clean flux were studied. The effects of different components on the copper electrode inhibition performance were studied using alternating current(AC)impedance; The surface corrosion of three main components on comb patterns was also observed through scanning electron microscope, the residue of flux and interaction were tested by thermal analysis. The results showed that the structure and their acidity of the activator had a certain effect on the corrosion resistance of the flux, the weaker of the acid, the weaker corrosion of the flux. The inhibition property of the flux can be improved with the addition of polyoxyethylene nonyl phenyl ether as nonionic surfactant. With benzotriazole (BTA) as inhibitor, the inhibition performance of the flux was promoted effectively. The flux, with combination of succinate and malic acid (w=1.6%) as activator, polyoxyethylene nonyl phenyl ether (50) (w=0.2%) as surfactant, BTA(w=0.06%) as corrosion inhibitor, showed optimum inhibition performance, high membrane resistance and lower inhibition on hygrothermal condition. The mechanisms of this interaction were discussed based on thermal analysis, which showed that the esterification reaction between organic acid and glycerol may play an important role for corrosion resistance performance improvement in welding process.The performance of water-based no-clean flux was tested according to the Chinese industry standard of SJ/T 11273-2002 (No-clean Liquid Flux). The results indicated that these new VOC-free fluxes were halogen-free, had low solid content(2.15%) and had low VOCs (below 5%), the maximum wetting force was 3.44mN, wetting time was 1.89 s. The expansion rate reached 71.09% and 82.56% over Sn-0.7Cu and Sn-3.0Ag-0.5Cu, respectively. The surface insulation resistance of the flux was beyond 5.9×108Ω. which meet the requirement of lead-free solder. The flux showed some other advantages such as nonflammable, environment friendly and no harm to health.