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108 SMT Magazine • July 2016 ginning of drop in G' value, and defined as the maximum deformation that can be applied to the solder paste prior to structure breakdown. The broader linear region and higher G' value of SP1 over SP2 indicate that SP1 has better structure resistance towards applied stress and can withstand higher stress without structure breakdown [11] . In addition, the stress at G'=G" can be used as an index for the cohesiveness of the solder paste. From the plot, SP1 showed the higher stress value at G'=G" of 248 Pa than SP2 of 153 Pa. The lower the stress at G'=G", the more liquid-like the solder paste which could contribute to fracture or leaking deposits dur- ing dispensing process. Therefore, based on the fundamental designation of flux formulas in solder paste and its rheological properties, SP1 is proved to be more applicable for dispensing process. Thermogravimetric Analysis of Solder Paste As stated, flux formulas in solder paste dic- tates the thermal behaviors of solder paste dur- ing soldering process, and there are numerous attempts to correlate the thermogravimetric analysis of flux in solder paste and soldering performance. Splash of flux or solder balling around solder joints is primarily concerned when using laser soldering due to the rapid heating-up and cooling rate. This phenomenon is considered to be caused by exploding of flux ingredients during soldering process. Accord- ingly, the splash issue can be reduced through proper selection of flux ingredients. The vola- tility, hygroscopic property and wetting speed during soldering were considered to have a great influence on splash phenomenon [13] . The results of TGA showed in Figure 5. A heating rate of 10°C/min in air atmosphere was performed in this study. The weight loss of flux at the temperature below 100°C is assumed to be volatilization of moisture, and the presence of moisture is mostly due to hygroscopic prop- erties of chemicals. Two stages of weight loss can be found from the TGA results, in which the first stage of weight loss is mainly caused by volatilization of solvents and a fraction of activator derivatives, and the following stage is related to chemical reactions of activator de- rivatives. The comparative value of initial tem- perature and total weight loss of each stage of the samples were showed in Table 3. For the results of the first stage, it was ap- parent to figure out that decrease in weight of flux in SP2 is greater than the flux in SP1 due to the different boiling point solvents and the activator derivatives. Typically, more than two solvents of different boiling points are used to formulate a flux in order to offer compatibil- ity to the other ingredients and avoid solvent evaporating simultaneously which can result in splash or solder balling issues. To discuss how volatility of solvent affects the flux splash in this research, three solvents of different boil- ing points were used to formulate two fluxes, as showed in Table 2, in which flux in SP1 contains solvent S1 and S2 while flux in SP2 contains S1 and S3. Solvent S2 has the highest boiling point followed by S1 and S3, indicating flux in SP1 is more stable at higher temperature with lower risk of flux splash than flux in SP2. On the other hand, the formation of activator derivatives oc- curred when flux manufactured. Solvent S1 is the common solvent whose functional group can react with activator in the formation of ac- tivator derivatives. For the concept of designa- tion, flux in SP1 was assumed to form more acti- vator derivatives due to the easier reactions be- tween solvent S1 and activator A1, resulting in less weight of solvents retained. That is the rea- Figure 5: Weight loss curve as a function of temperature of (a) flux in SP1 and (b) flux in SP2. A NEW DISPENSING SOLDER PASTE FOR LASER SOLDERING TECHNOLOGY