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July 2016 • SMT Magazine 109 son why the weight loss at first stage of flux in SP1 is smaller than the flux in SP2. Furthermore, the formation of activator derivatives can be an important factor of wetting behavior. It can in- dicate to have improvements on the splash or solder balling issues during laser soldering, and will be discussed in following sections. The second stage of weight loss is related to chemical reactions of activator derivatives. As mentioned, the carboxylic acid functional group of activator reacts with coexisting ingre- dients to form activator derivatives when flux was manufactured. Then during soldering pro- cess, the free carboxyl group can be regenerat- ed by thermal dissociation of activator deriva- tives, meanwhile, volatilization of the regen- erated coexisting ingredients occurred. These two causes bring the decreasing in weight at the second stage. The reaction diagram in de- tails was showed in Figure 6. In addition, it has been reported that wetting speed of activator derivatives could have influence on flux splash and concluded that this issue can be minimized by using activator which has a characteristic of slow wetting speed [14] . Therefore, in this re- search, different activators of A 1 and A 2 in these two samples are chosen to make a difference in the wetting behavior. The TGA results showed that the temperature of wetting behavior oc- curred of flux in SP1 is about 200°C and 183°C for flux in SP2 since activator derivatives begin to react after solvents vaporize, indicating that flux in SP1 has a slower wetting behavior than flux in SP2 during soldering process. In other words, SP1 could be contributed to mitigate the splash issue when soldering using laser. Investigation of Soldering Performance using Laser To discuss the relationship of paste char- acterization and soldering performance using laser for SP1 and SP2, the laser soldering trial on solder paste only was carried out. The best results was obtained in the case of using SP1, as can be seen in Figure 7a. It is obvious that no flux splash and solder balling were found around the solder joints. For the case of using SP2, few solder balls around solder joints can be observed, as shown in Figure 7(b). That is- sue could be a result of solder splash because of fundamentals of flux formulas, such as the properties of volatility, hygroscopic property and wetting speed. Therefore, as we expected, the splash or solder balling could be eliminated from a point of view of formulated solder paste by the modification of flux compositions. Reliability of Solder Joints SP1 was used in the second part of the re- search, to understand the reliability of solder joint after laser soldering, including voiding check, the microstructure of the interfaces and mechanical properties by a shear test. X-ray in- spection for the samples on both OSP and ENIG surface finishes was showed in Figure 8, that the Table 3. Summary of weight loss and temperature for fluxes in SP1 and SP2. Figure 7: Optical image illustrating solder joint appearance after laser soldering for (a) SP1 and (b) SP2. Figure 6: The reactions diagram when (a) flux manufactured; (b) soldering process. A NEW DISPENSING SOLDER PASTE FOR LASER SOLDERING TECHNOLOGY