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106 SMT Magazine • July 2016 was observed by X-ray machine (Dage 7600NT). Micrographs and chemical element analysis of the IMC layer in solder joints were performed on cross-sectioned samples using SEM and EDX, respectively. We dipped the cross-sectioned samples into 5 cc. HNO3 acid solution, in or- der to highlight the IMC layers. The mechanical properties of the joints were measured by shear test on a bond tester (Condor Sigma Lite) with the shear speed of 100 µm/sec. Results and Discussion Rheological properties of solder paste The rheological properties of the two sol- der pastes were determined via measurements obtained during rheological tests [11-12] . For paste dispensing, viscosity of solder paste plays an im- portant role because it should be controlled in the workable range for dispensing. For example, viscosity must be low enough to force the paste flowing through the dispensing needle, but high enough to support the deposit to retain the shape without slumping. The viscosity of sol- der paste for dispensing is recommended in the range of 100±30 Pa.s, technically. The viscosity of SP1 and SP2 was measured to be 107.5 Pa.s and 95.2 Pa.s by a viscometer, indicating suit- able for dispensing. A plot of viscosity against shear rate for the samples was presented in Fig- ure 3. As applied shear rate increased, both SP1 and SP2 showed the same trend of decreased viscosity. That means the solder paste sample shear-thinning behavior as non-Newtonian flu- ids. The difference of viscosity as a function of shear rate can be resulted from the flux system composition due to different solvents inside. Furthermore, the oscillatory stress sweep test, which basically applied at a constant fre- quency of 1 Hz and in the range of 0.5–500Pa, is to characterize the visco-elastic behavior that can help to understand the structures changes occurring under stress during process. The re- sults from the oscillatory test were shown in Figure 4a, 4b. Generally, two modulus were presented in the plot: the storage modulus G' measures the elasticity of solder paste, and loss modulus G" measures the plasticity. Linear visco-elastic region can be identified as the be- Figure 4: Storage and loss modulus as a function of shear stress for (a) SP1 and (b) SP2. Figure 3: Viscosity against shear rate of solder pastes. A NEW DISPENSING SOLDER PASTE FOR LASER SOLDERING TECHNOLOGY