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110 SMT Magazine • July 2016 solder joints realized using laser contain a very low volume of voids. This type of defect would lead to poor mechanical and electrical proper- ties. The voids are a result of evaporated gas from flux being constrained inside the solder joint due to the surface tension. Because of the fast outgassing using laser, this defect is mini- mized, so laser soldering produces good results in voiding. One of the major influences on reliability was the solder joint's microstructure. Therefore, investigation of the interfacial reactions is im- portant and crucial to the understanding of sol- der joint during laser soldering. Figure 9 showed typical SEM micrographs of the interfaces be- tween the SAC305 solder and two kinds of pads just after soldering using laser processes. From the images, a continuous and uniform IMC lay- er was found at the interface, and the thickness of IMCs was lower than 1 µm since laser sol- dering is a very rapid process, that joints were formed within few seconds as energy delivery. The difference in IMC thickness for both OSP and ENIG surface finishes was not so apparent though it has been reported that laser energy is sensitive to the color of surface finish due to the reflection rate of laser beam. The composition analysis results of EDX showed that the reaction products at the interface were close to Cu 6 Sn 5 . The Cu 6 Sn 5 IMC dispersed inside the bulk solder as the shape of rods and small particles. In the case of ENIG surface finish, Ni and P elements were also detected in the IMC layer, as shown in Figures 9(b) and 10, indicating a Cu-Sn-Ni-P with P-rich layer was formed. Although it can- not be seen on SEM images, this kind of IMC layer has been believed to be more brittle and could degrade the reliability of solder joints. Figure 11 is a SEM image of interface be- tween solder and OSP pads after the first reflow in a conventional hot air oven. The IMC layer was much thicker than the former one as dis- cussed when soldering using laser, because the longer heating time during reflow process to make IMCs gather and grow into a large flake [15] . For SMT assembly, the thickness of IMC layer is usually required to be in the range of 2–3 µm after the first reflow to be believed to the de- Figure 8: X-ray inspection of (a) OSP and (b) ENIG surface finishes. Figure 9: SEM images of the interfaces between the SAC305 solder and (a) OSP and (b) ENIG for laser soldering. A NEW DISPENSING SOLDER PASTE FOR LASER SOLDERING TECHNOLOGY