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July 2016 • SMT Magazine 111 veloping of a strong interconnection. However, the presence of excessive IMC layer will weaken the long-term reliability of solder joints, not only because most IMCs are brittle, but also because the coefficient of thermal expansion (CTE) mismatch exists between the IMCs and solder joints. Furthermore, numerous micro- voids were also seen in the solder matrix and the interface between the IMC layer and solder. These micro-voids comes from the unbalanced in diffusion rate of Sn, Cu, Ag and Ni during reflow process, and can provide initiation sites and propagation paths for cracks whose forma- tion degrades the reliability of solder joints. The study of the effects of isothermal aging on long- term reliability will be necessary in the future. Mechanical properties of BGA spheres sol- dered by laser soldering and hot air reflow pro- cess with same solder paste SP1 were shown in Figure 11. It was believed that the shear strength above 10N is strong enough for the connect- ing of microelectronic components. The results of shear strength revealed two important as- pects: first, in the case of using laser soldering, the shear strength of BGA sphere soldered on ENIG pads was inferior to soldered on the OSP pads, and also had a larger deviation of shear strength. As discussed before, a thin additional P-rich layer at the interface between solder and IMC layer could be the main reason. Secondly, it was found that the shear strength of solder joints by the laser soldering was slightly lower Figure 10: Composition analysis of IMC by EDX mapping for ENIG pads. Figure 11: SEM images of the interfaces between the SAC305 solder and OSP after the first reflow process. Figure 12: The results of shear strength for (a) OSP with laser soldering, (b) ENIG with laser soldering, and (c) OSP with reflow process. A NEW DISPENSING SOLDER PASTE FOR LASER SOLDERING TECHNOLOGY