Issue link: https://iconnect007.uberflip.com/i/989774
64 SMT007 MAGAZINE I JUNE 2018 In all cases when through-hole or unplugged vias were assessed, solder overfills the hole leading to the bumps on the bottom of the board (Figure 13). This uncontrolled solder wicking could result in insufficient thermal connection between PCB and heat sink. Reduced via diam- eter could limit the amount of solder wicking. With smaller vias, the surface tension of the liquid solder inside the via could oppose grav- ity and minimize solder wicking. However, the reduction in via diameter will result in higher overall thermal resistance, which could be one of the downsides of this approach. In the case of the 5.4 x 5.4 mm thermal pad, 48-pin component, board was designed with four vias per pad. Results similar to the Figure 12: X-ray images of different via design the medium QFN components assemblies built with (a) SPO and (b) PF+paste with different via design. Figure 13: Optical images of the backside of the PCB, pads with through vias.