Hello, in my experience SMD components doesn't need a presoldering operation at all, because pins are already stained or golded, thus very solderable. Plus the extra heat could let the pin move inside the plastic, or add somehow excessive thickness when positioned on the PCB. In my experience the best option is to clean well the PCB with a desoldering wick, then add some flux and touch the pads with a thin soldering iron and some AgPb wire, to dilute the no-Pb alloy on the pads, which is harder to melt than Pb alloy. Thanks to flux, surface tension will be low and Agpb alloy will form a thin layer with no oxide. Now position the new component, it should seat well in position, without excessive height over the PCB. Now with thin iron, heat two opposite pins letting the alloy to melt, and the component will be held in place. Now proceed in order with all pins.
Note: use lower temperatures (<380C, less is better), good quality flux and AgPb are required for comfortable operation with low risk of damage. Andrea
