Re: why scan chain is deleted before placement and reconnect
At initial stage , we dont have the placement information. So we just stich the flops register by register. But after placement it might be possible (and generally in pratice happens) that the two flops stiched at initial stage of a different block sits far or too far from each other when the placement is done. So if we keep the scan chanin stiched same as a initial stage, very long and wearied and inefficient scan path results(i.e. one flop at this end of chip and next flop in chain is on ohter corener of chip). So before we do placement, remove the scan chain stiching, and once you done with the placement we re-stich the whole scan chain. (called scan chain re-ordering as well). So tool will systematically stich the scan flops.
Hope this help.!!