求翻译：1.先将装好PCB板的塑料底板放入夹具内，在PCB正极端的镍片上套热缩管（￠4*24mm），注意PCB上的正负极位置；是什么意思？

1. the first installed within the plastic bottom of PCB Board into the fixture, fall into a trap of nickel in PCB is extreme heat shrinkable tube (DIA 4*24mm), attention to the positive and negative position on the PCB;