The reason why some double floor, others are not?

 The 4 layer design of the product, the reason why some double floor, others are not?
 The floor function has several aspects to consider:
 1, shielding;
 2, heat dissipation;
 3, strengthening;
 In 4, PCB processing needs.
  So whether several laminate flooring, first of all depends on its main reason. Here we mainly discuss the problem of high speed, so that the shielding effect. The surface of floor is good for EMC, but copper to be as complete as possible, avoid island.
  If the general surface device wiring is more, it is difficult to guarantee the copper foil complete, will bring the signals across the segmentation problem. So I suggest surface device or line board, not copper.