1. When the outer layers have GND and PWR stack ups, these boards should be used for short trace runs or fan outs only. The second layer is the signal layer for the purpose of HDI. These should run traces from fine pitch ball grid arrays. For HDI uses, a producer accesses the second layer through controlled depth drilling by utilizing a laser drill. 

 2. To get rid of or minimize warp-age, it is best to maintain the thickness of the laminate from the centerline of the PCB to between the layers. The laminate type and thickness must be set before doing the PCB layout by CAD. 

 3. To insure a control of impedance, it is very important to analyze the stack up with the board producer for determining copper weights, prepreg and core thickness before the board is laid out using CAD. 

 4. Stackup layers 2 through 16 should use 1.6 mm FR 4 material. Stack ups 10 through 20 layers should use 1.8 mm FR4 material. Stackup layers 10-32 should use 2.3 mm FR4 material. 

 5. The most common thicknesses for printed circuit boards are: 
  ▪　0.8mm (0.031”) D. 1.8mm (0.070”)
  ▪　1.0mm (0.040”) E. 2.3mm (0.090”)
  ▪　1.6mm (0.062”) F. 3.2mm (0.125”)