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40 DESIGN007 MAGAZINE I SEPTEMBER 2021 overall thickness restrictions, circuit width and spacing and impedance values required. Here are some guidelines that should be fol- lowed to assure design success: • Design multilayer boards with an even number of layers • Whenever possible, only one core thick- ness should be used • If specifying the dielectric thickness when, for example, it may be required for imped- ance reasons, the dimensions should be selected from available core or pre-preg thicknesses – Dielectric thicknesses made up of prepreg depend on the type or the combination of prepreg that is suitable and of achievable dimensions and tolerances – It is beneficial to discuss special dielectric requirements during the design stage with your PCB vendor if possible • Maintaining a balanced layup in relation to the Z-axis median of the board will assure minimum bow and twist. is balance includes the following: dielectric thickness of layer, copper thickness of layers and its distribution and location of circuit and plane layers. A higher number of layers normally will mean an increased number of plane layers. Planes should be balanced around the Z-axis median line of the layup, and ideally located internal to the board. If accepted multilayer design rules are adhered to, boards will meet a maximum allowable bow and twist specification of 0.010 inch per inch (1%) or better. • If the prepreg opening is greater than 0.016" use filler cores • Outer layer circuitry: Circuit area and dis- tribution between the front and back of the board should be balanced as closely as possible. e addition of plating thieving of low pattern density of external plane area should be considered. Each outer layer is typically a signal layer built on half-ounce copper • ickness tolerance: As the overall thickness of a multilayer board increases, the thickness tolerance should also increase. A good rule is to specify a toler- ance of +/– 10% of the overall thickness. Figure 2: An example of a FR-4/PTFE hybrid material set.