The manufacturing of multilayer PCBs is possible in our company in 4, 6 and 8 layers. For the production it is necessary to provide the data by layers in Gerber file (RS274-X format) and drill files (Excellon, Sieb & Meyer 1000 and 3000 format) and so it is important to describe the stack-up, which contains the order of layers, the type and thickness of used materials and the total thickness. The total thickness depends on the pressing parameters, the inner layer filling, the thickness of copper, the type and rsin content of pre-pregs. Because it is impossible to create the theoretical or calculated total thickness, we give the total thickness with ±10% tolerance.
The multilayer PCB is consists of 3 important element:
Copper foil: in case of normal built-up the 2 outer layers (top and bottom) is created from copper foil. Thicknesses of copper foils: 18µm, 35µm and 70µm. The plating of the holes and layout the copper thickness is increased by 20-25µm comparing the copper base thickness. The thickness of copper can be increased by longer galvanisation, plating process.
Core: different thickness both sides are FR4 copper clad. It realizes two conductive layers and the insulating layer between them. Possible Core thicknesses: 0.2mm, 0.36mm, 0.51mm, 0.71mm, 1.0mm, 1.2mm, 1.5mm, 2.0mm, 2.4mm. Copper thickness on the core: 18µm, 35µm, 70µm (an option can be even 105µm).
Pre-preg: "pre-impregnated" composite glass fibers and epoxy resing where the epoxy resing is B-Stage material it means that it is only partially cured to allow easy handling. During pressing cycle and heating this resin achieve complete polymerisation, which creates the stack-up, two conductive layers will be sticked together. We can use single pre-preg or more pre-pregs, two Cores or one Core and a copper foil can be conductive layers in a multilayer PCB. We use the following pre-preg types, medium and high Tg: 1080 (appr. 80µm), 2116 (appr. 120µm), 7628 (appr. 180µm).:
When our customer does not describe the stack-up because the thickness of each layers do not have influence on operation of the multilayer PCB, we manufacture the following standard stack-up in case of 1,5 mm total thickness, in case of 4-layers and 6-layers PCB:
In these normal cases we create the outer layer from copper foil. The different built-up of cores and copper foils determine the different total thicknesses, of which minimum value is 0,5-0,6 mm.
Blind via and buried via on multilayer PCBs.
The standard stack-up we talked about all had normal via, which crosses all the layers. The multilayers where there are vias connecting only certain layers but not all, we call blind via or buried via.
Blind via: the via connects the outer layer and the inner layer directly next to it.
Buried via: the via connects an inner layer and an other inner layer, which is next to it on the same Core.
These figures illustrate the traditional blind via and buried via layer stack-up in case of total thickness 1.5mm:
In our technology the blind via and buried via is possible between two conductive layers on the same Core. In case of 4 layers printed circuit board we cannot manufacture blind and buried via, for this technology the printed circuit board must be 6-layer printed circuit board.
Hybrid multilayer printed circuit boards
They are consist of the combination of FR4 and the better high frequency parameters (lower DK and DF values) base material. It is typically Rogers RO4003. The type of the Pre-preg between the two Cores is FR4. This figure illustrates a typical hybrid stack-up: