multi layer board (MLB)
Due to the large number of contacts that microprocessors and other electronic components have, single- and double-sided printed circuit boards are not sufficient to realize the required packing density. The resulting printed circuits have too many crossing points. For this reason, multilayer printed circuit boards were produced as early as the 1970s, in which the conductors are routed on several layers that are free of crossing points.
These multilayer printed circuit boards are multilayer printed circuit boards (MLB). They are constructed in layers and consist of copper foils, prepregs and carrier material. The layer structure of multilayer printed circuit boards is symmetrical with respect to the inner layers, the prepregs and the copper layers. The inner layers consist of carrier materials and have standard material thicknesses between 0.10 mm and 1.20 mm. There are usually two prepregs between the individual layers.
Thicker prepregs provide greater stability as far as the copper layers on the inner layers are concerned. Thicker copper requires more resin-rich prepreg to fill the copper areas that have been etched away. In addition to the multilayer PCBs described, HDI technology, High Density Interconnect, and for Surface Mounted Technology( SMT) are available and can consist of 4 to 10 individual layers. Up to 50 layers are feasible. The connection between the individual layers is made with vias or microvias. Such microvias can connect the outer layers with each other as Plated Through Hole( PTH), but they can also connect inner layers with each other or these with the outer layers.
On the inner layers of multilayer PCBs, passive and active components can be embedded using Embedded Component Technology( ECT). Thin film resistors, capacitors, transistors, striplines and also chips can be accommodated between the layers.
Multilayer PCBs there are still the Ultra Thin Multilayers( UTM), which are less than half as thick as the normal multilayer PCBs.