With the complexity of electronic circuits increasing, it is not always possible to provide all the connectivity that is required using just the two sides of the PCB. This occurs quite commonly when dense microprocessor and other similar boards are being designed. When this is the case multilayer boards are required.
The manufacture of multi-layer printed circuit boards, although it uses the same processes as for single layer boards, requires a considerably greater degree of accuracy and manufacturing process control.
The boards are made by using much thinner individual boards, one for each layer, and these are then bonded together to produce the overall PCB. As the number of layers increases, so the individual boards must become thinner to prevent the finished PCB from becoming too thick. Additionally the registration between the layers must be very accurate to ensure that any holes line up.
To bond the different layers together the board is heated to cure the bonding material. This can lead to some problems of warp. Large multi-layer boards can have a distinct warp on them if they are not designed correctly. This can occur particularly if, for example one of the inner layers is a power plane or a ground plane. While this in itself is fine, if some reasonably significant areas have to be left free of copper. This can set up strains within the PCB that can lead to warping.
