We talked about the basic material used to start the PCB manufacturing process before，next PCB multilayer fabrication will be discussed.The outer layers of our multilayer consist of sheets of glass cloth pre-impregnated with uncured epoxy resin (prepreg) and a thin copper foil.
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 lay-up operator has already placed a copper foil and 2 sheets of prepreg on the heavy steel baseplate. Now he places the pre-treated core carefully over the alignment pins. Then he adds 2 more sheets of prepreg, another copper foil and an aluminium press plate
He builds up 3 panels on the baseplate in the same way. Then he rolls the heavy stack under a press which lowers down the steel top plate. He pins the stack together and rolls the finished stack out of the clean room into a rack.
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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.
The press operator collects 3 stacks on a loader and loads them into the bonding press. This press uses heated press plates and pressure to bond the layers of the PCB together. The heat melts and cures the epoxy resin in the prepreg while the pressure bonds the PCB together. The process is computer controlled to build up the heat and the pressure correctly, hold it and then to cool the press down. In this way we ensure a permanent bond that will last the lifetime of the PCB. Our board has 4 layers but complex PCBs for defence, avionic and telecommunications applications can have more than 50. These may include sub-assemblies of cores, prepregs and foils drilled and plated before being assembled into the final PCB.