Difficulty Factors of Multilayer PCB boards

By definition, a multilayer circuit board has three or more circuit layers. Once, the main application of multilayer circuit board is limited to advanced industrial electronic products. However, they are now used in all mainstream electronic devices, including consumer products such as camcorders, mobile phones, and audio CD-ROMs.

Multilayer PCB’s represent one of the most complicates products in PCB industry. In order to design producible multilayer PCB’s, it is imperative that PCB designers understand the processes and difficulty factors

1) Layer count
As personal computers and workstations become increasingly powerful, mainframe computers and supercomputers are being replaced by these miniaturized machines in many applications. This is the result of the use of a highly complex multilayer PCB, which has more than 70 layers and, although its current use is decreasing, its production technology has been proven. Thin and high density multilayers at the other end of the layer range are mainstreams.

2) Build-up
The more layers in a multilayer PCB, the greater risk of misregistration during lamination process; the rejection rate, therefore, increase with an increasing layer counts.

3) Core thickness of rigid inner laminates
Inner layers are normally based on thin rigid laminate cores with copper cladding. The thinner the rigid laminates, the more difficult they are to handle, for example, in the lamination process, etching process etc. Therefore the rejection rate increases with decreasing thickness of the rigid laminated.

4) Thickness tolerance
When measuring the thickness of a multilayer board, it is common practice to include the copper foils of the outer layers. As a PCB user, we should measure the thickness across a clean-etched laminate, and add the nominal thickness of the copper foils of the outer layers. The rejection rate depends very much upon the thickness tolerance required, and indirectly upon the layer build-up. When thickness tolerance T=±10%, obviously rejection ratio trends to zero.