The Explanation and Development of Printed Circuit Board

Modern printed circuit board technology has led to a considerable reduction in conductor crossovers on different planes, resulting in a reduction in space requirements and increased packaging density of electronic components. However, the modern VLSI and other multi-pin configuration devices have tremendously increased the packaging density and consequently the concentration of inter-connecting lines.

This has given rise to complex design problems such as noise, cross-talk, capacitance and unacceptable voltage drops due to parallel signal lines. These problems could not be satisfactorily solved in single-sided or double-sided boards, thereby necessitating an extension to the multi-layer circuit board fabrication.

Multi-layer printed circuit board is, therefore, used in situations where the density of connections needed is very high to be handled by two layers or where there are other reasons such as accurate control of line impedance or for earth screening. The multi-layer board makes use of more than two printed circuit boards with a thin layer of what is known as 'prepreg' material placed between each layer, thus making a sandwich assembly. The printed circuit on the top board is similar to a conventional printed circuit board assembly except that the components are placed much closer to avoid having many terminals, which necessitates the use of additional board layers for the required interconnections. The electrical circuit is completed by interconnecting the different layers with plated through-holes, placed transverse to the board at appropriate places. Multi-layer boards have three or more circuit layers, while some boards have even more than 50 layers.

Multi-layer printed wiring has facilitated a reduction in the weight and volume of the interconnections commensurate with the size and weight of the components it interconnects.

The following areas of application necessitate the use of multi-layer printed wiring arrangements:

1) Wherever weight and volume savings in interconnections are the overriding considerations, as in military and air-borne missile and space applications;

2) When the complexity of interconnection in sub-systems requires complicated and expensive wiring or harnessing;

3) When frequency requirements call for careful control and uniformity of conductor wave impedance with minimum distortions and signal propagation, and where the uniformity of these characteristics from board-to-board is important;

Because of the developments in mass lamination technology, four-layer boards and even six-layer boards can be made with almost the same ease as double-sided boards. With the improvement in reliability and reduction in cost of printed circuit boards, the use of multi-layer boards is no longer limited to only high technology products, but has spread to some of the most common applications like entertainment electronics and the toy industry.