Printed circuit board can have single multiple layers of dielectric and conductive materials. These layers when attached with each other, they carry electrical circuits which sometimes linked with each other. According to applications like home electronics, alarm clocks, other household appliances, computers and mobiles. Printed circuit board has also extended its applications with industrial tools and machinery, in most of the medical devices, storage systems and aerospace industry. The number of layers and its dimensions determined by printed circuit board power distribution design.
It is often a question in users minds that how many layers will be sufficient, should they go with two layer PCB or three layer PCB? The functional needs and the economy of the application decide that how many layers one can employ? More emphasis on hindrance on PCB wiring after pre-positioning of devices. With proper tools it is required to find out the wiring density of the printed circuit board. It is also required to identify that what kind of signals are going to travel through this and then specific wiring demand will come into picture, whether it should be differential lines, sensitive signal lines etc. A term known as pin density will be explained later on, it plays very important role in terms of deciding the number of layers in printed circuit boards. Five factors are there which are required to be considered before selection of number of layers:
- 1) Applications: It is very important to identify the purpose. Types of machines, complexity of the electronics circuit and power demand.
- 2) Operation frequency:It is also important to determine the operation frequency of the application. Operation frequency of any application indicates the function and the capacity of the printed circuit board. For higher processing speed and capacity a multiple layer printed circuit board is required.
- 3) Economy:Manufacturing cost is another parameter in this list. As the layers are going to increase, so will the cost. If applications required a higher processing speed and capacity then the cost will also escalate.
- 4) Manufacturing time:this talk about the lead time, that means time to manufacture single or multiple layer printed circuit board. This lead time effect all above mentioned parameters.
- 5) Density:The density will also decide the selection of layers. There is another term which is known as pin density. If the pin density is 1, it suggests that printed circuit board must be of single layer. As pin density decreases the number of layers increases. For example; if the value of pin density is 0.2, there is need of at least ten layers in printed circuit board.
Single layer PCB is a lamellar structure. One layer of dielectric (soldered) and a conductive material are laminated. Single layer printed circuit boards are still in vogue over a range of applications. It is quite simple in construction compare to other multilayer Printed circuit board. It consists of one thermally conductive dielectric layer which is laminated with copper firstly and then with a soldermask. The single layer PCB is always shown with three strips of color. In which gray represents dielectric layer, brown for copper and green to indicate the soldermask.
Since the design is very simple, the single layer printed circuit board are very cos-effective. Since it is limited technological options, but it still provides the following benefits:
- 1- Design is quite easy to understand
- 2- Less for complicated compare to all other Printed circuit boards in terms of production
- 3- Cost-effective in comparison.
Single layer printed circuit board are nowadays are produced with copper lamination which has a thickness for 1 to 20 ounce. And the temperature they can withstand with is of order 130 to 230oC. In earlier decades single layer PCB’s were used in many devices. Such developments were taken place because of the capabilities afforded by the printed circuit board, to pull-off many complicated tasks, many of them were remote.
Before the design of multi-layer printed circuit boards, first concern is of deciding the circuit board structure and scale of circuit and the size of circuit and the most important is requirement of capability. It suggests that designer has to decided how many layers are required is it 2,4 or 6 or more layer of circuit boards. If the printed circuit boards demand for high density BGA circuit, then minimum number of layers required for this purpose should be considered carefully. It is not the only the cost factors which decides that how many number of layers should be there, and people often confuse that lesser the number of layer lower would be the cost. But it will get compensated somewhere else, or by other means. In past few years the cost of multi-layers printed circuit boards has been reduced considerably, because of their higher demand for various applications. At designing stage when decision of number of layer is done, the next step is the placement of layer and how these signals will travel ahead in the circuit.
PCB stack up principle:
After the required numbers of layers are determined, the following tasks are required to be completed carefully. So that all circuits are arranged reasonably. There are two required factors:
- Special signal layers distribution.
- Proper distribution of power layer and ground layer.
There could be more possible arrangements for ground layer, power layer and special signal layer as the number of layers. But there are certain principles to make these arrangements of layers:
- 1- The special signal layer should be next to the power layer. Shielded with copper power layer.
- 2- Two avoid the direct contact between two signal layers, a ground layer can be placed between them.
- 3- Multiple ground layer and power layers can reduce impedance effectively.
- 4- The symmetry of layer structure.
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