This sol- der only connects the components to the solder pads. When making only one or two boards with a simple design, a solder mask may not be needed. Addition- ally, the solder mask adds corrosion protection to the printed circuit board traces.
In theory, with only through-hole passive compo- nents and discrete transistors, a single layer is all that is necessary . Most radio-frequency (RF) printed circuit boards have traces on only one side, although a second layer is used as a ground plane to improve performance. Standard integrated circuit pin outs used with analog ICs—such as op-amps, timers, and so on—can get by with a single layer. This is because there is plenty of room for traces to pass between the leads of the pas- sive components. When digital logic or other designs that use high- pin-count chips are used, two trace layers are required. Digital IC chips with a large number of pins are impos- sible to connect to other chips with a different layout without crossing and shorting the traces. Therefore, a second layer is required. In theory, only two layers are ever necessary with high-pin-count components.
In practice, more than two layers are used for dense, highly populated boards or for performance considerations. For high-speed or high-precision designs, more than two layers are used to improve performance. With four trace layers, the two middle ones are usually power and ground. This evens out the flow of power and reduces voltage fluctuations at different points on the printed circuit board. The ground plane layer provides controlled impedance for high-speed signals. The close proximity of the ground and power planes also acts as a capacitor to reduce noise. One problem with multilayer printed circuit boards is the reliability of connections between the layers. It is sometimes dif- ficult to detect which layer is defective. Two sides can be etched at the same time; there- fore, the cost of a double-sided PCB is only slightly higher than that of a single-sided PCB . Multilayer boards are built up by laminating layers together. A four-layer board costs about twice as much as a dou- ble-sided printed circuit board. The printed circuit board is often the most expensive part of a design.
LAYOUT OF PCB Once the decision to design a printed circuit board has been made, a number of questions must be ad- dressed before the printed circuit board is laid out: 1. What is the power source? Will it be part of the board design or external to it? 2. What oddly shaped or sized devices will be mounted? Devices such as relays, switches, and large capacitors must be identified.
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- Winter '18
- Printed circuit board, Printed circuit boards