As a preliminary step of PCB manufacturing process, PCB layout is one of the most significant phases in PCB design since its quality essentially determines that of PCB routing, which further affects the final reliability and functionality of PCBs. Therefore, it can be concluded that reasonable PCB layout paves the way for high-quality PCB boards. Unreasonable PCB layout, however, may lead to issues in terms of functionality and reliability. Well-designed PCB layout will bring forward more conveniences that it not only saves real estate of PCB surface but guarantees the performance of circuit as well.
PCB layout primarily comes in two types, interactive layout and automatic layout. Generally speaking, automatic layout accounts for the framework based on which adjustment will be carried out by interactive layout. During PCB layout, redistribution can be implemented on gate circuit according to the specific situation of routing. Two gate circuits are exchanged, which will then become optimal layout that is most user-friendly for routing.
After the completion of PCB layout, some information can be labeled on PCB design files or schematics so that concerning information or data on PCB is consistent with that depicted in schematics. As a result, synchronous change can be maintained in both profiling and modification of PCB design. Furthermore, updating is made on analog data and board-level verification can be implemented on electrical performance and functions.
Basically speaking, PCB layout should conform to two fundamental rules:
1). PCB layout should ensure high quality.
2). PCB layout should look neat and clear in appearance, which allows component evenly laid on board surface.
Once a product performs agreeably in terms of the two aspects mentioned above, it can be regarded as perfect.
Guideline#1. Loop should be as short as possible.
Loops, especially high-frequency loop, should be as short as possible. Small loops usually feature lower inductance and resistance and can help to cut down signal count coupled into node deriving from external source or transmitted by node. Inductance can be lowered if loop is located on ground plane. You can also keep loop of Op-amp circuit as short as possible to prevent noise coupled into circuit.
Guideline#2. Thermal via should be appropriately located.
Vias transfers heat from one end of PCB to the other side, which is especially useful when board is mounted on heat sink on the chassis. Under such condition, chassis will further dissipate heat. Big vias perform better than small vias on thermal dissipation efficiency. Multiple vias perform more efficiently than single vias in terms of thermal dissipation and reduce components' operating temperature. Lower operating temperature leads to higher reliability.
Guideline#3. Via size and count should be reasonably arranged.
Vias feature both inductance and resistance. If you plan to arrange routing from one end of PCB board to the other end and call for relatively low inductance or resistance, multiple vias can be relied on. Large vias feature lower resistance. This method works especially usefully when filter capacitor and high-current node are connected to the ground.
Guideline#4. Take care to heat sensitive components.
Heat sensitive components should be located far away from components generating heat. Heat sensitive components include thermocouple and electrolytic capacitor. Temperature measurement will possibly be affected when thermocouple is located near heat source. Electrolytic capacitor will suffer from a reduced operating life when electrolytic capacitor is located near components generating heat. Heat-generating components possibly include diodes, inductor, diodes, bridge rectifier, MOSFET and resistor whose generated heat depends on current flowing through them.
Guideline#5. Decoupling capacitor should be carefully located.
Decoupling capacitor should be located near IC power or ground pins in order to maximize decoupling efficiency. Stray capacitance will be caused when capacitor is placed at a far place. Multiple vias should be arranged between capacitor pins and ground plane so that inductance can be reduced.
Guideline#6. Thermal pad should be smartly located.
Thermal pad setting aims to make the distance as small as possible between traces or fill and component pins, which is beneficial for soldering. Small connection is short when it comes to resistance reduction. Once thermal pads on components pins are not applied, the temperature of components will be lower. A better thermal connection is available connecting traces or fill, which helps thermal dissipation. However, it is more difficult to solder or desolder.
Guideline#7. Digital and noise traces should be away from analog circuits.
Parallel traces or conductors can lead to the generation of capacitance. Signals tend to be coupled on circuits when traces are located too near to each other, which is especially true for a relatively high frequency. High-frequency and noise traces should be far away from those that you don't want to be disturbed by noise.
Guideline#8. Distance between traces and mounting via should be appropriately arranged.
Sufficient space should be maintained between copper traces or fill and mounting vias in order to prevent shock hazard. Solder mask isn't a reliable inductor so sufficient distance should be also kept between copper and any mounting hardware.
Guideline#9. Ground may be dangerous if you pay little attention to it in PCB layout.
Ground isn't an ideal conductor so care should be taken when placing noise ground away from quiet signals. Ground traces should be large enough in order to carry flowing current. Placing a ground plane under signal traces can help reducing trace impedance, which is an ideal condition.
Guideline#10. PCB board should be regarded as a heat sink.
More copper should be placed around surface mount components so that extra surface area can be provided to dissipate heat, which is a method bringing forward higher efficiency. Similar guidelines are even mentioned in datasheets of some components.
Once PCB layout is completed, before moving forward to the next step, please carefully examine your PCB layout based on the following tips.
1). Board size should be checked to make sure whether it's compatible with that depicted in schematics or PCB manufacturing technique requirement and whether there are fiducial marks.
2). Components should be guaranteed that there's no conflict in two-dimensional and three-dimensional space.
3). Components should be checked to ensure that all components are neatly and evenly distributed.
4). Components that require consequent replacement should be examined to make sure they are accessible to replacement or modification.
5). Sufficient distance has been maintained between heat sensitive components and heat generating components.
6). Adjustable components have to be guaranteed to be conveniently adjusted.
7). Thermal dissipation area should contain heat sink and feature smooth air flow.
8). Signal flow should be smooth and interconnection should be as short as possible.
