PCB design 90% in the device layout, 10% in the wiring, this is indeed a big truth. Begin to take great pains to finely place the device can be twice as effective, but also to improve the electrical characteristics of the PCB.
Before placing the components, you first need to know exactly where the board's mounting holes, edge connectors, and the board's mechanical size limitations.
Because these factors affect the size and shape of your circuit board. I have seen a certain design of the board can not fit into the board fixed area, had to redesign.
To avoid making silly, you can intentionally set a clear zone for those mechanical restrictions (mounting holes, outer contour of the circuit), so you can rest assured that you can create within the allowed range.
Again, before placing circuit components, you would do well to clarify several key pieces of information from the circuit manufacturer.
1. the circuit assembly process and testing procedures.
2. the need to reserve space for PCB V-slotting.
3. the component soldering process: wave soldering, partition soldering or hand soldering?
Circuit board production process will affect the gap size between components needs. Also, if your board will be soldered on the assembly line in the future, you will need to leave additional space (greater than 20 mil) at the edge of the board for the board to be fixed on the conveyor belt. The extra fixing plate on the circuit board, it is broken off after the board is soldered.
Note that the "air" here is not air, as to what, read the following you will know .
In the layout of any components, you need to leave as much as possible between them at least 350mil distance, for more pins of the chip, the space left need to be larger.
Nowadays, the more pins of the chip turns out to be more and more dense. If the integrated chips are too close to each other, there is a high probability that their leads will not be easily led out of the wiring. Often the later the wiring is more difficult, and sometimes the effort to lay through a line will consume 100 of your hair, even to the point of calling the day not to answer, calling the ground not to work dilemma. (Had I known, why bother at the beginning)
For the same device as far as possible, let them line up and maintain a consistent formation.
This is mainly to facilitate the assembly, inspection and testing of the board later, especially for the surface package devices in the wave soldering process, the board evenly after melting the solder wave. Uniform placement of the device heating process is uniform and can ensure high consistency of the solder joint.
Reduce lead crossing by adjusting the device position and orientation.
Many PCB design software now provides a function that shows the connection between pairs of pins without a cloth pass. By changing the position and orientation of the device to minimize lead crossings between devices, you can save a lot of effort for the later wiring.
The devices that cannot be moved arbitrarily due to mechanical constraints should be placed first, such as external connectors, switches, USB ports, etc. on the board.
These devices are often the overall mechanical design of the system to determine the location, can not be changed. After placing these devices, it also makes you have a glorious starting point for the arrangement of devices in the back. Fix the edge of the board devices, the rest is your imagination and creativity of the high light of the moment.
Absolutely avoid overlapping the pads of devices to share in order to route a small board, or make the edge of the device overlap. It is best to maintain a distance of 40 mil (1mm) between all devices.
The most important reason for this is to avoid short-circuit faults between pads in the subsequent circuit fabrication process. Don't forget that tight placement can also make wiring more difficult.
Likewise, avoid too dense when placing the vias. These small round holes may also expose the copper skin in the future, causing a short circuit.
If you design a two-layer circuit board, the most common advice is to place the device on the same side.
If you do not put the device on the same side of the board, it will make the later circuit board production laborious and difficult. The following tells you why, usually, the devices on the board is done by automatic device placement machine, the device is only on one side, the production of PCB process only need to be done once. Otherwise, it takes two device placements. A waste of production time is a waste of money and life.
Each integrated chip has a sign giving the starting position of pin 1. For the orientation of the chip where pin 1 is located, or have polarity of the device (motor capacitors, diodes, transistors, LEDs, etc.) direction to maintain consistent, will also bring convenience to the board production.
If you have personally soldered or debugged the board, you will not doubt this. Think about it, when you solder the components on the board polarity and direction is very messy, is not for the successful welding of the board you have no bottom in mind?
In the placement of components, the brain in accordance with the relationship between the location of your schematic to place.
In fact, you have optimized the relationship between the location of the device when designing the schematic (the shortest connection, the least cross), so it is, according to the schematic device location until the PCB device placement has a natural rationality. Especially, in the later manual wiring, the schematic in your head will secretly help you choose a reasonable short path to wiring.
Constraint under the creation
The best PCB design originates from the extraordinary layout of the device, do not easily muddle through. You should always insist on focusing on the reasonable placement of devices, the process is worth all the effort, which is perhaps the most worthy of your full effort in PCB design. When you see the design PCB into the finished circuit board, it will be a happy time to taste the fruits of your labor.
