PCB design is both a science and an art. There are a lot of technical specifications about routing line widths, routing stack-ups, schematics, etc., but when you come to the layout of artistic components in PCB design, the problem becomes interesting.
In fact, there are few restrictions on the placement of components, and there is no "absolutely correct" specification requirement, which also makes beginner electronic engineers like a full "secondary" when they arrange components on the circuit board, yearning for Personal ambition and creativity, how to place it is entirely up to you and your design ideas.
But this does not mean that you can do whatever you want. The design in the computer still needs to fall into the dust and form something specific. Therefore, the following ten suggestions for placing PCB components can guide electronic beginners to complete the initial stage of electronic design smoothly.
Why is component placement so important?
There is an old saying that 90% of PCB design is device layout and 10% is wiring. This is indeed a big truth. Beginning to put effort into placing components carefully can do more with less, and it can also improve the electrical characteristics of the PCB. What happens if you just randomly place components on the circuit board?
Time wasted: It is very likely that during the wiring process, you will find that there is not enough space for wiring in some places, or even the entire wiring needs to be overturned.
The board doesn't work: You'd think you'd be good to go by placing the device and routing all the leads. Send the design files to the board manufacturer and wait a few days to receive a brand new board. When I was about to happily solder the circuit, I was slapped in the face by reality and found that some components could not be soldered at all (either the packaging was wrong, or they were in conflict with each other).
Aesthetics are broken: we must admit that even if we are humble engineers, we still agree with symmetry and detail in the pursuit of aesthetics. The kind of circuit board that was initially placed by people who lack love and carelessness will make people feel more blocked in the later welding and debugging process.
If you run into a bully, you will argue with you about where the components should be placed. With the same circuit diagram, 100 electronic engineers will have 1,000 wiring schemes. This is why designing circuit boards is more of an artistic process.
An elegant, artistic, carefully designed circuit board is easy to catch the eye. Symmetrical, tidy, and the device placement is so beautiful!
If you're always trying to figure out what's right and what's wrong, here's a little trick to help you judge whether your circuit board components are properly placed. After the components are placed, use the automatic routing in the circuit board design software (such as provided in Autodesk EAGLE) to route. If the final circuit failure rate is less than 85%, it means that you need to spend more time optimizing the components. placed.
Let's talk a little more about device placement.
1. Understand the physical limitations of the circuit board
Before placing components, you first need to know exactly the mounting holes of the board, the location of the edge connectors, and the mechanical size constraints of the board. why?
Because these factors affect the size and shape of your board. I have seen a circuit board designed by a certain person that cannot fit in the board mounting area and had to be redesigned.
In order to avoid being silly, you can intentionally set a clear area for those mechanical constraints (mounting holes, circuit outlines), so that you can rest assured that you can create within the allowable range.
Quadcopters, or other flexible wearable circuits often require the board to have an interesting look. This is the appearance design of a signal receiving circuit used on a mini quadrotor
2. Understand the circuit board manufacturing process
Again, before placing circuit components, you'd better get a few key pieces of information from the circuit manufacturer:
Circuit assembly process and testing process; whether to reserve space for PCB V-grooving; component soldering process: wave soldering, partition soldering or manual soldering?
The circuit board manufacturing process will affect the gap size requirements between components. Also, if your board will be soldered on the assembly line in the future, you will need to leave extra space (greater than 20mil) on the edge of the board for the board to be attached to the conveyor belt. Additional mounting plate on the board, which is broken off after the board is soldered.
3. Leave breathing room for integrated chips
Note that the "gasp" mentioned here is not air. As for what it is, you will be clear after reading the following. When arranging any components, it is necessary to leave a distance of at least 350mil between them as much as possible. For chips with many pins, the space needs to be larger. Why?
Now the chip pins are more and more dense. If the integrated chips are too close together, there is a high chance that their leads cannot be routed easily. It is often the more difficult the wiring is later, and sometimes it will consume 100 hairs of your hair if you bother to route a single wire, and even reach the predicament of being called every day and not working. (I knew this earlier, why did it happen in the first place)
See, this BGA packaged chip is so densely pinned. If you don't leave enough space around it, you can look good when wiring
4. The same device direction is the same
For the same devices, line them up as much as possible to maintain a consistent formation. Do you have obsessive-compulsive disorder? This is mainly to facilitate the assembly, inspection and testing of the circuit board in the later stage, especially for the surface-mounted devices in the wave soldering process, the circuit board passes through the melting solder wave at a uniform speed. Evenly placed devices have a uniform heating process, which can ensure high consistency of solder joints.
The following example shows that evenly placed components are suitable for a uniform speed wave soldering process. For example, the components are placed evenly, and the direction of the components is consistent, which is suitable for the uniform wave soldering process.
If the following messy arrangement is followed, the quality of the pads of different devices will be different for wave soldering. Especially when some small devices are located between large devices, a "shadow" effect will be formed, and the small devices may be poorly soldered. .
Example of messy placement of components: components with different orientations are prone to bad solder joints when wave soldering.
5. Reduce lead crossing
Reduce lead crossing by adjusting device position and orientation. What do you do?
Many PCB design software now provide a function that shows the connection relationship between pin pairs that are not routed. For example, the following figure shows the connection relationship between all the device pins in the schematic diagram, which is represented by a thin gray straight line in the figure (this kind of line is called ratsnest:y flying line, pre-drawn line).
Displaying the PCB interface of the flying leads, by changing the position and direction of the device, minimizes the crossover of the leads between the devices, which can save a lot of energy for the subsequent wiring.
6. Place circuit edge devices first
Devices that cannot be moved arbitrarily due to mechanical constraints should be placed first, such as external connectors, switches, USB ports, etc. on the circuit board. Why?
These devices are often located in the overall mechanical design of the system and cannot be changed. After placing these devices, it also allows you to arrange a glorious starting point in the rear device. Get the edge components of the board, and the rest is the highlight of your imagination and creativity.
The positions of peripheral interface devices on the computer motherboard are closely related to the chassis design, and their positions need to be determined in advance.
7. Avoid conflict between devices
Absolutely avoid overlapping device pads for routing in small boards, or overlapping device edges. It is best to maintain a distance of 40mil (1mm) between all devices. Do you have an intensive phobia?
The most important reason is to avoid short circuit faults between pads in the subsequent circuit fabrication process. Don't forget, tight placement also makes wiring more difficult. Likewise, avoid too dense when placing vias. These small round holes may also expose the copper skin in the future, resulting in a short circuit of the circuit.
8. Put the devices on the same side as much as possible
If you are designing a two-layer board, the most common recommendation is to place the components on the same side. why?
Not placing components on the same side of the board can make post-board fabrication laborious. Let me tell you the reason below. Usually, the components on the circuit board are completed by the automatic device placement machine. The components are only on one side, and the PCB production process only needs one pass. Otherwise, two device placements are required. Wasting production time is a waste of money and life.
9. Keep the chip pin and device polarity consistent
Each integrated chip has a flag giving the starting position of pin 1. The orientation of pin 1 of the chip, or the direction of polar devices (motor capacitors, diodes, triodes, LEDs, etc.) is consistent, which will also bring convenience to circuit board fabrication. Is this necessary?
If you've soldered or debugged the board yourself, you won't doubt this. Think about it, when you solder the polarity and orientation of the components on the circuit board is very messy, do you have no confidence in the successful soldering of the circuit board?
Usually there will be a small dot on the integrated chip package to indicate the position of pin 1. Keep all chips in the same orientation for easy soldering and inspection
10. The device location is similar to the schematic
When placing components, place them in your mind according to the positional relationship on your schematic. What is the reason?
In fact, when you design the schematic diagram, you have already optimized the positional relationship between the components (the shortest connection and the least crossover), so it is natural to follow the position of the components on the schematic diagram until the placement of the PCB components. . In particular, in the later manual wiring, the schematic diagram in your mind will secretly help you choose a reasonable short path for wiring.
The best PCB design originates from the extraordinary layout of the device, don't be fooled easily. You have to keep focusing on the reasonable placement of components. All the efforts in this process are worth it. This may be the most worthy process in PCB design. When you see the design PCB turned into a finished circuit board, it will be a happy time to taste the fruits of labor.
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