PCB Design

Schematic Diagram

The circuit design drawn in the CAD program is not only the lines on the paper, but also the circuit connection data drawn. When drawn correctly, it contains the parts and their interconnections.

It is important to connect the ends of the pins with "wires", otherwise, they may appear to be connected but not connected. Therefore, it is recommended that the grid and snap to grid always remain open.

A good layout starts with a good components library

Before using tools to draw schematics, you need to have a library containing component symbols and packages and use them in your project. Many people try to find a library on the Internet, and even ask a library on a forum. It must be time-consuming, and you never know what you will get. Usually, the symbols of these libraries are not the largest, and take up space and may not even be approved. I suggest you make your own library.

Spend a lot of time thinking about schematic symbols. It may be good to use the same symbols as in the data sheet. Maybe not. It is recommended to group the pins, and these pins are usually connected together (via wires or other components) or used together, such as data bus pins. It is not good to create a symbol similar to a physical IC pin. The "direction" of the pins should be correct. It can be in, out, passive, power, etc. This may help reveal the fault.

Draw the shape of the part correctly. If you fail to do this, people who have to assemble the circuit board in the future will ask you many questions. Maybe you are this person, and you don't remember your thoughts when you remember your footprints. The direction should be very obvious. A good footprint is very useful.

The placement is very important 

After drawing the schematic, you need to create a circuit board file. This is usually easy. Just click an icon to enter the PCB editor.

Firstly,these parts were placed outside the PCB of standard size (for example: 100mm x 160mm). These placements usually don't make any sense . The thin lines between the pins are called rubber bands. They are connections created by drawing schematics. The layout is to create appropriate copper traces on the top and bottom layers (or some inner layers) of these rubber bands. But first, the parts must be placed in a meaningful way.

The time spent in a decent location is usually not wasted. There are several standards that must be met, and sometimes contradict each other.

Generally, some connectors may have to be on one side of the board, otherwise the electrolytic capacitors should not be too close to the heat sink, as they tend to dry out and start to lose their capacity. Sometimes the track must be as short as possible, etc.

For more complex and difficult pcb circuits, it is best to place the components in groups first, and then move these groups to the board in the second step. After that, you can further modify the placement. Sometimes, there are similar groups in the circuit, such as several equal input channels. It also makes sense to place them equally.

Also it is a good practice is to connect the power supply voltage and ground to a wider rail. Both, especially grounding, should be low impedance. Grounding is a reference, and a significant voltage drop on ground may cause malfunctions. This is the reason why it is a good habit to route GND and VCC (or other power supply potentials) before other traces. as the picture shows.

Layout Optimization

There is not much difference in the optimized layout. In any case, this is a very simple circuit. Using diagonal lines will free up some space, which can also be used for other shorter tracks. The main reason is to modify the VCC track here. One audio track (the highest VCC path) is torn apart and routed again.

SMD means "Surface Mount Device".

Today, those are the preferred parts for industrial assembly. The advantages are obvious:

SMD components can be placed by machine
SMD components are smaller
Before placing the parts, use the stencil to screen print the solder
Soldering is carried out in a reflow oven or gas phase oven

Fiducials

The SMD assembly machine uses the camera reference circuit board. The benchmark lightweight structure is easy to find.
Larger fine-pitch ICs should also have two reference points near them.

Gerber files

Generally, PCB manufacturers do not accept CAD design files (some). After the layout is completed and checked, a Gerber file must be generated.

Gerber files contain drawing commands for standard plotters. These photo plotters can "draw" the film used in PCB production. There is one film for each layer, so there is one Gerber file for each layer. The preferred Gerber format is RS-274X. There are more advanced data formats, these formats are gradually established in PCB production, but not all CAD software can use them, and by 2020, Gerber is still very popular.
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