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Some basic concepts of PCB design

Posted:02:17 PM May 14, 2018 writer: G

1. Layer

Protel's "layer" is not virtual, but is the real copper foil layer of the printing plate material itself. Nowadays, components of electronic circuits are densely installed. With special requirements such as anti-interference and wiring, printed boards used in some newer electronic products not only have upper and lower sides for wiring, but also have interlayer copper foils that can be specially processed in the middle of the board, for example, now used by computer motherboards. The printing plate material is more than 4 layers. These layers are mostly used for setting the power wiring layers (such as Ground Dever and Power Dever in software) that are relatively difficult to process because they are relatively difficult to process, and are often used for wiring with large area filling methods (such as ExternaI P1a11e and Fill in software). ). The surface layer in the upper and lower positions and the intermediate layers need to communicate with each other using the so-called "Via" mentioned in the software. With the above explanation, it is not difficult to understand the concept of "multilayer pad" and "wiring layer setting". To give a simple example, many people complete the wiring, only to find out that many terminals have no pads when printing out. In fact, this is the concept of “layer” that is ignored when adding device libraries. It does not draw the package itself. The pad characteristics are defined as "Mulii-Layer". It is to be reminded that once the number of layers of the used printing plate is selected, it is necessary to close those unused layers.

2. Vias

In order to connect the lines between the layers, a common hole is drilled in each layer where the wires need to be connected. This is the via hole. In the process, a layer of metal is deposited on the cylindrical surface of the via hole by chemical deposition, so as to connect the copper foils in the intermediate layers, and the upper and lower surfaces of the via hole are formed into a common pad shape. Connected with the upper and lower lines, it may not be connected. In general, the following principles apply to the processing of vias when designing a circuit:

(1) Minimize the use of vias. Once vias are selected, be sure to handle the gaps between them and the surrounding entities, especially the gaps between lines and vias that are easily overlooked and that are not connected to the vias. Automatic routing can be automatically resolved by selecting the “on” item in the “Minimal Via Number” submenu (Via Minimiz8tion).

(2) The larger the required ampacity, the greater the size of the required vias, and the larger vias used for the connection of the power plane and the formation to other layers.

3. Overlay

In order to facilitate the installation and maintenance of the circuit, the required logo patterns and character codes and the like are printed on the upper and lower surfaces of the printed board, such as component labels and nominal values, component outline shapes, and manufacturer's marks, production dates, and the like. When many beginners design the silkscreen layer, they only pay attention to the neat appearance of the text symbols, and ignore the actual PCB effects. On the printed boards they designed, the characters were not blocked by the components or they were invaded by the soldering area, and the components were indexed on the adjacent components. All of these designs would bring about great assembly and maintenance. inconvenient. The correct layout principle of the silk screen layer is: "No ambiguity, no seams, beautiful appearance."

4. Particularities of SMD

Protel package library has a large number of SMD packages, surface mount devices. The biggest feature of this type of device is that it is a single-sided distributed elemental pinhole. Therefore, the surface of the device should be defined so as to avoid "missing pins". In addition, the relevant text labels of such components can only be placed along the surface of the component.

5. External Plane and Fill

As the name of both, the network-like filling area is to treat a large area of copper foil as a mesh, and the filling area is only a complete reserved copper foil. In the process of designing beginners, you often cannot see the difference between the two in the computer. In essence, as long as you enlarge the drawing, you can see it at a glance. It is because it is not easy to see the difference between the two, so the use of time does not pay much attention to the distinction between the two, to emphasize that the former has a strong role in inhibiting high-frequency interference in the circuit characteristics, apply to need to do Areas where large areas are filled are particularly appropriate when certain areas are used as shielding, partitions, or high-current power lines. The latter is often used in places where small filling is required, such as a general line end or a transition area.

6. Pads

Pads are the most common and most important concept in PCB design, but beginners tend to overlook their choices and fixes, and use circular pads in the design. The pad type of the selected component must consider the shape, size, layout, vibration and heat conditions, and force direction of the component. Protel offers a series of pads of different sizes and shapes in the package library, such as circles, squares, octagons, circles, and positioning pads, but sometimes this is not enough and you need to edit it yourself. For example, for a pad that generates heat and is subject to greater force and current, it can be designed as a “teardrop”. In the familiar color TV PCB output transformer pin pad design, many manufacturers are Adopt this form. In general, besides the above, the following principles should be considered when editing the pad by yourself:

(1) When the length of the shape is inconsistent, consider that the difference between the width of the connecting line and the length of the specific side of the pad should not be too large;

(2) The need to use asymmetrical pads of long and short lengths when routing between component lead angles is often more effective;

(3) The size of each component pad hole should be edited and determined according to the component pin thickness. The principle is that the hole size is 0.2-0.4 mm larger than the pin diameter.

7. Various types of Mask

These films are not only indispensable in the PCB manufacturing process, but also the necessary conditions for component soldering. According to the "membrane" position and its role, "film" can be divided into component surface (or welding surface) Top film (Top or Bottom) and component surface (or welding surface) solder mask (Top or Bottom Paste Mask Two types. As the name implies, the flux film is a layer of film that is applied to the pad to improve the solderability, that is, light colored round spots that are slightly larger than the pad on the green board. In the case of solder masks, the opposite is true. In order to adapt the fabricated board to wave soldering and other forms of soldering, it is required that the copper foil on the non-pads of the board cannot be soldered. Therefore, a layer of paint must be applied to all parts other than the solder pads. Used to stop tin on these parts. It can be seen that the two films are complementary. From this discussion, it is not difficult to determine settings for items such as "solder Mask En1argement" in the menu.

8. Flying line

Flying line has two meanings:

In the case of automatic cabling, a network connection similar to a rubber band is used for observation. After the components are transferred to the network table and a preliminary layout is performed, the “Show” command can be used to see the cross-over status of the network connection under the layout and adjust it continuously. The position of the components minimizes this crossover to achieve maximum automatic routing of the wiring. In addition, the end of the automatic routing, which networks have not been deployed, can also be found through this function. Can be used manually compensated, can not really use the "flying line" of the second meaning, is to connect these networks with wires in the future printing plate. To explain is that if the circuit board is a large-volume automatic line production, Think of this flying line as a 0 Ohms resistance element with uniform pad pitch for design

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