PCB Tech - Basic Concepts of PCB Design

Basic Concepts of PCB Design
1. The concept of "Layer"
Similar to the concept of "layer" introduced in word processing or many other software to realize the nesting and synthesis of graphics, text, colors, etc., Protel's "layer" is not virtual, but the printed board material itself is real. The actual layers of copper foil. Nowadays, because of the dense devices of electronic circuits. Special requirements such as interference prevention and wiring. The printed boards used in some newer electronic products not only have upper and lower sides for routing, but also have a laminated copper foil that can be specially processed in the center of the board. For example, the current accounting machine Most of the printed board materials used by the main board are more than 4 layers. Because these layers are relatively difficult to process, they are mostly used to set up the power wiring layers with simpler wiring (such as Ground Dever and Power Dever in the software), and often use large-area filling methods for wiring (such as ExternaI P1a11e and Fill in the software). ). The upper and lower outer surface layers and the central layers need to communicate with the so-called "vias" mentioned in the software. With the above explanation, it is not difficult to understand the related concepts of "multi-layer pad" and "wiring layer setting". To give a simple example, many people have completed the wiring, and when it was printed out, they found that many of the connected terminals did not have pads. In fact, this is to add the equipment library by themselves.

I ignored the concept of "layer", and did not define the characteristics of the pads that I made packaged as "multi-layer (Mulii-Layer). It should be reminded that once the number of layers of the printed board used is selected, it must be closed Those unused layers, avoid making detours.

2, via (Via)

is the line connecting the layers, and a common hole is drilled at the Wenhui of the wires that need to be connected on each layer, which is a via. In the process, a layer of metal is plated on the cylindrical surface of the hole wall of the via by chemical deposition to connect the copper foils of the central layers that need to be connected, and the upper and lower sides of the via are made into ordinary pad shapes. It is directly connected to the upper and lower sides of the line, or not connected. Generally speaking, there are the following guidelines for the treatment of vias when planning lines: (1) Use as little as possible

Hole, once a via is selected, be sure to handle the gap between it and the surrounding entities, especially the gap between the lines and the vias that are simply ignored in the center layers and the vias. If it is active wiring, you can Select the "on" item in the Via Minimiz8tion submenu to take the initiative to solve it. (2) The larger the current-carrying capacity required, the larger the size of the required vias. For example, the vias used to connect the power layer and ground layer to other layers will be larger.

3, silk screen layer (Overlay)

In order to facilitate the installation and repair of the circuit, the required logo patterns and text codes are printed on the upper and lower surfaces of the printed board, such as component label and nominal value, component outline shape and manufacturer logo, production date, etc. When planning the content of the silk screen layer, many beginners only pay attention to the neat and beautiful placement of the text symbols, ignoring the PCB effect produced by practice. On the printed board they planned, the characters were either blocked by the component or invaded the soldering area and wiped off. Some of the components were marked on the adjacent components. Such various planning will bring great difficulties to assembly and repair. Great inconvenience. The correct guidelines for the layout of characters on the silk screen layer are: "No ambiguity, beautiful and generous".

4, the characteristic of SMD

There are many SMD packages in the Protel package library, that is, external soldering equipment. In addition to the compact size, the biggest feature of this type of equipment is the single-sided dispersion of pin holes. Therefore, when choosing this type of equipment, you must define the equipment location and avoid "missing pins (Missing Plns)". In addition, the relevant text labels of this type of component can only be placed along with the component location.

5, grid-like filling area (External Plane) and filling area (Fill)

Just like the names of the two, the network-shaped filling area is to process a large area of copper foil into a net shape, and the filling area only keeps the copper foil intact. In the process of planning for beginners, the difference between the two is often not seen on the accounting machine. In fact, you only need to zoom in and see at a glance. It is precisely because it is usually not easy to see the difference between the two, so when using it, it is even more careless to distinguish between the two. It should be emphasized that the former has a strong effect of suppressing high-frequency interference in circuit characteristics and is suitable for needs. Large-area filled areas are especially suitable when certain areas are used as shielding areas, cutting areas, or high-current power lines. The latter is mostly used in general line ends or turning areas where a small area is required to be filled.

6. Pad

The pad is the most frequently touched and most important concept in PCB planning, but beginners simply ignore its selection and correction, and use circular pads in the planning. The selection of the pad type of the component should comprehensively consider the shape, size, layout, vibration and heating conditions, and force direction of the component. Protel provides a series of pads of different sizes and shapes in the package library, such as round, square, octagonal, round and positioning pads, but sometimes this is not enough and needs to be modified by yourself. For example, for pads that generate heat, are subject to greater stress, and have a greater current, they can be designed into a "teardrop shape". In the familiar color TV PCB line output transformer pin pad planning, many manufacturers are just This way of choice. Generally speaking, in addition to the above, the following criteria should be considered when modifying the pad by yourself:

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

(2) It is often necessary to use asymmetric pads with asymmetrical length when wiring between component lead angles;

(3) The size of the pad hole of each component should be modified and confirmed according to the thickness of the component pin. The criterion is that the size of the hole is 0.2 to 0.4 mm larger than the pin diameter.

7, various types of film (Mask)

These films are not only indispensable in the PcB production process, but also a necessary condition for component welding. According to the position of the "membrane" and its effect, the "membrane" can be divided into component surface (or welding surface) soldering mask (TOp or Bottom) and component surface (or soldering surface) solder mask (TOp or BottomPaste Mask). As the name suggests, the soldering film is a layer of film that is applied to the pad to improve the solderability. It is also the light-colored round spots on the green board that are slightly larger than the pad. The situation of the solder mask is just the opposite, in order to To adapt the finished board to soldering methods such as wave soldering, it is required that the copper foil at the non-pads on the board cannot be tinned. Therefore, a layer of paint must be applied to all parts other than the pads to prevent tin from being applied to these parts. It can be seen that these two kinds of membranes are in a complementary relationship. From this discussion, it is not difficult to confirm the settings of similar items such as "solder Mask En1argement" in the menu.