PCB Technical - One of the high-speed PCB design guidelines: PCB basic concepts

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, color, etc., Protel's "layer" is not virtual, but the actual printed board material itself In the various copper foil layers. Nowadays, due to the dense installation of electronic circuit components. Special requirements such as anti-interference and wiring. The 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 boards. For example, the current computer motherboards are used Most of the printed board materials 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). ). Where the upper and lower surface layers and the middle layers need to be connected, the so-called "vias" mentioned in the software are used to communicate. 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 found that many of the connected terminals have no pads when they are printed out. In fact, this is because they ignored the concept of "layers" when they added the device library and did not draw and package themselves. The pad characteristic is defined as "Multilayer (Mulii-Layer). It should be reminded that once the number of layers of the printed board used is selected, be sure to close those unused layers, so as not to cause troubles and detours.

2. Via (Via)

In order to connect the lines between the layers, 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 foil that needs to be connected to the middle layers, and the upper and lower sides of the via are made into ordinary pad shapes, which can be directly It is connected to the upper and lower lines, or not connected. Generally speaking, there are the following principles for the treatment of vias when designing a circuit:

(1) Minimize the use of vias. 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 easily overlooked in the middle layers and the vias. If it is Automatic routing can be solved automatically by selecting the "on" item in the "Via Minimiz8tion" submenu.

(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. Pad

The pad is the most frequently contacted and most important concept in PCB design, but beginners tend to ignore its selection and modification, and use circular pads in the design. When selecting the pad type of the component, it is necessary to comprehensively consider the shape, size, layout, vibration, heat, 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 edited by yourself. For example, for pads that generate heat, are subjected to greater stress, and are current, they can be designed into a "teardrop shape". In the familiar color TV PCB line output transformer pin pad design, many manufacturers are just In this form. Generally speaking, in addition to the above, the following principles should be considered when editing the pad by yourself:

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

(2) 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;

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

4. Silk screen layer (Overlay)

In order to facilitate the installation and maintenance 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 many beginners design the relevant content of the silk screen layer, they only pay attention to the neat and beautiful placement of the text symbols, ignoring the actual PCB effect. On the printed board they designed, the characters were either blocked by the component or invaded the soldering area and wiped off, and some of the components were marked on the adjacent components. Such various designs will bring a lot to assembly and maintenance. inconvenient. The correct principle for the layout of the characters on the silk screen layer is: "no ambiguity, stitches at a glance, beautiful and generous".

5. The particularity of SMD

There are a large number of SMD packages in the Protel package library, that is, surface soldering devices. The biggest feature of this type of device in addition to its small size is the single-sided distribution of pin holes. Therefore, when choosing this type of device, it is necessary to define the surface of the device to avoid "Missing Plns". In addition, the relevant text annotations of this type of component can only be placed along the surface where the component is located.

6. Grid-like filling area (External Plane) and filling area (Fill)

Just like the names of the two, the network filled area is to process a large area of copper foil into a network, and the filled area only keeps the copper foil intact. Beginners often can't see the difference between the two on the computer in the design process, in fact, as long as you zoom in, you can see it at a glance. It is precisely because it is not easy to see the difference between the two in ordinary times, 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 applications that need to be made. Places filled with large areas, especially when certain areas are used as shielded areas, partitioned areas, or high-current power lines are particularly suitable. The latter is mostly used in places where a small area is required such as general line ends or turning areas.

7. Various types of membranes (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 function, 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 implies, the soldering film is a film that is applied to the pad to improve the solderability, that is, the light-colored round spots on the green board are slightly larger than the pad. The situation of the solder mask is just the opposite, for To adapt the finished board to wave soldering and other soldering methods, it is required that the copper foil at the non-pad on the board cannot be tinned. Therefore, a layer of paint must be applied to all parts other than the pad to prevent tin from being applied to these parts. It can be seen that these two membranes are in a complementary relationship. From this discussion, it is not difficult to determine the menu

Items like "solder Mask En1argement" are set up.

8. Flying line, flying line has two meanings:

A rubber band-like network connection used for observation during automatic routing. After importing components through the network table and making a preliminary layout, you can use the "Show command to see the crossover status of the network connection under the layout, and continue to adjust The position of the component minimizes this crossover to obtain the maximum automatic routing rate. This step is very important. It can be said to sharpen the knife and not cut the wood by mistake. It takes more time and value!

In addition, after the automatic wiring is completed, which networks have not yet been deployed, you can also use this function to find out. After finding the unconnected network, it can be compensated manually. If it can't be compensated, the second meaning of "flying line" is used, which is to connect these networks with wires on the future printed board. It should be confessed that if the circuit board is mass-produced automatic line production, this flying lead can be designed as a resistance element with a 0 ohm resistance value and a uniform pad spacing.