High-speed PCB Design: PCB basic concepts

High-speed PCB design:  PCB basics

1. Layer

Protel "layer" is not virtual, but the printing plate material itself is the real layer of copper foil. Today, components are densely installed due to electronic circuits. Interference and cabling and other special requirements, some of the newer electronic products used in the printed board are not only the upper and lower sides for the alignment, but also in the middle of the plate can be specially processed laminated copper foil, for example, but the computer motherboard is also used the material more than 4 layers. 

Most of these layers are used to set up simpler power cabling (such as Ground Dever and Power Dever in the software) and are often populated with large area pads (such as ExternaI P1a11e and Fill in software). The upper and lower surface layer and the middle of the need to communicate with the place "Via (Via)" to communicate. With the above explanation, it is not hard to understand the concept of "multi-layer pad" and "wiring layer settings." Cite a simple example, many people completed the wiring, to print out just found a lot of connection terminals have no pads, in fact, this is their own add-on library ignored the concept of "layer", did not draw their own package Pad features are defined as multiple layers. To be reminded, once you have selected the number of layers to be used, be sure to turn off any unused layers.

2. Via

To connect the lines between the layers, drill a common hole in the sink of the wires that need to be connected to each layer. This is the via hole. A metal layer is deposited on the surface of the hole wall of the via hole by a chemical deposition method to connect the copper foils that need to be connected in the middle layer. The upper and lower surfaces of the via hole are formed into an ordinary pad shape, The two sides of the same lines, but also not even. In general, the design of the vias to deal with the following principles:

(1) minimize the use of holes, once the selection of the vias, be sure to deal with it and the surrounding entities in the gap, especially in the middle layer is easily overlooked and the vias are not connected lines and vias, if it is Autorouting is automatically solved by selecting the "on" entry in the "Via Minimization" submenu.

(2) The greater the required current carrying capacity, the larger the required via size, the larger the via holes used to connect the power and ground planes with other layers.

3. Pad 

Pads are the most common and most important concept in PCB design, but beginners can easily overlook their options and fixes by using circular pads in their designs. Select the type of pad components to comprehensive consideration of the shape of the component, size, layout, vibration and heat conditions, the direction of force and other factors. Protel in the packaging library gives a series of different sizes and shapes of the pads, such as round, square, anise, round and positioning pads, but sometimes this is not enough, you need to edit. For example, on the heat and stress larger, the larger current pad can be designed as a "teardrop-like", in the familiar color TV PCB output transformer pin pad design, many manufacturers are Used in this form. In general, in addition to the above when editing your own welding pad, but also consider the following principles:

(1) The need to use the length of the asymmetric pads tend to do more with less;

(2) When the length of the shape is inconsistent to consider the connection width and the length of the specific side of the pad size difference can not be too large;

(3) The size of each component pad hole is determined by the pin size of the component pin, the principle is that the size of the hole is 0.2-0.4 mm larger than the pin diameter.

4. Silkscreen layer (Overlay)

In order to facilitate the installation and maintenance of the circuit, the printed logo on the upper and lower surfaces of the printing plate is required by the logo and text code, such as component labels and nominal values, component shape and manufacturer logo, production date and so on. Many beginners to design the silkscreen layer content, only pay attention to the text symbol placed neatly beautiful, ignoring the actual effect of the PCB system. They designed the printing plate, the characters are not blocked by the components is invaded the welding area is wiped out, and some of the components labeled hit in the adjacent components, so a variety of designs will bring great assembly and maintenance inconvenient. Correct silk screen character layout principle is: "no ambiguity, see the pin, beautiful and generous."

5. SMD's particularity

Protel package library has a large number of SMD packages, the surface of the welding equipment. In addition to these devices other than small size is the most prominent feature of the single-sided distribution of pinholes. Therefore, the choice of such devices to define the device surface, in order to avoid "missing pin" In addition, the relevant text labels of such components can only be placed with the surface of the component.

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

As the names of the two, the network-like filling area is a network of large areas of copper foil, the filling area is only fully preserved copper foil. Beginners often do not see the difference between the design process, in fact, as long as you zoom in on the picture at a glance. It is precise because the common is not easy to see the difference between the two, so useless pay attention to the distinction between the two, it is emphasized that the former in the circuit characteristics have a strong inhibitory role of high-frequency interference, apply to be done Large areas filled, especially when certain areas are shielded, split or high-current power cords are particularly suitable. The latter is used for general line ends or transition areas that need a small area filling the place.

7. All kinds of the membrane (Mask)

These films are not only indispensable in the production of PCB, but are also necessary components for soldering. According to the location of "membrane" and its function, "membrane" can be divided into the component surface (or welding surface) flux film (Top or Bottom and component surface (or welding surface) solder mask (Top or BottomPaste Mask) two As the name implies, the solder mask is applied to the pad to improve the solderability of a layer of film, which is slightly larger than the pad on the green board each of the light-colored spots. The situation is the opposite of the solder mask, The board made to adapt to wave soldering and other forms of welding, requiring non-pad copper foil at the board can not stick tin, so all parts of the pad should be coated with a coating to prevent these parts of the tin. Visible, these two kinds of the membrane are a complementary relationship. From this discussion, it is not difficult to determine the menu

Similar "solder Mask En1argement" and other projects set up.

8. Fly line

Has two meanings:

Automatic wiring for observation with a similar rubber band network connection, through the network table, into the components and made a preliminary layout, use the "Show command you can see the layout of the network connection under the cross conditions, and constantly adjusted The location of the components minimizes this crossover for maximum automatic routing of the patch. This step is important!

In addition, the end of automatic routing, which networks have not yet posted, but also through the function to find. To find out the network after the cloth can be manually compensated, it is necessary to compensate will not be used, "Flying Line" means the second layer, that is, in the future printed circuit board connected with these wires. To bear in mind is that if the circuit board is a high-volume automatic line production, this line can be regarded as 0-ohm resistance, with uniform pad spacing of the resistance element to design.