Basic concept of PCB design

1.“Layer” The concept of "layer" is introduced in word processing or in many other software to achieve nesting and compositing of graphics, text, color, etc. Protel's "layer" is not virtual, but the actual copper foil layer of the printed board material itself. Today, components of electronic circuits are densely mounted. Special requirements for anti-interference and wiring, some of the newer electronic products used in the printed board not only have the upper and lower sides for the trace, but also in the middle of the board is a special processing of laminated copper foil, for example, used in computer motherboards The printing plate material is more than 4 layers. These layers are mostly difficult to process and are used to set up a simpler power wiring layer (such as Ground Dever and Power Dever in software), and are often routed by large-area filling (such as ExternaI P1a11e and Fill in software). ). The surface layer of the upper and lower positions and the intermediate layers need to communicate with each other by the so-called “Via” mentioned in the software. With the above explanation, it is not difficult to understand the concept of “multi-layer soldering disk” and “wiring layer setting”. To give a simple example, a lot of people have finished wiring. When they print out, they find that many connected terminals have no pads. In fact, this is the concept of "layer" when you add the device library. You don't draw yourself. The pad characteristics of the package are defined as "multiple" (Mulii-Layer). It should be reminded that once the number of layers of the used plates is selected, it is important to close those unused layers so as not to cause trouble. 2. Via is the line connecting the layers. A common hole is drilled in the intersection of the wires that need to be connected at each layer. This is the via. In the process, a layer of metal is deposited by chemical deposition on the cylindrical surface of the via hole of the via hole to connect the copper foil to be connected in the middle layer, and the upper and lower sides of the via hole are formed into a common pad shape, which can be directly It can also be connected to the lines on the upper and lower sides. In general, the following principles apply to the processing of vias when designing a circuit: (1) Use vias as little as possible. Once the vias are selected, be sure to handle the gaps between them and the surrounding entities, especially those that are easily overlooked. If there is a gap between the line and the via that is not connected to the via, if it is auto-routed, you can select it by selecting the “on” option in the “Sub Minimiz8tion” submenu. (2) The larger the required current carrying capacity, the larger the required via size, such as the vias used to connect the power layer and the ground plane to other layers. 3. Overlay layer is used to facilitate the installation and maintenance of the circuit. The required logo pattern and character code 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, and so on. 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 plates they designed, the characters are not blocked by the components or invaded the soldering area, and the component numbers are placed on adjacent components. Such designs will bring great assembly and maintenance. inconvenient. The correct silk screen layer character layout principle is: “No ambiguity, see the stitches, beautiful appearance”. 4, SMD's speciality Protel package library has a large number of SMD packages, namely surface soldering devices. The biggest feature of this type of device in addition to its small size is the single-sided distribution element pin hole. Therefore, to select such devices, you must define the surface of the device to avoid “Missing Plns”. In addition, the relevant text markup for such components can only be placed with the face of the component. 5. The External Plane and Fill (Fill) are just like the names of the two. The network-like filling area is to process a large area of ​​copper foil into a mesh, and the filling area is only a complete copper foil. In the design process of beginners, the difference between the two is often not seen on the computer. In essence, as long as you enlarge the surface, you can see at a glance. It is precisely because it is not easy to see the difference between the two, so the use of it does not pay attention to the distinction between the two, it should be emphasized that the former has a strong role in suppressing high-frequency interference in the circuit characteristics, suitable for the need to do Large areas of filling, especially when using certain areas as shields, partitions, or high-current power lines. The latter is often used in places such as general line ends or turning areas where small area filling is required. 6. Pad The pad is the most common and most important concept in PCB design, but beginners tend to ignore its choices and corrections, using circular pads in the design. The type of the pad of the selected component should take into account factors such as the shape, size, arrangement, vibration and heat, and direction of the force. Protel presents a range 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 subjected to a large force and a large current, it can be designed in a “teardrop shape”. In the design of the output transformer pin pad of the familiar color TV PCB, many manufacturers It is the form that is adopted. In general, in addition to the above, the following principles should be considered when editing the pad by itself: (1) When the length of the shape is inconsistent, the difference between the width of the wire and the length of the specific side of the pad should not be considered too large; (2) The need to use long and short asymmetric pads when routing between component leads is often twice the result; (3) The size of each component pad hole should be edited according to the component pin thickness. The principle is that the hole size is smaller than the pin diameter. Large 0.2-0.4 mm. 7. All kinds of membranes (Mask) These membranes are not only indispensable in the PcB manufacturing process, but also necessary for component welding. According to the position and function of “film”, “film” can be divided into component surface (or soldered surface) solder mask (Top or Bottom and component surface (or soldered surface) solder mask (Top or BottomPaste Mask). As the name suggests, the soldering film is a film that is applied to the pad to improve solderability, that is, a light colored spot on the green board that is slightly larger than the pad. On the contrary, in order to adapt the fabricated board to soldering forms such as wave soldering, it is required that the copper foil on the non-pad on the board cannot be tinned, so a layer of paint is applied to each part of the pad to prevent these. The tin on the part. It can be seen that the two membranes are a complementary relationship. From this discussion, it is not difficult to determine the settings of items such as “Solder Mask En1 argement” in the menu. 8. Flying lines and flying lines have two meanings. :(1) For the rubber cable-like network connection for observation during automatic wiring, after the components are transferred through the network table and the preliminary layout is made, the network connection under the layout can be seen by using the “Show command. Cross-over condition, constantly adjusting components The position minimizes this crossover to obtain the maximum rate of automatic routing. This step is very important. It can be said that it is not a mistake to cut the firewood, it takes more time, value! In addition, the automatic wiring is over, what else? The network has not been laid-through, and can also be found through this function. After finding out the network that has not been laid, you can use manual compensation. If you can't compensate, you need to use the second layer of “flying line”, which means printing in the future. These wires are connected to these networks. It is to be noted that if the board is a large-volume automatic line production, the flying line can be regarded as a resistance element with a 0-ohm resistance value and a uniform pad pitch for design.

One, the concept of vias Via is one of the important components of multi-layer PCB, the cost of drilling usually accounts for 30% to 40% of the cost of PCB board. In short, on the PCB Each hole can be called a via. In terms of function, the via can be divided into two types: 1. Used as an electrical connection between layers. 2. Used as a device for fixing or positioning. These vias are generally divided into three categories, namely blind holes (b Lind via), buried via, and through via. The blind via is located on the top and bottom surfaces of the printed circuit board and has a depth for the connection of the surface line and the underlying inner layer. Usually does not exceed a certain ratio (aperture). Buried holes refer to the connection holes in the inner layer of the printed circuit board, which do not extend to the surface of the circuit board. Both types of holes are located in the inner layer of the circuit board, and are used before lamination. The via forming process is completed, and several inner layers may be overlapped during the via formation process. The third type is called a via hole, which passes through the entire circuit board and can be used for internal interconnection or as component mounting. Positioning holes. Since the vias are easier to implement in the process and less expensive, most printed circuit boards use it without the need for two other vias. The via holes described below are considered as through holes unless otherwise specified.