PCB News - What is a PCB board and how to inspect it

PCB design, PCB (Printed Circuit Board) abbreviation for printed circuit board

The specific method is as follows

1. Purpose and Function

1.1 Standardize design operations, improve production efficiency and improve product quality.

2. Scope of application

1.1 VCD super VCDDVD audio and other products of the development department of XXX company.

3. Responsibility

3.1 All electronic engineers, technicians and computer drafters in the XXX Development Department.

4. Qualifications and training

4.1 Have a foundation in electronic technology;

4.2 Have basic computer operation knowledge;

4.3 Familiar with the use of computer PCB drawing software.

5. Work instruction (the length unit is MM)

5.1 Minimum line width of copper foil: 0.3mm for the panel, 0.2mm for the panel, and 1.0mm minimum for the edge copper foil

5.2 Minimum gap of copper foil: panel: 0.3MM, panel: 0.2MM.

5.3 The minimum distance between the copper foil and the board edge is 0.55MM, the minimum distance between the component and the board edge is 5.0MM, and the minimum distance between the plate and the board edge is 4.0MM

5.4 The size (diameter) of the pad of the general through-hole mounting component is twice the aperture, the minimum of double-sided board is 1..5MM, and the minimum of single-sided board is 2.0MM. If you can't use round pads, use waist circle. Shaped pad, as shown in the figure below (if there is a standard component library,

The standard component library shall prevail)

The relationship between the long side and short side of the pad and the hole is:

5.5 Electrolytic capacitors cannot touch heating components, high-power resistors, varistors, voltages, heaters, etc. The minimum distance between the decapacitor and the radiator is 10.0MM, and the minimum distance between the component and the radiator is 2.0MM.

5.6 Large-scale components (such as transformers, electrolytic capacitors with a diameter of 15.0 mm or more, high-current sockets, etc.) increase the copper foil and upper tin area as shown in the figure below; the area of the shaded part must be the same as the pad area.

5.7 There can be no copper foil (external grounding) components within the screw hole radius of 5.0MM. (as required by the structure drawing).

5.8 There should be no silk screen oil on the upper tin position.

5.9 If the center distance between the pads is less than 2.5MM, the adjacent pads must be wrapped with silkscreen oil, and the width of the ink is 0.2MM (0.5MM is recommended).

5.10 Do not place jumpers under the IC or under the components of motors, potentiometers and other large-volume metal casings.

5.11 In large-area PCB design (more than 500CM2), prevent the PCB board from bending when passing through the soldering furnace, and leave a 5 to 10mm wide gap in the middle of the PCB board without placing components (wiring), which is used to pass the soldering furnace. When adding a pressure strip to prevent the PCB board from bending, the shaded area in the following figure::

5.12 Each transistor must be marked with e, c, b pins on the silk screen.

5.13 For components that need to be soldered after passing through the tin furnace, the plate must be driven away from the tin position, and the direction is opposite to the passing direction. The size of the viewing hole is 0.5MM to 1.0MM as shown in the figure below:

5.14 When designing a double panel, pay attention to the metal shell components, the shell is in contact with the printed board when plug-in, the top pad cannot be opened, and it must be covered with green oil or silk screen oil (for example, two-pin crystal oscillator).

5.15 In order to reduce the short circuit of the solder joints, all double-sided printed boards have no green oil windows in the via holes.

5.16 A solid arrow must be used to mark the direction of the tin furnace on each PCB:

5.17 The minimum distance between holes is 1.25MM (double panel is invalid)

5.18 During the layout, the placement direction of the DIP packaged IC must be perpendicular to the direction through the soldering furnace, not parallel, as shown in the figure below; if the layout is difficult, it is allowed to place the IC horizontally (the OP packaged IC is placed in the opposite direction to the DIP ).

5.19 The wiring direction is horizontal or vertical, and it takes 45 degrees to enter from vertical to horizontal.

5.20 The placement of components is horizontal or vertical.

5.21 The silk-screened characters are placed horizontally or rotated 90 degrees to the right.

5.22 If the width of the copper foil into the round pad is smaller than the diameter of the round pad, then teardrops need to be added. As shown in the figure:

5.23 The material code and design number should be placed on the empty space of the board.

5.24 Reasonably use the place without wiring as grounding or power supply.

5.25 The wiring should be as short as possible, paying special attention to the shorter wiring of clock lines, low-level signal lines and all high-frequency loops.

5.26 The ground wire and power supply system of analog circuit and digital circuit should be completely separated.

5.27 If there is a large area of ground wire and power wire area (more than 500 square millimeters) on the printed board, the window should be partially opened. As shown in the figure:

5.28 The positioning holes of the electrical plug-in printed board are specified as follows. No components can be placed in the shaded parts, except for hand-plugged components. The range of L is 50 to 330mm, and the range of H is 50 to 250mm., If it exceeds 330X250, it will be changed to a manual board. The positioning hole must be on the long side.

Basic Concepts of PCB Design

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 easily overlooked in the middle layers and the vias. If it is automatic routing, you can Select the "on" item in the Via Minimiz8tion submenu to automatically 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

3, 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".

4. 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 pins (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.

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 network, and the filling 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 normal 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 needs. 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.

6. 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 same design. 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 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) When the shape is inconsistent in length, consider the difference between the width of the wire and the specific side length of the pad not too large;

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

(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.

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 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, the copper foil on the non-pad on the board must not 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:

(1) A rubber band-like network connection for observation during automatic wiring. After loading 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, Constantly adjust the position of the components to minimize 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 firewood by mistake. It takes more time and value! In addition, automatic routing At the end, you can find out which networks have not been deployed yet. You can also use this function to find out. After finding out the networks that have not been deployed, you can manually compensate. If you can’t compensate, you will need to use the second meaning of "flying line", that is, in the future The printed circuit board uses wires to connect these networks. It should be confessed that if the circuit board is mass-produced by automatic line, this kind of flying wire can be regarded as a resistance element with a resistance of 0 ohm and a uniform pad spacing.

Pieces to design.

The above is an introduction to what a PCB board is and how to inspect it. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.