PCB Tech - Engineer's summary: PROTEL technology encyclopedia (One)

1. Common errors in schematic diagrams:

(1) There is no signal connected to the ERC report pin:

a. I/O attributes are defined for the pins when the package is created;

b. Inconsistent grid attributes are modified when components are created or placed, and the pins and wires are not connected;

c. When creating the component, the pin direction is reversed, and the non-pin name end must be connected.

(2) The component went out of the drawing boundary: no component was created in the center of the diagram paper of the component library.

(3) The network table of the created project file can only be partially imported into the PCB: when the netlist is generated, global is not selected.

(4) When using multi-part components created by yourself, never use annotate.

2. Common errors in PCB:

(1) It is reported that NODE is not found when loading the network:

a. The components in the schematic diagram use packages that are not in the PCB library;

b. The components in the schematic diagram use packages with inconsistent names in the PCB library;

c. The components in the schematic diagram use packages with inconsistent pin numbers in the PCB library. For example, a triode: the pin numbers in sch are e, b, and c, while those in PCB are 1, 2, and 3.

(2) It can't always be printed on one page when printing:

a. It is not at the origin when creating the PCB library;

b. The components have been moved and rotated many times, and there are hidden characters outside the boundaries of the PCB board. Choose to show all hidden characters, shrink the PCB, and then move the characters to the boundary.

(3) The DRC reporting network is divided into several parts:

Indicates that this network is not connected. Look at the report file and use CONNECTED COPPER to find it.

In addition, remind friends to use WIN2000 as much as possible to reduce the chance of blue screen; export the file several times to make a new DDB file to reduce the file size and the chance of protel deadlock. If you make a more complicated design, try not to use automatic wiring.

In PCB design, wiring is an important step to complete product design. It can be said that the previous preparations are done for it. In the entire PCB, the wiring design process has the highest limit, the finest skills, and the largest workload. PCB wiring includes single-sided wiring, double-sided wiring and multilayer wiring. There are also two ways of wiring: automatic wiring and interactive wiring. Before automatic wiring, you can use interactive to pre-wire the more demanding lines. The edges of the input end and the output end should be avoided adjacent to parallel to avoid reflection interference. If necessary, ground wire should be added for isolation, and the wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is easy to occur in parallel.

The routing rate of automatic routing depends on a good layout. The routing rules can be preset, including the number of bending times, the number of vias, and the number of steps. Generally, explore the warp wiring first, quickly connect the short wires, and then perform the labyrinth wiring. First, the wiring to be laid is optimized for the global wiring path. It can disconnect the laid wires as needed. And try to re-wire to improve the overall effect.

The current high-density PCB design has felt that the through-hole is not suitable, and it wastes a lot of valuable wiring channels. In order to solve this contradiction, blind and buried hole technologies have emerged, which not only fulfill the role of the through-hole It also saves a lot of wiring channels to make the wiring process more convenient, smoother, and more complete. The PCB board design process is a complex and simple process. To master it well, a vast electronic engineering design is required. Only when personnel experience it by themselves can they get the true meaning of it.

1 Treatment of power supply and ground wire

Even if the wiring in the entire PCB board is completed very well, the interference caused by the improper consideration of the power supply and the ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of the electric and ground wires must be taken seriously, and the noise interference generated by the electric and ground wires should be minimized to ensure the quality of the product.

Every engineer engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only the reduced noise suppression is described:

It is well known to add decoupling capacitors between the power supply and ground. 7 X2 B3 K) Y/ ?" e( A1 F/ t# Y4 x, n

Try to widen the width of the power and ground wires, preferably the ground wire is wider than the power wire, their relationship is: ground wire>power wire>signal wire, usually the signal wire width is: 0.2～0.3mm, the smallest width can be reached 0.05～0.07mm, power cord is 1.2～2.5 mm

For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground net to use (the ground of the analog circuit cannot be used in this way)

Use a large area of copper layer for grounding, and connect the unused places on the printed circuit board to the ground for grounding. Or it can be made into a multi-layer board, and the power supply and ground wire occupy one layer each.

2. Common ground processing of digital circuit and analog circuit

Many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground wire.

The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line should be as far away as possible from the sensitive analog circuit device. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated and they are not connected to each other, but at the interface (such as plugs, etc.) connecting the PCB to the outside world. There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.

3. The signal line is laid on the electric (ground) layer

In the multi-layer printed board wiring, because there are not many wires left in the signal line layer that have not been laid out, adding more layers will cause waste and increase the production workload, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second. Because it is best to preserve the integrity of the formation.

4. Treatment of connecting legs in large area conductors

In large-area grounding (electricity), the legs of common components are connected to it. The treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better to connect the pads of the component legs to the copper surface. There are some undesirable hidden dangers in the welding and assembly of components, such as: 1. Welding requires high-power heaters. 2. It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, called heat shields, commonly known as thermal pads (Thermal), so that virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the power (ground) leg of the multilayer board is the same.

5. The role of the network system in cabling

In many CAD systems, wiring is determined by the network system. The grid is too dense and the path has increased, but the step is too small, and the amount of data in the field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the computer-based electronic products. Great influence. Some paths are invalid, such as those occupied by the pads of the component legs or by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate. Therefore, there must be a well-spaced and reasonable grid system to support the wiring.

The distance between the legs of standard components is 0.1 inches (2.54 mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or an integral multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches etc.

6. Design rule check (DRC)

After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer, and it is also necessary to confirm whether the established rules meet the requirements of the printed board production process. The general inspection has the following aspects:

Whether the distance between line and line, line and component pad, line and through hole, component pad and through hole, through hole and through hole is reasonable, and whether it meets the production requirements.

Is the width of the power line and the ground line appropriate, and is there a tight coupling between the power line and the ground line (low wave impedance)? Is there any place in the PCB where the ground wire can be widened?

Whether the best measures have been taken for the key signal lines, such as the shortest length, the protection line is added, and the input line and output line are clearly separated.

Whether there are separate ground wires for analog circuit and digital circuit.

Whether the graphics (such as icons and annotations) added to the PCB will cause signal short circuit.

Modify some undesirable line shapes.

Is there a process line on the PCB? Whether the solder mask meets the requirements of the production process, whether the solder mask size is appropriate, and whether the character logo is pressed on the device pad, so as not to affect the quality of the electrical equipment.

Whether the outer frame edge of the power ground layer in the multi-layer board is reduced, for example, the copper foil of the power ground layer is exposed to the outside of the board and it is easy to cause a short circuit. Overview The purpose of this document is to explain the process of using pads' printed board design software PowerPCB for printed board design and some precautions, to provide design specifications for designers in a working group, and to facilitate communication and mutual inspection between designers.

2. Design process

The PCB design process is divided into six steps: netlist input, rule setting, component layout, wiring, inspection, review, and output.

2.1 Netlist input

There are two ways to enter the netlist. One is to use PowerLogic's OLE PowerPCB Connection function, select Send Netlist, and use the OLE function to keep the schematic diagram and PCB diagram consistent at any time to minimize the possibility of errors. Another method is to load the netlist directly in PowerPCB, select File->Import, and input the netlist generated by the schematic diagram.

2.2 Rule settings

If the PCB design rules have been set in the schematic design stage, there is no need to set them again q# W- C& ^, f! N

These rules are gone, because when the netlist is entered, the design rules have been entered into PowerPCB along with the netlist. If the design rules are modified, the schematic diagram must be synchronized to ensure that the schematic diagram is consistent with the PCB. In addition to the design rules and layer definitions, there are also some rules that need to be set, such as Pad Stacks, which need to modify the size of the standard vias. If the designer creates a new pad or via, layer 25 must be added.

Note: PCB design rules, layer definitions, via settings, and CAM output settings have been made into the default startup file, named Default.stp. After the netlist is entered, according to the actual situation of the design, assign the power network and ground to the power layer And stratum, and set other advanced rules. After all the rules are set, in PowerLogic, use the Rules From PCB function of OLE PowerPCB Connection to update the rule settings in the schematic to ensure that the rules of the schematic and PCB are consistent.

2.3 Component layout

After the netlist is entered, all components will be placed at the zero point of the work area and overlapped together. The next step is to separate these components and arrange them neatly according to some rules, that is, component layout. PowerPCB provides two methods, manual layout and automatic layout.

2.3.1 Manual layout

1. Draw the board outline for the structural dimensions of the printed board of the tool.

2. Disperse the components (Disperse Components), the components will be arranged around the edge of the board.

3. Move and rotate the components one by one, put them inside the edge of the board, and place them neatly according to certain rules.

2.3.2 Auto Layout

Power PCB provides automatic layout and automatic local cluster layout, but for most designs, the effect is not ideal and it is not recommended. 2.3.3 Matters needing attention

a. The first principle of the layout is to ensure the wiring rate, pay attention to the connection of the flying leads when moving the device, and put the connected devices together

b. Separate digital devices from analog devices and keep them as far away as possible

c. The decoupling capacitor is as close as possible to the VCC of the device

d. When placing the device, consider future soldering, not too dense

e. Use the Array and Union functions provided by the software more to improve the efficiency of layout,

2.4 Wiring.

There are also two ways of wiring, manual wiring and automatic wiring. The manual wiring function provided by PowerPCB is very powerful, including automatic pushing and online design rule checking (DRC). Automatic wiring is performed by Specctra's wiring engine. Usually these two methods are used together. The common steps are manual-automatic-manual.

2.4.1 Manual wiring

1. Before automatic wiring, first hand-lay some important networks, such as high-frequency clocks, main power supplies, etc. These networks often have special requirements for wiring distance, line width, line spacing, and shielding; in addition, some special packaging, Such as BGA, it is difficult to arrange automatic wiring regularly, and manual wiring must be used.

2. After automatic routing, the PCB routing must be adjusted by manual routing.

2.4.2 Automatic wiring

After the manual wiring is over, the remaining network is handed over to the automatic router for cloth. Select Tools->SPECCTRA, start the interface of the Specctra router, set the DO file, and press Continue to start the automatic wiring of the Specctra router. After the completion, if the wiring rate is 100%, then you can manually adjust the wiring; if not If it reaches 100%, it indicates that there is a problem with the layout or manual wiring, and the layout or manual wiring needs to be adjusted until all the connections are made.

2.4.3 Matters needing attention

a. Make the power cord and ground wire as thick as possible

b. Try to connect the decoupling capacitor directly to VCC

c. When setting the DO file of Specctra, first add the Protect all wires command to protect the manually clothed wires from being redistributed by the automatic router

d. If there is a mixed power layer, you should define the layer as Split/mixed Plane, divide it before wiring, and after wiring, use Pour Manager's Plane Connect for copper pour

e. Set all device pins to thermal pad mode by setting Filter to Pins, select all pins, modify properties, and tick the Thermal option

f. When manually routing, turn on the DRC option and use dynamic routing (Dynamic Route)

2.5 Inspection

The items to be checked include Clearance, Connectivity, High Speed and Plane. These items can be selected by Tools->Verify Design. If the high-speed rule is set, it must be checked, otherwise you can skip this item. If an error is detected, the layout and wiring must be modified.

Note: Some errors can be ignored. For example, when a part of the outline of some connectors is placed outside the board frame, errors will occur when checking the spacing; in addition, each time the traces and vias are modified, the copper must be re-plated.

2.6 Review

The review is based on the "PCB checklist", which includes design rules, layer definitions, line widths, spacing, pads, and via settings; also focus on reviewing the rationality of the device layout, the routing of power and ground networks, and high-speed clock networks. The wiring and shielding, the placement and connection of decoupling capacitors, etc. If the recheck is unqualified, the designer shall modify the layout and wiring. After passing, the rechecker and the designer shall sign separately.

2.7 Design output

The PCB design can be exported to a printer or a gerber file. The printer can print the PCB in layers, which is convenient for designers and reviewers to check; the gerber file is handed over to the board manufacturer to produce the printed board. The output of the gerber file is very important. It is related to the success or failure of this design. The following will focus on the matters needing attention when outputting the gerber file.

a. The layers that need to be output include wiring layers (including top, bottom, and middle wiring layers), power layers (including VCC layers and GND layers), silk screen layers (including top and bottom silk screens), and solder masks (including top solder masks) And bottom layer solder mask), and also generate a drilling file (NC Drill)

b. If the power layer is set to Split/Mixed, then select Routing in the Document item of the Add Document window, and each time the gerber file is output, you must use Pour Manager's Plane Connect to pour copper on the PCB diagram; if it is set to CAM Plane, select Plane. When setting the Layer item, add Layer25, and select Pads and Viasc in the Layer25 layer. In the device setting window (press Device Setup), change the value of Aperture to 199

c. When setting the Layer of each layer, select the Board Outline

d. When setting the Layer of the silk screen layer, do not select Part Type, select the top layer (bottom layer) and Outline, Text, Line 9 of the silk screen layer

e. When setting the layer of the solder mask layer, select vias to indicate that no solder mask is added to the vias, and not to select vias to indicate solder masks, depending on the specific situation.

f. When generating drilling files, use the default settings of PowerPCB and do not make any changes

g. After all gerbera files are output, open and print them with CAM350, and check them according to the "PCB checklist" by the designer and reviewer

Via is one of the important components of multi-layer PCB, and the cost of drilling usually accounts for 30% to 40% of PCB manufacturing cost. Simply put, every hole on the PCB can be called a via. From the point of view of function, vias can be divided into two categories: one is used for electrical connections between layers; the other is used for fixing or positioning devices. In terms of process, these vias are generally divided into three categories, namely blind vias, buried vias and through vias. Blind vias are located on the top and bottom surfaces of the printed circuit board and have a certain depth. They are used to connect the surface line and the underlying inner line. The depth of the hole usually does not exceed a certain ratio (aperture). Buried hole refers to the connection hole located in the inner layer of the printed circuit board, which does not extend to the surface of the circuit board. The above-mentioned two types of holes are located in the inner layer of the circuit board, and are completed by a through-hole forming process before lamination, and several inner layers may be overlapped during the formation of the via. The third type is called a through hole, which penetrates the entire circuit board and can be used for internal interconnection or as a component mounting positioning hole. Because the through hole is easier to implement in the process and the cost is lower, most of the printed circuit boards use it instead of the other two types of through holes. The following via holes, unless otherwise specified, are considered as via holes.

From a design point of view, a via is mainly composed of two parts, one is the drill hole in the middle, and the other is the pad area around the drill hole, as shown in the figure below. The size of these two parts determines the size of the via. Obviously, in high-speed, high-density PCB design, designers always hope that the smaller the via hole is, the better, so that more wiring space can be left on the board. In addition, the smaller the via hole, the parasitic capacitance of its own. The smaller it is, the more suitable it is for high-speed circuits. However, the reduction of hole size also brings about an increase in cost, and the size of vias cannot be reduced indefinitely. It is limited by process technologies such as drilling and plating: the smaller the hole, the more the hole is drilled. The longer the hole takes, the easier it is to deviate from the center position; and when the depth of the hole exceeds 6 times the diameter of the drilled hole, it cannot be guaranteed that the hole wall can be uniformly plated with copper. For example, the thickness (through hole depth) of a normal 6-layer PCB board is about 50Mil, so the minimum drilling diameter that PCB manufacturers can provide can only reach 8Mil.