Printed Circuit Board Sketch Design

The so-called printed circuit board sketch is a design drawing that can accurately reflect the position and connection of components on the printed circuit board, and refers to the basis for drawing a black and white base map. In the sketch, it is required that the position of the pads, the spacing of the pads, the interconnection between the pads, the direction and shape of the printed wires, and the overall size of the entire board, etc., should be based on the actual size of the printed board (or according to a certain The ratio) is drawn as a basis for the printed board. Drawing sketches is the key and main workload for patterning printed boards. The design of the sketch can be designed by hand or by computer-aided design. Manual design requires the design of the sketch on the drawing. Using a computer to design a printed board, although you can draw a black and white base map directly on the computer without designing a sketch on paper, the design principle of the sketch still needs to be reflected in the application process of the CAD software.

Manual design is generally to accurately mark a series of requirements such as the position, size, conductive pattern, wire width and spacing of the connection pad (pad) and hole on the fixed-size grid paper, and can add text descriptions. Computer-aided design (CAD), with the help of CAD software, according to the circuit schematic diagram and wire list, the design data such as the structural parameters, electrical requirements, design rules and component catalogs of the printed circuit board are input into the computer. According to these instructions, the computer automatically completes functions such as wiring, graphics compilation, netlist generation, and design specification verification. Its structural graphics can be displayed by the terminal, and the design can be modified by human-computer interaction. After completion, a series of data tapes (disks) will be provided.

Sketches usually include the following types of drawings: layout sketches, original graphics (or conductive graphics, in the form of drawings or CAD graphics data, handed over to printed circuit board manufacturers), solder mask drawings, marked character drawings, mechanical processing drawings and assembly drawings etc. The layout sketch is the key to designing a printed circuit board. According to the layout sketch, design the mechanical processing drawing, electrical assembly drawing and photographic base drawing. Design the solder resist map and mark character map (referred to as character map) according to the photographic base map.

The layout sketch is a technical document that marks all the size ranges and grid positions on the printed circuit board according to the requirements of the electrical schematic diagram or logic diagram. It includes conductive patterns, component sizes and types, hole locations, and instructions necessary to manufacture printed circuit boards, which can be indicated by one or more sketches. In general, printed circuit board graphics and design data are expressed on a sketch as much as possible.

1. Principles of printed circuit board sketch design

Once the prep work is done, it's time to start sketching. In addition to paying attention to dealing with various types of interference and solving grounding problems, the main principle of sketch layout design is to ensure that the printed wires do not cross the ground connection. It is not easy to do this well, and the main workload of sketch design is also to draw non-intersecting one-line diagrams. By rearranging the position of the components, the components are connected according to the circuit on the same plane, and the connections between each other cannot cross. If you encounter a cross, you need to re-adjust the arrangement position and direction of the components to solve or avoid this situation. Therefore, when designing a sketch, first draw a single-line diagram that does not intersect.

1.1 Points to note when drawing non-intersecting one-line diagrams

1) Components are installed on the component surface, which forms a mirror image relationship with the soldering surface, and special attention should be paid to multi-lead components.

2) Printed wires have certain width and spacing requirements. The wires of discrete components are generally about 1.5 to 3.0 mm, and the wire spacing is determined by the insulation resistance and breakdown voltage between wires. When the insulation resistance exceeds 20 MΩ, the distance between the two lines is 1.5mm, the allowable working voltage is 300V, and the distance between the two lines is 1.0mm, the allowable working voltage is 220V. Therefore, when drawing a single-line non-intersecting diagram, the spacing between wires should not be too dense.

3) The drawing of the schematic diagram is generally based on the principle of signal flow process and reflecting the function of components in the diagram, so as to facilitate the analysis and reading of the circuit, and never consider the size, shape and arrangement order of the lead wires of the components, so the principle There are many lines crossing in the figure. This has no effect on the reading diagram, but the crossover phenomenon is not allowed in the printed board. Therefore, in typesetting, first draw a single-line non-intersecting diagram. By rearranging the positions of the components, the connections between the components on the same plane do not cross. In case of crossing, you can rearrange the positions and directions of the components. to solve.

When it is difficult to avoid wire crossing, the spacing and span of components can be appropriately increased, and the "short wiring" method can also be used. When it is really complicated, double-sided panels can be used. When drawing single lines that do not intersect, first draw with a pencil, and gradually adjust the layout until it conforms to the electrical schematic diagram.

At the beginning of the design process, do not prematurely determine the arrangement position and direction of each component to solve or avoid this situation (wire crossing). In a more complex circuit, sometimes it is difficult not to cross at all. For example, in order to solve the problem that two wires do not cross, one turns around and becomes very long. This should be avoided in the design, because it not only increases the printed line. Density, and it is very likely that there will be interference in the circuit due to the length of the line. For this reason, "flying wires" can be used to solve the problem of too long printed wires, but the "flying wires" should not be too many. "Flying wire" means cutting off one at the intersection of the printed lines and connecting it with a short wire from the component surface of the board. This phenomenon is only used accidentally when it is absolutely necessary. If there are too many "flying lines", it will affect the quality of the board, and it cannot be regarded as a successful work.

After the non-intersecting one-line diagram is basically finished, you can start to draw the typesetting sketch. The position and size of components are required to be roughly fixed. The printed wires are arranged, and try to be short, few, and sparse. It usually takes several iterations to adjust the position and direction of components several times to achieve a satisfactory result.

In order to make a black and white base map of the printed board, a formal layout sketch should be drawn. It is required that the layout size, pad position, connection and direction of printed wires, size and position of each hole on the board, etc., must be the same as the actual layout and clearly marked. The ratio of the figure can be determined according to the density and precision of the graphics on the printed board, and can take different ratios such as 1:1, 2:1, 4:1, etc.

1.2 Analysis circuit schematic diagram

The degree of analysis of technical data such as principle design ideas and the use of the whole machine determines the initiative in the process of printed circuit design. The following objectives should be achieved through the analysis of the schematic diagram.

1) Find possible sources of interference in the line and sensitive components that are susceptible to interference.

2) Familiarize yourself with each component that appears in the schematic diagram, master the outline size, package form, lead mode, pin arrangement sequence, function and shape of each pin, etc. of each component, and determine which components need to be installed due to heat generation Heat sink and calculate the heat dissipation area, determine which components are mounted on the board, which are off the board, etc.

3) Determine the type of printed board: single-sided or double-sided. Single-sided: It is often used in discrete component circuits, because the discrete components have fewer leads and the arrangement position is easy to change flexibly. Double-sided: It is mostly used in circuits with more integrated circuits, especially dual-in-line packaged devices. Because the lead spacing of the device is small and the number is large (as few as 8 pins, as many as 40 pins or more), it is very difficult to lay out printed lines on one side without crossing, and it is almost impossible to realize more complex circuits.

4) Determine the installation method, arrangement method and pad routing form of components. According to different characteristics, there are often the following corresponding relationships: horizontal installation of components → regular arrangement → circular pads, vertical installation of components → irregular arrangement → island-shaped pads.

5) Determine the external connection method.

2.The steps to draw a PCB sketch

2.1 Drawing of a single-sided printed board sketch

According to the size of the sketch, take graph paper or coordinate paper (there should be a certain margin).

Draw the outline size of the layout, and leave a certain space under the outline to explain the technical requirements of the drawing.

Leave a certain blank distance (usually 5-10mm) around the board without pads and wires. Draw each process hole on the board (including the fixing holes of each component on the printed board substrate), and the center of the process hole is generally taken at the intersection of the coordinate grid.

Use a pencil to draw the outline of each component (should be drawn in the order of the position of the components on the single-line non-intersecting sketch), pay attention to make the outline size of each component correspond to the real thing, and the spacing of the components should be uniform. It is not necessary to draw outlines for small components that are used more, such as resistors, small capacitors, and low-power transistors, but you must be aware of them. Determine and mark the position of each pad. The pads with precision requirements should be marked strictly according to the size. If there is no size requirement, the components should be arranged evenly and neatly (more attention should be paid to the regular arrangement). The position of the pads should not be considered whether the pads are neat and consistent, but should be determined according to the size and shape of the components. In the end, it should be ensured that the components are evenly spaced, neat and moderately dense after assembly.

For the sake of simplicity, to draw the printed wire, it is only necessary to mark the direction and path of the wire with a thin line. It is not necessary to draw according to the actual width of the printed wire, but the distance between the wires must be considered.

After repeatedly checking the sketch drawn by the pencil is correct, retrace the solder joints and printed wires with a drawing pen, and wipe off the physical outline of the component after tracing to make the sketch clear.

Indicate the size of the pad and the width of the wire, and indicate the technical requirements of the printed board.

The technical requirements are as follows: 1) pad outer diameter, inner diameter, wire width, pad spacing and tolerance; 2) board material and board thickness; board dimensions and tolerances; 3) board coating requirements (referring to gold, silver, lead tin alloy, etc.); 4) the use of flux and flux on the board surface; 5) other specific requirements, etc.

2.2 Drawing of double-sided printed board sketch

In addition to the design and drawing process of the double-sided printed board sketch above, the following points should also be considered.

The components are arranged on one side (side A), and the main printed wires are arranged on the non-component side (side B). The printed wires on both sides should try to avoid parallel layout, and should try to be perpendicular to each other to reduce interference.

It is best to arrange the printed wires on both sides separately. If drawing on one side, use two colors to show the difference, and indicate which side they are on. The corresponding pads on both sides should be strictly one-to-one. The method can be to lead the center of one side of the pad to the other side through the needle punching method. When drawing the wires on the component surface, pay attention to avoiding the component casing, shielding cover, etc.

The printed lines that need to be connected between the two sides need to be realized with metallized holes.

2.3 Precautions for circuit board sketch drawing

When drawing a printed circuit board sketch, the following should be noted.

1) Familiar with the circuit principle. The so-called familiarity with the circuit principle means knowing the composition and working principle of the circuit, such as the ins and outs of the signal, the flow of the working current, and the relationship between components and unit circuits, so as to ensure the electrical performance during wiring.

2) Collect component data. In order to ensure the location, size and correct arrangement of components, the external dimensions, installation dimensions, and pin arrangement of components and accessories in the circuit should be collected as comprehensively as possible for reference when implementing wiring.

3) Determine the position of the fixture. The fixing part refers to the relative position between the printed circuit board and the casing. The position of the potentiometer, variable capacitor, battery pole piece, pulley and printed circuit board fixing hole in the circuit cannot be changed arbitrarily to prevent them from being damaged after wiring. Printed line damage or rewiring For the position of the fixture, in addition to marking its shape or outline and locating its size, the solder joints should also be marked. When part of the "site" is "occupied" by the fixture, an appropriate amount of margins with a certain width should be reserved.

4) Select the sketch scale and sketch proof. The sketch of the printed circuit board is the basis for making the printed circuit board. In order to make the sketch convenient and accurate, the sketch of the printed circuit board is usually enlarged than the real thing. After drawing, it will be photographed and copied and reduced to a ratio of 1:1, so that the deficiencies in the drawing can be reduced and compensated accordingly. Commonly used ratios are doubled (ie 2:1) or magnified 5 times (ie 5:1). The magnification is based on the distance between wires or pads. In order to draw accurate printed circuit board sketches, reasonable and clean wiring, and beautiful appearance, it is best to use light-colored coordinate paper. When the coordinate paper has a large grid of 10mm×10mm and a small grid of 2.5mm×2.5mm, when the ratio of 2.5:1 is selected, the large grid is equivalent to 4mm×4mm, the small grid is equivalent to 1mm×1mm, and other ratios are as follows And so on.

Summarize

To sum up, we can design a sketch of a printed circuit board that meets the requirements, and improve the PCB design process.

The above is the sketch design method of the printed circuit board introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. for you. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad: Songhan, Yingguang, Jieli, Ankai, Quanzhi, realtek, with MCU, voice IC, Bluetooth IC and module, wifi module. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS earphones, the development of Bluetooth earphone speakers, the development of children's toys, and the research and development of electronic education products.