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PCB Design

Commonly Used Terms in PCB Circuit Boards

Process PCB circuit board process a lot, so what are the terms commonly used in the process of process circuit board?

1, Gold Fingers: Exposed metal pads on the side of the board, generally used as a connection between two circuit boards, such as memory sticks, old game cards.

2, Hard Gold: Electroplated alloy.

3, Soft Gold: Electroplated pure gold.

4, Hole Ring: PCB metallization holes on the copper ring.

5, Through-Hole: Through the entire circuit board, can be used to achieve internal interconnections or as a component of the mounting and positioning holes, lower cost, easy to realize.

6, Blind Holes: Located in the top and bottom surface of the printed circuit board, connecting the surface layer of the line below the inner layer of the line, there is a certain depth, the depth of the hole is usually no more than a certain ratio (hole diameter).

7, Buried Holes: Printed circuit board connection holes in the inner layer, it does not extend to the surface of the board.

8, Stamp Holes: Split-board design method, with some continuous holes to form a weak connection point, the board from the collage to split out.

9, Solder Pads: PCB surface of the exposed part of the metal, used to solder devices.

10, Solder Resist: Copper covered with a protective film to prevent short circuits, corrosion and other problems.

11, The Board: A large circuit board composed of many small circuit boards can be divided. Automated circuit board production equipment in the production of small boards often have problems, the combination of several small boards together, you can speed up production.

12, Stencil: A thin metal template (can also be plastic), in the assembly, will be placed on the PCB so that the solder through some specific parts.

13, Pick-and-place: Components to the circuit board on the machine or process.

14, Silkscreen: Group soldering layer on the white is silkscreen layer.

15, Solder Paste Layer: Before placing components on the PCB, will be through the stencil in the surface paste device pads formed on a certain thickness of the solder paste layer. In the reflow soldering process, the solder paste melts, in the pad and device pins between the establishment of reliable electrical and mechanical connections.

16, Soldering Furnace: The furnace for soldering plug-ins. Generally there is a small amount of molten solder inside, the board card on the top of the rapid passage, you can expose the pins on the tin welded.

17, Soldermask: In order to prevent short circuits, corrosion and other problems, copper will be covered with a protective film. Protective film is generally green, may be other colors (SparkFun red, Arduino blue, or Apple black). Generally known as "soldermask.

18, Linked Tin: Two connected pins on the device, by a small drop of solder incorrectly connected to each other.

19, Surface Mount: A method of assembly, the device simply placed on the board, do not need to device pins through the board on the hole.

20, Hot Pads: Refers to the connection pads to the plane between a short section of the alignment. If the pads do not do the proper thermal design, soldering is difficult to heat the pads to a sufficient soldering temperature. Improper heat dissipation pad design, the pad will feel more sticky, and reflow time is relatively long. (Translator's note, generally hot pads are made on the side of the insert that contacts the wave solder. I do not know why this article will mention reflow, reflow is mainly to consider the thermal balance, to prevent the monument.)

21, V-score: The board will be an incomplete cut, you can break the board through this straight line. (Translator's note: often called "V-CUT").

22, Through-hole: A hole in the card, generally used to switch the signal from one layer to another.

23, Plug Holes: Refers to the over-hole covered with soldermask to prevent being soldered. Connector or device pin holes, because of the need to solder, generally not plug holes.

24, The Panel: A large circuit board composed of many small circuit boards can be divided.

25, Slotting: PCB on any hole that is not round, can be plated or not plated.

26, Wave Soldering: Molten solder to form a wave of solder to the components of the welding, applicable to chip electronic components.

27, Reflow Soldering: High-temperature hot air to form a reflow of molten solder on the components to be soldered, applicable to the pin electronic components.

28, Lead Spacing: The center distance between adjacent leads.

29, DRC: Design Rule Check.

PCB Design

PCB Design

General PCB basic design flow is as follows: Pre-preparation, PCB structural design, PCB layout, wiring, Wiring optimization and Silkscreen network and DRC inspection and Structural inspection, Board making.

A. Pre-preparation

This includes the preparation of component libraries and schematics. "To make a good board, in addition to designing a good principle, but also to draw well. Before PCB design, first of all, you should prepare the component library of schematic SCH and the component library of PCB. Component library can use peotel comes with the library, but in general it is difficult to find a suitable, it is best to do their own according to the standard size of the selected device information to do their own component library in principle, the first to do the PCB component library, and then to do the SCH component library. PCB component library requirements are higher, it directly affects the installation of the board; SCH component library requirements are relatively loose, as long as attention to defining the pin attributes and correspondence with the PCB components on the right. Just pay attention to define the pin attributes and the correspondence with the PCB components.

Pay attention to the hidden pins in the standard library. After that is the schematic design, ready to start doing PCB design.

B. PCB Structure Design

PCB design first consider the external dimensions, which is the final assembly of the PCB size. After verifying the size of the PCB installation shell with the size of the PCB board to determine the outer contour dimensions, the outer contour dimensions drawn in CAD and imported into the PCB drawings, this step has been determined in accordance with the size of the circuit board and the mechanical positioning, according to the positioning requirements to be placed in the required connectors, keys / switches, screw holes, assembly holes and so on.

Secondly, we should consider the prohibition of wiring size, change the size of the device affects the installation and insulation voltage problems, to reserve enough space for testing and adjustment. Fully consider and determine the wiring area and non-wiring area (e.g., screw holes around how much belongs to the non-wiring area . Finally, consider the size of the shaped board as far as possible using the regular shape, which is conducive to the board to reduce production costs, reduce material waste.

C. PCB Layout.

Layout is to put the device on the board. At this time, if the preparatory work mentioned earlier are done, you can generate a network table on the schematic ( Design - "Create Netlist), after the PCB map into the network table ( Design - "Load Nets."), you can see the device smoothly all stacked. You can see the device smoothly stacked up, there are flying wires between the pins prompted to connect. Then you can layout the device.

General Layout According to The Following Principles:

  1. 1, According to the electrical properties of a reasonable partition, generally divided into: digital circuits (i.e., fear of interference, but also produce interference analog circuits (fear of interference) power drive area (thousands of sources of interference).
  2. 2, To complete the same function of the circuit, should be placed as close as possible, and adjust the components to ensure that the most concise connection, at the same time, adjust the relative position between the functional blocks so that the connection between the functional blocks of the most concise.
  3. 3, For the quality of the components should consider the installation location and installation strength, heat-sensitive components should be placed separately from the temperature-sensitive components, if necessary, should also consider thermal convection measures.
  4. 4, I / 0 drive components as close as possible to the edge of the printed board, close to the lead connector.
  5. 5, The clock generator (such as: crystal or clock oscillator) should be as close as possible to the use of the clock device.
  6. 6, In each integrated circuit between the power input pin and ground, need to add a go capacitor (generally use high-frequency performance of a good monolithic capacitor); circuit board space is dense, but also in a few integrated circuits around a Vantage capacitor.
  7. 7, The relay coil to add a discharge diode (1N4148 can be).
  8. 8, The layout requirements to be balanced, sparse and orderly, can not be head-heavy or head to sink one by one need to pay special attention to the placement of the meta-components, be sure to consider the actual size of the size of the components (the area and height of the components) the relative position of the components in order to ensure that the electrical performance of the circuit board and the feasibility of production and installation of the ease of use and convenience at the same time, it should be in the context of ensuring that the principles can be embodied in the above, and appropriately modify the placement of the device At the same time, the placement of devices should be modified appropriately under the premise of ensuring that the above principles are reflected. This step is related to the board so that the whole production of beautiful, such as the same device to be placed in the whole production, the same direction, can not be placed "wrongly" the overall image and the next step of the wiring degree of difficulty, so a little effort to consider. Layout, not too sure of the place can be made first preliminary wiring, full consideration.

D. Cabling:

Cabling is the most important process in the whole PCB design. This will directly affect the performance of the PCB board.

In the PCB design process, wiring is generally divided into three realms: First of all, the cloth through, which is the most basic requirements of PCB design. If the line is not through, everywhere is flying line, it will be a piece of unqualified board, can be said to have not started.

Second, the electrical performance to meet. This is a measure of a printed circuit board is qualified. This is after the cloth through, carefully adjust the wiring, so that it can achieve the best electrical performance. Next is aesthetics.

If the wiring through the cloth, there is nothing to affect the electrical performance of the place, but at a glance over the haphazard plus colorful, colorful, that how good the electrical performance, or a useless piece of waste PCB, so that the test and maintenance to bring great inconvenience. Wiring should be neatly organized, not crisscrossed without chapter. These are to ensure that the electrical performance and meet other individual requirements to achieve, otherwise it is the end of the matter.

1, The Main Principles of Wiring:

  1. 1) In general, the first should be wired for power lines and ground lines to ensure the electrical performance of the circuit board. Within the conditions allow, try to widen the power supply, ground line width, it is best to ground line than the power line width, their relationship is: ground > power line > signal line is usually signal line width: 0.2 ~ 0.3mm, the thinnest width of up to 0.05 ~ 0.07mm, the power line is generally 1.2 ~ 2.5mm.
  2. 2) The PCB of digital circuits can be wide ground wire to form a circuit, that is, constitute a ground network to use (analog circuit), otherwise it can not be used in this way. In advance of the more stringent requirements of the line (such as high-frequency line) for wiring, the input and output side lines should be avoided adjacent to parallel to avoid reflective interference. If necessary, ground isolation should be added, the wiring of the two neighboring layers should be perpendicular to each other, and parallel is prone to parasitic bad combination.
  3. 3) Oscillator shell grounding, clock line should be as short as possible, and can not lead everywhere. Clock oscillator circuit below, special high-speed logic circuit part to increase the area of the ground, and should not go to other signal lines in order to make the surrounding electric field tends to zero.
  4. 4) As far as possible, the use of 45-degree angle folding line wiring, not to use 90-degree angle folding line to reduce the radiation of high-frequency signals: (high requirements of the line but also with a double-arc line)
  5. 5) Any signal line do not form a loop, if unavoidable, the loop should be as small as possible; signal line over the hole to minimize the number of critical wires are as short and thick as possible, and add protective measures on both sides.
  6. 6) Through the flat cable transmission of sensitive signals and noise field band signals, to use the "ground - signal - ground" way out.
  7. 7) Key signals should be reserved for testing points to facilitate production and maintenance testing.
  8. 8) Schematic wiring is completed, the wiring should be optimized; at the same time, after the initial network check and DRC check is correct, the un-wired area for the ground fill, with a large area of copper layer for the ground, in the printed circuit boards are not used on the place are connected to the ground as a ground. Or made into a multilayer board, power supply, ground occupied by a layer.

2, Wiring Process Requirements:

  1. 1) Line in general, the signal line width of 0.3mm (12mil), power line width of 0.77mm (30mil) or 1.27mm(50mil); between the line and the line and the distance between the line and the pad is greater than or equal to 0.33mm (13mil) in practice, conditions allow should be considered to increase the distance; wiring density is higher, can be considered (but is not recommended) to use the IC feet to go between the two wires, the width of the line is 0.254mm (10mil) line spacing is not less than 0.254mm (10mil) special circumstances When the device pin is denser when the width is narrower, you can press the appropriate reduction in line width and line spacing.
  2. 2) Pad (PAD) pad (PAD) and transition holes (VIA) of the basic requirements are: disk straight than the diameter of the hole to be.

Greater than 0.6mm; for example, general-purpose pin resistors, capacitors and integrated circuits, etc., using disk / hole size 1.6mm/0.8mm63mil/32mil), sockets, pins and diodes 1N4007, etc., using 1.8mm/1.0mm (71mil/39mil.). Practical applications, should be based on the actual size of the components to determine the conditions, can be appropriate to increase the size of the pad; PCB board design component mounting aperture should be larger than the actual size of the component pins 0.2 ~ 0.4mm or so.

3, Wiring Optimization and Screen Printing:

The General Design Experience Is: The time to optimize the wiring is twice as long as the initial wiring. After checking that nothing needs to be modified, it can be laid ( Place-"polygon Plane. Place-"polygon Plane." Generally, place the ground lines (be careful to separate analog and digital ground), and the power supply may be required for multilayer boards. For silkscreen, be careful not to be blocked by the device or removed by the vias and pads. At the same time, the design is looking at the component surface, the bottom of the word should be done to mirror processing, so as not to confuse the level.

Note: Formal wiring must be set up first wiring rules, the use of wiring rules to check the wiring in the error, real-time use of online inspection function to adjust the wiring.

4, Network and DRC Check and Structure Check:

First of all, in determining the circuit schematic design under the premise of no error, the generated PCB network file and schematic network file for physical connection relationship between the network check (NETCHECK, and according to the results of the output file in a timely manner to amend the design to ensure that the wiring connection relationship between the correctness; network check correctly through the PCB design of the DRC check, and according to the results of the output file Timely corrections to the design to ensure that the electrical performance of the PCB wiring. Finally, the mechanical mounting structure of the PC should be further checked and confirmed.

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