LED display unit board PCB design rules

LED display unit board PCB design rules

1. General principles

(1) When the design rules are met, the production cost should be reduced as much as possible. For example, if two-layer boards can be used, try to use two-layer boards; when the cost conflicts with the design rules, the design rules should be guaranteed.

(2) The components are arranged neatly, and the same chips are arranged in rows and columns according to certain rules; the spacing between components must consider the production process and cannot affect welding.

(3) For PCB design, automatic routing cannot be used for design.

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2. Details

(1) Use of wiring layer

When designing a circuit board with more than 2 layers, it is prohibited to use the inner electrical layer. Use inner layer definitions like normal layers.

For unit boards that require blind and buried via design, for four-layer boards, punch holes 1-2, 3-4, 1-4; for six-layer boards, punch holes 1-2, 2-5, 5-6; specifically The process must be confirmed with the PCB manufacturer.

(2) Digital devices and driving devices are powered separately

It is necessary to use separate wiring methods for digital ground and analog ground. The power supply of the digital part is only connected to the power supply of the driving part through a jumper at the entrance of the power socket.

If there is space on the board, it is recommended to separate the column driver power supply and the row driver power supply.

(3) Power cord and line cord:

The power supply trunk width of the digital part is >=50mil.

The width of power lines and row lines should take into account the current, width, and distance of the power branches. Ensure that the voltage drop on the line from the power inlet to the module is not greater than 0.01V.

It is recommended that the power socket be placed in the middle of the board. When the power base is placed to one side, reserve a power base or pad on the symmetrical side to facilitate flying wires during production.

When wiring is difficult, power supply pads are reserved on the board to facilitate flying wires during production.

The circuit board where the row driver is located must have a separate power socket or pad.

The circuit board where the column driver is located must have a separate power socket or pad.

When using non-soldering methods (pin headers, hole rows and cable rows) to connect power supplies and row lines, the current of each pin should not be greater than 1A, and there should be at least two pins for each connection.

For users who already have light boards, we will discuss them separately.

It is recommended that the row pipe be placed in the middle of the row line within 1/3 of the entire unit board floor.

For outdoor screens, if the light panel driver board is separated, the row tube should be placed closest to the row cable introduction pin.

(4) Ground wire requirements

The width of the ground trunk line in the digital part is >=50mil.

The ground line of the column driver should be wider than the power line of the row driver. It is recommended to be at least 1.5 times the width of the power line.

The ground wires of the unit board should be laid out in a checkerboard shape, with each branch connected.

The driving part is paved with copper ground, and the digital part is paved with grid ground. The distance between the ground and other parts is set to more than 20mil. The line width of the grid ground is 25mil, and the center spacing is 40mil.

When paving the floor, avoid dead corners without loops. After the paving is completed, the divided parts will be connected by manual paving.

To simulate the ground, place the test ground pin at the power inlet.

The digital ground places the test ground pin between the input pin header, output pin header and 9702.

(5) Capacitor requirements

At the power inlet of the drive part and digital part, place 100u electrolytic capacitors and 104 capacitors.

Place 100u electrolytic capacitors at the entrance and end of the power cord branch.

It is recommended to leave a 100u electrolytic capacitor near the driver chip.

Leave a 104 capacitor between the power pin of the serial converter and parallel driver chip and ground.

(6) Chip power pin outlet

The power pin outlet of the serial-to-parallel chip and row driver chip is at least as wide as the pin pad.

(7) Column lines and signal lines:

The width of the light panel, column lines and signal lines of the outdoor screen is >=12mil. When the PCB size is large, the signal line width is recommended to be 15mil.

In other situations, it is recommended that the width of column lines and signal lines be 12mil, or at least 10mil.

The line spacing is not less than the minimum line width.

(8) Via:

For the light panel of the outdoor screen, the via parameters are 50 mil and 28 mil.

In other situations, the recommended via parameters are 50 mil and 28 mil. At least 40 mil, 24 mil.

The distance between vias and lines should not be less than the minimum line width.

The outer diameter of the power trunk via is the power line width, and the inner diameter of the power trunk via is >= power line width*1/3. Recommended via inner diameter: 28mil, 40mil.

Large power and ground wires can increase the number of vias.

To ensure welding, do not place vias on the pads. At least 5mil away from the pad.

(9) Plug-in soldering device pad:

The via holes of the soldering pads of the device must be inserted to ensure that there is no problem with the device insertion.

The inner diameter of the ordinary pin header hole is 40mil, and the power socket pad hole is 60mil.

(10) Installation holes:

For unit boards using LED modules, the mounting holes should not be located at the junction of the two modules, and there should be no copper foil around the holes to prevent the mounting holes from being connected to the DC ground of the unit board (for last resort requirements required by the customer, the customer’s signature is required ).

General components should be placed 3.5mm away from the center of the fixing hole, and taller components (such as capacitors, plug-in chips) should be placed more than 5mm away from the center of the fixing hole.

When the user does not have clear needs, the cascade pin header and power socket should be placed 2cm away from the horizontal and vertical connections of the fixing holes.

When designing a PCB for a specific customer, it is necessary to clarify the customer’s fixing hole location and fixing structure to determine the placement of pin headers, power sockets and other devices.

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(11) Module

Socket-soldered components cannot be placed at the module boundary.

It is recommended to use “queue placement” or grid placement for modules to avoid errors.

The PCB edge should be at least 20mil smaller than the module.

The module is built from the front and positioned on the back when placed. This can prevent the front pads of other devices from being covered by the silk screen on the module boundary.

The module placement center distance should be point spacing * 8. When the module accuracy is too low to guarantee this accuracy, the center distance of module placement should be confirmed with the customer.

(12) Optimization:

After the layout design is completed, it should be reviewed and the routing optimized.

The width of the power cord should be increased and the ground wire should be laid out in a checkerboard shape as much as possible.

(13) Board layout sequence:

Pre-wiring should be carried out before board layout, and the routing of column lines should be adjusted. You can go back and modify the schematic diagram and network table to make the routing of column lines the most convenient. When laying out the board, first lay out the routing wires, power wires, ground wires, cascade signal wires, and then other wires.

(14) After the PCB design is completed, there should be a description of the unit board PCB processing process requirements, including: PCB version thickness requirements, PCB version copper foil thickness requirements, PCB version minimum line width, PCB version minimum spacing, PCB minimum via parameters. It is convenient to select the board casting manufacturer during the board casting process.

(15) After the PCB design is completed, a component list should be made and the various component packages should be clearly marked. If there are reserved positions for components that do not need to be soldered during production, this should be specially indicated. The components list should also include a brief description of soldering precautions.

(16) Silk screen layer

After the design of the unit board is completed, the unit board information, including the board number, basic parameters of the unit board, and completion date, is marked on the TOP layer of the unit board.

The unit board should have obvious cascading direction signs at prominent positions at the cascade entrance and cascade exit; there should be obvious signs indicating the cascade pin header and pin 1 of other pin headers; for the positive and negative plugs of the power socket There should be an easy-to-observe +- mark next to the pin, and there should be obvious indications of the welding direction; for empty pads designed for jumpers, obvious network name identification should be placed; for unit boards with separate light boards and driver boards , there should be obvious connection direction indications on both the lamp board and the driver board, or asymmetry should be made in the design.

For components with positive and negative poles, the direction of the positive and negative poles must be reflected on the layout. Such as diodes, voltage regulator tubes, electrolytic capacitors, power sockets, etc., and the placement direction indicates that they cannot be covered after welding is completed;

For unit boards with virtual pixels, in addition to the welding direction of the LED, there must also be obvious R/G/B marks. For unit boards with surface-mounted LED lights that cannot be screen-printed on the bottom layer, the LED lights must be explained in detail in the production instructions. arrangement.

In order not to affect the welding, the characters on the silk screen layer cannot overlap with the pads.

After the drawing is completed, place the name of the IC under the IC; for 62706 or 595, place its control color logo next to the IC;

After the PCB design is completed, place optical positioning points on the diagonal line of the PCB.

(17) Testing

DRC testing is required after the design is completed. When converting to PROTEL2.8 after using PROTEL99 and POWERPCB to make drawings, DRC testing must also be done.

When drawing using PowerPCB software and importing it into Protel 2.8 format, it should be noted that sometimes the via holes on the board will change size. After drawing the picture using Protel99 software and importing it into Protel2.8 format, the vertically placed FILL may be changed to horizontally placed. These are things to pay attention to when drawing.

When drawing a PCB, for components with positive and negative poles, the direction of the positive and negative poles must be reflected on the layout. Such as diodes, voltage regulator tubes, electrolytic capacitors, power sockets, etc.

For two-pin components, you must no longer use the ordinary 2-pin component library of SIP2 or IDC2. You must use a dedicated component library.

For the power socket or power pad, the network name (or the positive and negative polarity signs) must be marked on the silk screen layer.

For single row needles and double row needles, there must be an obvious mark on one leg.

For other components, there must be an obvious pin mark or welding direction mark.

For unconventional usage, it must be explained in the production instructions.

For unit boards with virtual pixels, in addition to the LED welding direction, there must also be obvious R/G/B marks. For surface-mounted unit boards that cannot be screen-printed on the bottom layer, this must be explained in detail in the production instructions.

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