In the design of Printed Circuit Board, apart from the schematic design made by hardware engineers and the layout, layering and wiring design made by pcb engineers, the structural engineers should not only participate in the design at the beginning, but also plan and constrain the board size, shape, installation method, location and number of installation holes, etc., and form a drawing file, which will be used as input condition files with the board design files output by hardware engineers and pcb engineers. The design process will be repeated until the final production version of the board is determined.
Some design points in this process are summarized as follows:
Board size and shape
The board size is determined by two factors, one is the size of the whole equipment; Second, the design of board function principle. For example, a board needs a certain number and types of components in order to realize a certain function, so the size of these components puts forward a basic requirement for the board size. If the board size is small, these components will not fit.
The shape of the board, if there are standard requirements, will be based on the standard. For example, the size of 3U board of standard VPX bus is 100mm * 160mm, and the size of 6U board is 233.35mm*160mm, etc. If it is a non-standard board, the shape of the board can be determined by combining the shape of the chassis or the cold plate.
open holes
The installation holes shall be arranged evenly as far as possible, and the spacing between holes is generally designed to be about 40mm, in order to improve the rigidity and stress uniformity of the board.
Position of connector, guide sleeve/pin
The connector that plugs into the motherboard or backplane should be placed according to relevant standards. If the connector itself does not have a guide device, it is necessary to design the guide device on the daughter board and the back board respectively. For example, cpci connector has its own guide device, while vpx connector has no guide device.
External connector location and installation
The position of the external interface connector led out by the board depends on a. the size of the connector itself and b. the wall thickness of the chassis. The local wall thickness of the chassis here should also consider the spacing requirement of connector plug and socket.
The installation of the connector depends on whether it has its own mounting bracket and mounting hole. If not, a mounting bracket should be designed structurally to a. fix the connector, and b. serve as a tool for connector welding to ensure the accuracy of its welding height.
The spacing of connectors should be considered so as not to interfere when plugging in the plug, and a certain operating space should be left. When placing the connector, try to make the direction of pin 1 uniform.
Position of power socket on board
Generally, the power socket on the board is designed for two purposes, one is to be the power input for single board debugging, and the other is to provide power for the cooling fan.
The structure should consider the position of the power socket, which is generally placed at the edge of the board. If the heat dissipation mode of the board is to install the whole cold plate, then it should be considered that the power socket should have an avoidance hole at the corresponding position of the cold plate. If the debugging power socket is not well arranged on the front of the board, you can ask the hardware engineer to place it on the back of the board. The power socket of the fan is close to the fan position.
Board positioning hole
If the installation accuracy of the board is very high, the positioning hole of the structure can be designed on the board, and the requirement of metallization pad is not required.
Device height limit requirement
Some areas on the board should be marked in the drawing if there is a requirement for the height of components. For example, in the travel area of self-made non-standard extractor, the height of components should not interfere with the handle of extractor.
If the socket on the board needs cold plate opening, whether the opening size is too large, and whether it affects the layout of cold plate reinforcing ribs.
In addition to keeping a safe distance from the electrical components, it is also necessary to consider the dispensing position of the components, especially the fluidity of the glue, and do not cover the pad of the mounting hole and other areas with overlapping surfaces with the structure.
Whether there are cables on the board, if there are cables, binding holes should be reserved on the board, and the bending radius of cables, smooth routing, interference with other components, and the length of cables should be reserved with allowance for assembly and disassembly.
Devices with large calorific value should be placed close to the cold end of the heat dissipation path. When pcb layout is reviewed, heat dissipation should be considered emphatically.
Use surface-mounted devices as much as possible to improve the mechanical resistance of the board.
Copper laying requirements, such as the area where cold plates are installed on both sides of the board, and the copper laying rate is an important factor affecting the thermal conductivity of the board in the plane direction.
Thickness requirements, the standard board thickness is generally 1.6mm；; The thickness of non-standard board depends on the number of board layers and board size.
The cloth is forbidden, and the area overlapped with the structural parts; The mounting hole requires that the size of the metal pad be larger than the outer diameter of the flat washer, etc.