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Component layout rules and safety distance considerations

Component layout rules

Under the condition that the design permits, the layout of same components should be arranged in the same direction, and the modules of the same function should be arranged together; the components of the same package should be placed at equal distance to facilitate component mounting, welding and testing.


PCB board size considerations

The key factor limiting the size of the PCB board is the processing capability of the board cutter.

When the selected processing process involves a milling cutter, the PCB board size: 70mm × 70mm -- 310mm × 240mm.

When the selected processing process involves a round knife type board cutter, PCB board size: 50mm × 50mm (taking into account the processing capacity of other equipment) - 450mm × 290mm. Board thickness: 0.8 mm -- 3.2 mm.


Process edge

PCB board should have at least one pair of edges to leave enough space for the location of the conveyor belt, that is, the process edge. PCB board processing, usually with a longer pair of edges as the process edge, left for the equipment conveyor belt, within the scope of the conveyor belt can not have components and lead interference, otherwise it will affect the normal transmission of the PCB board.

* The width of the process edge is not less than 5 mm. If the layout of the PCB board can not meet, you can use the method of increasing the auxiliary edge or panelize board, see " panelize board".

* PCB test impedance process edge is greater than 7MM.

* PCB board made of rounded corners, right-angle PCB board in the transmission is easy to produce a stuck board, so in the design of PCB board, the board frame to do rounded corners, according to the size of the PCB board to determine the radius of the corner of the arc (5mm). panelize board and add auxiliary edge of the PCB board in the auxiliary edge is easier to do rounded corners.


The safety distance between the component body


Considering that there is a certain error when mounting by SMT machine, and considering that it is convenient for maintenance and visual appearance inspection, the adjacent two component bodies cannot be too close, and there should be a certain safety distance.



The common characteristic of these two kinds of devices is four-sided lead package, the difference is the lead shape is different. QFP is gull wing lead, PLCC is J-shaped lead. Because of the four-sided lead package, wave soldering process cannot be used.


QFP and PLCC devices are usually laid on the component side of the PCB board. If they are to be laid on the soldering side for the secondary reflow soldering process, their weight must meet the requirement that the weight of the contact surface should be less than or equal to 30 grams per square inch of solder angle.


The safety distance between the components


BGA devices

BGA and other surface array devices are increasingly used, generally 1.27mm, 1.0mm and 0.8mm ball pitch devices are commonly used. Under normal circumstances, BGA and other surface array devices are not allowed to be laid out on the soldering surface, and when the layout space is limited, BGA and other surface array devices must be laid out on the soldering surface, and their weight must meet the aforementioned requirements.

BGA and other surface array devices can not be used wave soldering process.


SOIC devices

Small form factor packages are available in various forms, including SO, SOP, SSOP, TSOP, etc. The common feature is that they are all edge-to-edge lead packages. These devices are suitable for reflow soldering process and have the same layout design requirements as QFP devices. SOIC devices with lead spacing 1.27mm (50mil) and device standoff 0.15mm can be used in wave soldering process, but attention should be paid to the relative direction of the SOIC device and the wave.


SOT, DPAK devices

SOT devices are suitable for reflow soldering process and wave soldering process, in the layout can be placed on the component side and soldering side. When using the wave soldering process, the height of the device lift (Standoff) should be 0.15mm. Standoff greater than 0.2mm can not pass the wave soldering