Component layout rules
Under the condition of design permission, the layout of components should be arranged in the same direction as much as possible, and modules with the same function should be arranged together; The same packaged components are placed equidistantly to facilitate component mounting, welding and testing.
Consideration of PCB size
The key factor to limit the size of PCB board is the processing capacity of cutting machine.
When the milling cutter cutting machine is involved in the selected processing technology, the size of PCB panel is 70mm × 70mm ―― 310mm × 240mm.
When the selected processing technology involves the circular knife cutting machine, the size of PCB panel: 50mm × 50mm (considering the processing capacity of other equipment) ― 450mm× 290mm. Thickness: 0.8mm-3.2mm.
PCB component layout rules
There should be at least one pair of edges on the PCB with enough space for the conveyor belt, that is, the process edge. When PCB is processed, the longer opposite edge is usually used as the process edge, which is reserved for the conveyor belt of the equipment. There must be no interference between components and leads within the conveyor belt, otherwise the normal transmission of PCB will be affected.
* The width of the process edge is not less than 5mm. If the layout of PCB can't meet the requirements, you can add auxiliary edges or panels. See "Panels".
* PCB test impedance technology edge is greater than 7MM.
* PCB board is made into circular arc angle, and the right-angle PCB board is prone to jamming during transmission. Therefore, when designing PCB board, the board frame should be treated with circular arc angle, and the radius of circular arc angle (5mm) should be determined according to the size of PCB board. The mosaic board and the PCB board with the auxiliary edge make the arc angle on the auxiliary edge.
Safe distance between components
Considering that there is a certain error in machine mounting, and considering the convenience of maintenance and visual appearance inspection, two adjacent components should not be too close, and a certain safe distance should be left.
The common feature of these two devices is four-sided lead package, but the difference is that the lead shapes are different. QFP is gull-wing lead and PLCC is J-lead. Because it is a four-sided lead package, the wave soldering process cannot be adopted.
QFP and PLCC devices are usually laid on the component surface of PCB board. If they are to be laid on the welding surface for secondary reflow soldering, their weight must meet the requirement that the bearing weight per square inch of the contact surface of the welding corner should be less than or equal to 30g.
Safe distance between components
BGA Isoplanar Array Device
BGA array devices are used more and more. Generally, 1.27mm, 1.0mm and 0.8mm ball pitch devices are commonly used. The layout of BGA array devices mainly considers its maintainability. Because of the space limitation of the hot air hood of BGA rework station, there can be no other devices within 3mm around BGA. Under normal circumstances, BGA isoplanar array devices are not allowed to be arranged on the soldering surface. When the layout space limits, BGA isoplanar array devices must be arranged on the soldering surface, and their weight must meet the aforementioned requirements.
BGA array devices can't adopt wave soldering process.
There are many kinds of devices in small package, such as SO, SOP, SSOP, TSOP, etc. Their common feature is edge-to-edge lead packaging. This kind of device is suitable for reflow soldering process, and the layout design requirements are the same as those of QFP devices. For SOIC devices with lead spacing ≥ 1.27mm (50mil) and device stand-off height ≤ 0.15mm, wave soldering process can be adopted, but the relative direction between SOIC devices and wave crest should be paid attention to.
Standoff greater than 0.2mm can't pass the peak.
SOT and DPAK devices
SOT devices are suitable for reflow soldering process and wave soldering process, and can be placed on the component surface and soldering surface during layout. When the wave soldering process is adopted, the Standoff height of the device should be ≤ 0.15mm