Whether vias can be placed on pads in PCB design depends on the specific application scenarios and design requirements.
For personal DIY projects, placing vias on pads may not cause significant issues.
However, in SMT assembly production, this practice can lead to the tombstoning effect. The surface tension of the solder will pull the component upright, resulting in cold soldering, desoldering, poor contact and other problems. This phenomenon is more likely to occur with small package components such as SMD capacitors and SMD resistors. To prevent this, it is sometimes necessary to open windows for the copper-poured pins.
In addition, poor via plugging can cause solder leakage, which further exacerbates the tombstoning effect. Therefore, the position of vias must also be considered from a process perspective.
To ensure the reliability of PCB design and the smooth progress of the manufacturing process, vias should be avoided from being directly placed on pads as much as possible. Nevertheless, placing vias on pads is acceptable and poses no major issues in the following two scenarios:
Scenario 1
Drilling vias on the back of large pads is permissible – for example, drilling vias on the pad of a MOSFET to improve its heat dissipation performance. It should be noted that vias on large pads must be arranged evenly to ensure uniform heating of the pad during soldering.
Scenario 2
Drilling vias on thermal pads is allowed – such as thermal vias for IC heat dissipation. Since there are no solder pins in the central area of the chip body, vias on IC thermal pads do not need to consider problems such as solder leakage and cold soldering.
In summary, the decision to place vias on pads requires evaluation and judgment based on specific design requirements and process conditions. Meanwhile, parameters such as the position, size and quantity of vias must be taken into account to meet the circuit performance and reliability requirements. In actual operation, it is recommended to consult professional PCB design engineers or PCB manufacturers for accurate suggestions and guidance.
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