In the intricate world of PCB design, space constraints and component density can sometimes force us to make tough decisions—like whether to place vias directly on pads. This practice, while convenient for routing, raises questions about potential issues it might introduce. Let's explore when this approach is acceptable, when it's not, and what alternatives we have if necessary.In the complex realm of PCB design, space constraints are a significant consideration.
When is it acceptable to place vias on pads?
- Personal DIY Projects: In individual DIY scenarios, vias on pads may not pose significant problems, offering a straightforward solution for compact designs.
- The back of large solder Pads: For example, to improve the heat dissipation of MOSFET, it is possible to punch holes in the bonding pad of MOSFET. However, it should be noted that via processing requires uniform hole distribution to ensure uniform heating of the pad.
- Thermal Vias: Thermal vias, in this case, are thermal vias made to dissipate heat from the IC. Because there are no pins to be soldered in the middle of the chip body, there is no need to consider tin leakage, virtual soldering and other issues in the vias on the IC heat dissipation pads. *Note: The diameter of the via hole placed in the design should be as small as possible, and the pad with electrical network (such as GND net) is still forbidden to place the via hole, and the buried blind hole mentioned above can be placed.
When should you avoid placing vias on pads?
- Small Packaged Components assemble on the vias: Resistors and capacitors in small packages should not have vias on their pads due to the risk of tombstoning—a common issue in the reflow soldering process for chip components, especially with 1005 or smaller 0603 components.
What considerations should be made if vias on the pad are absolutely necessary?
- Design Blind or Buried Vias: Opt for blind or buried vias to prevent solder leakage, as they do not extend through the entire PCB. However, be mindful of the increased manufacturing costs associated with these more complex via structures.
- Wave Soldering: Consider wave soldering over reflow soldering for the component assembly process, as the larger amount of solder used in wave soldering can reduce the likelihood of cold solder joints.
In summary, the decision to place vias on solder pads should be based on a careful evaluation of design requirements and manufacturing conditions. It's essential to consider the via's location, size, and quantity to meet electrical performance and reliability standards.
*For precise guidance, consult with a PCB design engineer donnali[at]pcbonline.com at PCBONLINE to ensure your design is both innovative and reliable.
