For a vertically mounted PCB with a MOSFET dissipating around 5 W, the enclosure has no fan. Should the heatsink fins run vertically or horizontally for better natural convection? Are there any general guidelines for fin spacing, and how does the recommendation change with forced airflow?
For natural convection on a vertical PCB, orient the fins so the channels run vertically. This allows cooler air to enter from below and warm air to rise through the channels (chimney effect). With horizontal channels, the rising air is blocked, which reduces cooling. The size of the difference depends on the heatsink geometry.
For spacing, natural convection generally works best with wider gaps. With forced airflow, you can use denser fins, but align the channels with the airflow direction and watch the pressure drop.
Also keep in mind that in an enclosed case, trapped warm air will hurt performance more than fin orientation. Vents at the bottom and top help.