The motherboard manual for my VIA board cautioned that proper airflow is a requirement for fanless.
So I have the board in a vertical mount, with lots of case holes above/below the board to allow a chimney effect to circulate fresh air across the heat sinks.
This was a big reason why I used a nice, roomy cookie tin, rather than some pre-built case that crams everything into a tight space with no potential for fanless airflow.