Start with the Resistance Chain
Heat flows from the junction through the package, thermal interface material, heat sink body and finally to ambient. These resistances add up: Rth(total) = Rth(junction-case) + Rth(interface) + Rth(sink-ambient). Define your maximum junction temperature and ambient first — the difference between them, divided by the dissipated power, gives the maximum total thermal resistance your design can allow.
Fin Geometry: Spacing, Height and Thickness
Natural convection needs wide fin spacing (roughly 6–10 mm) so air can circulate freely; forced-air designs can go tighter (2–4 mm) for more surface area per volume. Taller and thinner fins add area, but only up to the point where the fin efficiency starts to fall — a fin that conducts heat poorly at its tip is dead weight. Extrusion shines here: 6063 alloy produces thin, dense fins in one pass at low tooling cost.
Surface Area vs. Airflow: Two Cooling Regimes
In natural convection, performance is governed almost entirely by surface area and orientation — vertical fins beat horizontal ones. With a fan, the air channel geometry matters as much as area: keep fin channels aligned with the flow and watch the pressure drop. An anodized black surface also raises emissivity, adding a radiation component that helps in enclosed or outdoor enclosures.
Specifying a Heat Sink That Meets Its Number
Give your supplier four numbers: dissipated power (W), maximum junction temperature, ambient temperature, and the available envelope (L x W x H). From these we size the profile, fin pattern and surface finish, and confirm the simulated or measured Rth before tooling. Send your thermal spec to Siken Aluminum — our engineers will propose a profile, validate the thermal path, and quote the die and part together.