Heatsink Design A practical Approach. Sridevi Iyengar Global Application Engineer Sapa Profiles. Agenda. Introduction Heat sinks and Heat Transfer mechanisms Why use a heatsink Some facts you (N)ever wanted to know about heatsink Thermal Interface materials Liquid coolers
Global Application Engineer
Aluminium alloys are the
dominating materials for air-cooled heat sinks
Copper (pure): 395 W/mK
The governing equation which correlates the total power, temperature difference and the thermal resistance can be expressed as
The thermal resistance is analogous to the electrical resistance used in Ohm’s law.
Rj-c is the Junction to case thermal resistance. Usually a parameter that is published by the component manufacturer
Rc-s is the thermal resistance across the thermal interface material between the heatsink and the component.
Rs-a is the thermal resistance of the heatsink.
Junction to Ambient is the sum of the resistances
Tj, Rjc and Q will be provided by the component manufacturer.
Rcs – Thermal resistance of the interface material
Ta – Ambient temperature
Ta and Rcs are parameters that we can control to a certain extent
Rsa is the number that will help us identify a heatsink that will meet our criteria.
Optimal operating region
Characteristic curve of the fanAir-cooled heat sinks forced convection - fan curve
Air flow ∝ n (rpm)
Pressure drop ∝ n2
Noise ∝ n3
Heat flowFin efficiencyApparent cooling area vs. effective cooling area
q = h·A ·(Ths-Tair)
When there is a significant gap between the heatsink and the top surface of the enclosure air will bypass the heatsink. This reduces the performance of the heatsink. Bypass effect is more pronounced in heatsinks with closely packed fins.
Here the air is forced to go through the heatsink and in this case the performance of the heatsink is optimised.
Heat sourceConical fins vs. rectangular fins
Conical fins seems have some advantages when only heat flow is considered
Die casting always need a relief angle !
When both air flow and heat flow are considered, rectangular fins are better
Heat sourceConduction, contact surface
Actual contact area
< 2% of apparent contact area
Circulating fluid driven by pump
Heat absorbed at source by “cold plate! Or “water block”
Heat rejected to ambient by “heat exchanger” or “radiator”
Multiple heat sources possible in series or parallel
May also include two phase flow, evaporating at heat source, e.g.
ThermsyphonWhat is “liquid cooling”?
15Liquid cooling: Channel design is important.
A rotating tool is plunged into the joint line and moved along the joint. Neither flux nor filler material are used.
Friction Stir welding method of joining is based on the fact that the metal is subjected to heavy plastic deformation at high temperatures, but lower than the melting point.
When the rotating tool is plunged into the metal, friction heat is generated. The tool produces severe plastic deformation under high pressure, during which the weld interfaces are stirred together and a homogenous structure is formed.
Process results in completely pore-free,tight joints with a high strength
Minimum heat influence on the material
Good mechanical properties