Light-emitting diodes (LEDs) suffer
from heat problems that understandably
can limit their success as a light
source. Much attention is given to
the heatsink, and less is given to the
layers and barriers between the LED
and the heat-dissipating surface.
A change of concept and material
allows for significant gains in
thermal management and reliability
in addition to a simplified system.
Using ceramics as a heatsink, circuit
carrier, and part of the product design
requires some fresh thinking and the
willingness to overcome traditional
patterns.
A simulation process based on computational fluid dynamics
(CFD) supports thermal optimization and technical product
design. This article will explain the theoretical approach, the
proof of concept, and how to ultimately achieve those improvements
with ceramic heatsinks.
WHAT'S HOT
LEDs are known to be efficient and are favorites among designers
for being tiny. But they’re only really “tiny” as long as heat
management isn’t involved. While incandescent light sources
work with temperatures up to 2500°C, LEDs are much colder.
Thus, many designers ultimately realize that heat is such an issue.
Being relatively cold, LEDs still do produce heat, which isn’t
yet a problem. However, they’re based on semiconductors that,
roughly speaking, allow temperatures below 100°C.
According to the law of energy conservation, the thermal energy
must be transferred to the surrounding area. The LED can only use
a small temperature gap between 100°C of the hot spot and 25°C
ambience temperature, offering just 75 Kelvin. Consequently, a
larger surface and powerful thermal management are needed.
TWO OPTIMIZATION BLOCKS
Looking at Figure 1, Group 1 is the LED itself and mainly
remains untouchable. Its center is a die and a heat slug—a copper
part that connects the die with the bottom of the LED. Thermally,
the ideal solution is to bond the die directly to the heatsink. Due
to mass production, this concept is commercially unrealistic. We
consider the LED a standardized “catalog” product that can’t be
modified. It’s a black box.
Group 2 comprises the heatsink, which transmits energy from
a heat source to a heat drain. Usually, the surrounding air is either
free or forced convection. The less aesthetic the material, the more
it needs to be hidden. Yet the more you hide it, the less efficient the
cooling. Alternatively, pleasing and worthy materials can be used.
These can be directly exposed to the air and become part of the
visible product design.
In between Groups 1 and 2 is Group 3, which provides mechanical
connection, electrical isolation, and thermal transmittance.
That seems contradictory, since most materials with good thermal
conductivity conduct electricity, too. Vice versa, almost every
electrical isolation material translates into a thermal barrier.
The best compromise is soldering the LED to a printed-circuit
board (PCB) that’s glued on the metal heatsink. The original
function of a PCB as a circuit board can be maintained. Although
PCBs exist with various thermal conductivities, they remain an
obstacle to thermal transfer.
THERMAL RESISTANCE FOR VALID SYSTEM COMPARISON
The thermal resistance of LEDs
(die to heat-slug pad) and heatsinks
can be obtained from the
manufacturer. However, there’s
little focus on Group 3 and its
significant influence on the total
thermal performance. When adding
all thermal resistances but the
LED (Group 1), you get the total
thermal resistance (RTT) (Fig. 2).
The RTT allows a real comparison
of heat.
CERMAICS: TWO JOBS IN ONE MATERIAL
It’s common to optimize only the heatsink. Hundreds of designs
are available, essentially consisting of aluminium. But for further
improvement, it’s necessary to advance or even eliminate the third
group. Electrical isolation has to come from the heatsink itself via
other materials. Our conclusion is ceramic. Ceramics, e.g., Rubalit
(Al2O3) or Alunit (AlN), combine two crucial characteristics—
they are electrically isolating and thermally conductive.
Rubalit has a lower conductivity than aluminium, while
Alunit’s is slightly higher. On the other hand, Rubalit is less
expensive than Alunit (Fig. 3). Their thermal expansion coefficient
is adapted to semiconductors. Also, they are rigid and
corrosion-resistant, and they comply with the European Union’s
Restrictions on Hazardous Substances (RoHS). Completely inert,
they are the last part of a system to give out.
The simplified construction (without glues, insulation layers,
etc.) combined with a direct and permanent bond between the
high-power LED and the ceramic heatsink create ideal operating
conditions for the entire assembly. Put simply, what isn’t there
won’t wear out, and materials that expand in proportion to each
other won’t separate. The result is excellent long-term stability,
secure thermal management, and high reliability. A patent has
been filed for this concept, named CeramCool.
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