AN1149-4 Lumileds Lighting, LLC, AN1149-4 Datasheet - Page 3

• Manufacturer Part Number: AN1149-4
• Description: Thermal Management Considerations for SuperFlux LEDs
• Manufacturer: Lumileds Lighting, LLC

Change in Dominant Wave-length (Color) as a Function
of Junction Temperature
The junction temperature of LEDs also affects
their dominant wavelength, or perceived color.
The equation for dominant wavelength,
a function of temperature is:
Where:
λ
temperature T
λ
temperature T
Temperature-Induced Failures of LEDs
LEDs are typically encapsulated in an optically
clear epoxy resin. At a certain elevated
temperature, known as the glass transition
temperature, T
from a rigid, glass like solid to a rubbery
material. A dramatic change in the coefficient of
thermal expansion (CTE) is generally associated
with the T
of the temperature range at which this change
in CTE occurs, see Figure 4.2.
To avoid catastrophic failure of LED packages,
the junction temperature, T
kept below the T
Lumileds specifies a maximum junction
temperature, T
epoxy encapsulant used. For SuperFlux LEDs,
T
of the epoxy encapsulant will permanently and
dramatically change. A higher CTE causes the
d (T
d (T
j (max)
1
2
= 125 °C. If the T
)= dominant wavelength at junction
)= dominant wavelength at junction
g
. The T
1
2
g
j (max)
, these epoxy resins transform
g
g
of the epoxy encapsulant.
is calculated as the midpoint
, which is below the T
j (max)
is exceeded, the CTE
j
, should always be
λ
g
d , as
of the
3
A rule that is easy to remember is the dominant
wavelength will increase one nanometer for every
10°C rise in junction temperature. In most
designs of red automotive signal lamps, this
change in color is not important because the
allowed color range is very large (approximately
90 nm). However, for some amber automotive
signal lamps, this color shift can be a concern
and should be accounted for where the allowed
color ranges are small (approximately 5 to 10 nm
depending on the regional specifications).
epoxy encapsulant to expand and contract more
during temperature changes. This causes more
displacement of the wire bond within the LED
package, resulting in a premature wear out and
breakage of the wire. Wire bond breakage results
in an open failure.
Figure 4.2 Expansion-Temperature relationship for
clear, epoxy, LED encapsulants.