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.
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).
The equation for dominant wavelength, λd , as
a function of temperature is:
Where:
λd (T1)= dominant wavelength at junction
temperature T1
λd (T2)= dominant wavelength at junction
temperature T2
Temperature-Induced Failures of LEDs
LEDs are typically encapsulated in an optically
clear epoxy resin. At a certain elevated
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 wearꢀout and
breakage of the wire. Wire bond breakage results
in an open failure.
temperature, known as the glass transition
temperature, Tg, these epoxy resins transform
from a rigid, glassꢀlike solid to a rubbery
material. A dramatic change in the coefficient of
thermal expansion (CTE) is generally associated
with the Tg. The Tg is calculated as the midpoint
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, Tj , should always be
kept below the Tg of the epoxy encapsulant.
Lumileds specifies a maximum junction
temperature, Tj (max) , which is below the Tg of the
epoxy encapsulant used. For SuperFlux LEDs,
Tj (max) = 125 °C. If the Tj (max) is exceeded, the CTE
of the epoxy encapsulant will permanently and
dramatically change. A higher CTE causes the
Figure 4.2 Expansion-Temperature relationship for
clear, epoxy, LED encapsulants.
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