AN1149-4 datasheet(3/14 Pages) LUMILEDS

Part #	AN1149-4

Description	Thermal Management Considerations for SuperFlux LEDs

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Manufacturer	LUMILEDS [Lumileds Lighting Company]

Direct Link	http://www.luxeon.com

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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,

λd , as

a function of temperature is:

Where:

λd (T

1)= dominant wavelength at junction

temperature T

1

λd (T

2)= dominant wavelength at junction

temperature T

2

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).

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

g, these epoxy resins transform

from a rigid, glasslike solid to a rubbery

material. A dramatic change in the coefficient of

thermal expansion (CTE) is generally associated

with the T

g. 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, T

j , should always be

kept below the T

g of the epoxy encapsulant.

Lumileds specifies a maximum junction

temperature, T

j (max) , which is below the Tg of the

epoxy encapsulant used. For SuperFlux LEDs,

T

j (max) = 125

°C. If the T

j (max) is exceeded, the CTE

of the epoxy encapsulant will permanently and

dramatically change. A higher CTE causes the

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.

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AN1149-4 Datasheet(PDF) 3 Page - Lumileds Lighting Company

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