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Characterization of die-attach thermal interface of high-power LEDs: An inverse approach

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The light output as well as the lifetime of high power light emitting diodes (LEDs) decreases under a given operating current as the junction temperature increases.  A die-attach layer often creates the most significant resistance to the flow of heat from the junction to the heat spreader in high power LEDs.  Thus, characterization of the die-attach thermal interface (DTI) in high power LEDs is one of the most important tasks for assessing performance range and reliability.

The thermal resistance of the die-attach layer is governed by the bond line thickness (BLT), the thermal conductivity of the die-attach material, and the contact resistance at the die attach interfaces.  In addition, the existence of undesired but inevitable voids created by the die bonding processes serves to increase the thermal resistance between the die and the substrate, and thus gives rise to a further junction temperature increase.

It is well known that interfacial thermal resistance can be evaluated using the transient behavior of the device junction temperature, which can be expressed analytically using a model based on the thermal resistance and the thermal capacitance by proceeding carefully through a series of empirical steps.  This approach does not require the thermal properties of the materials used in the device, which is a very attractive advantage in practice.  Yet, the mathematically complex process used in this approach (i.e., differentiation and deconvolution) can lead to considerable measurement uncertainties, especially for the thermal resistance of the die attach layers.  Moreover, it is worth noting that the lateral heat spreading resistance in a high power LED is significant, and not easily captured by a 1-D model, as used in this approach.

An advanced inverse approach, based on the transient junction temperature behavior, was proposed and implemented to more accurately determine the resistance of the DTI in high power LEDs.

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