Multiple heat path dynamic thermal compact modeling for silicone encapsulated LEDs
Introduction
Efficient cooling of high performance solid-state lighting systems allows luminaire manufacturers to sufficiently extend product's lifetime, decrease associated manufacturing costs, boost performance, and ensure high reliability.
Detailed thermal characterization of LEDs' packages is a key step for enhancing design of cooling solutions for lighting applications. LED manufacturers do not disclose confidential information like material type, and LED structure architecture (among others including thermal architecture). This restricts LEDs and lighting systems thermal modeling. Therefore, a need for a reliable way of LEDs thermal models generation is arising.
Thermal boundary condition independent CMs were developed by DELPHI consortium to predict temperature of critical components for IC packages [1]. They remain high accuracy but require significantly smaller computation resources than full models [2,3]. These CMs' properties enable fast and reliable system level simulations. Nevertheless, these models are applicable for steady state conditions only.
The next step of compact mode development was introduction of DCTMs by adding thermal capacitances to steady state CMs [4]. ICs' DTCMs enable transient thermal behavior simulations remaining superior computational efficiency. Thermal transient analysis is a common technique for DTCM extraction [[5], [6], [7]].
The LEDs transient characterization requires advanced methodology [8,9]. LEDs are opto-electric devices. Unlike the majority of IC packages they transform significant part of the applied power to light. The efficiency of the light conversion is dependent on applied current and temperature. Therefore, special test methods and equipment were developed. Thermal testing methodology for LEDs is defined by standards from JEDEC (JESD51-51 and 51-52) [10].
A thermal transient analysis yields a thermal structure function which is a one-dimensional representation of the device's junction-to-ambient heat path. A structure function can be used to evaluate the partial thermal resistances of the heat path elements and such heat flow features as properties of radial heat spreading [11]. It enables such techniques as transient dual interface method [12] allowing to determine steady-state junction-to-case Rth of ICs.
Nevertheless, as shown in works [13,14] parasitic parallel heat paths can introduce a significant error to the structure function evaluation. Works [15,16] demonstrate that LEDs' dome is able to store significant amount of heat by acting similar to a parallel heat path. To the best knowledge of the authors there is no methodology shown to account for the structure function evaluation error caused by the dome heat storage yet.
This paper demonstrates a procedure providing multiple heat path DTCM extraction for LEDs with silicone domes. Unlike the previous publication [16] this paper offers a purely physically based approach for the silicone dome and main heat path characterization.
The proposed DTCM extraction procedure is based on thermal transient analysis of two LED's configurations. The derived DTCM can be used to enhance accuracy of the dual interface method and LEDs' main heat path characterization.
The proposed DTCM allows to predict a temperature response of the LED's main heat path elements for an arbitrary input power with significantly higher accuracy compared to the one-dimensional DTCM generated by a classical transient analysis. The increased accuracy of the main heat path modeling enables a better prediction of LEDs' failure modes related to thermo-mechanical stress such as delamination at the die attachment interface, delamination between the encapsulant and the die, ohmic contact degradation [[17], [18], [19]].
The work is done in a scope of DELPHI4LED European project. The project was established to define multi-domain LED compact models. One of its goals is to determine an optimal topology for LEDs' DTCMs [20].
Section snippets
Typical architecture of a mid-power LED
Modern LEDs dissipate significant amount of energy as heat [[21], [22], [23]]. The major heat source inside of an LED package is a p-n junction. Mid power LEDs p-n junctions are cooled primarily by thermal conduction through the sapphire/die attach/thermal pad heat path. Significantly lower amount of heat leaves the junction through other ways due to relatively low thermal conductivity of silicone and poor heat transfer from the LEDs' outer surfaces to the ambient air. Nevertheless, as will be
Finite element analysis
ANSYS software was used to perform steady-state and transient FEA. A finite element model of a typical mid-power illumination LED was used. Due to the proprietary nature of the product, the specific LED type and package details will not be described in this paper. Two LEDs dome configurations were considered: silicone dome and bare chip. The configurations are presented in Fig. 4.
The following boundary conditions were applied for the FEA. The PCB bottom side temperature was set to 22 °C.
Results
The de-embedding procedure was applied to the results of the transient thermal numerical simulations. The main and the secondary heat paths were derived. The resulting heat paths are presented in Fig. 8 and Fig. 9 in the form of structure functions.
Fig. 8 compares the de-embedded main heat path structure function with the raw structure function of the Silicone Dome LED configuration. Reference steady-state model junction-to-thermal pad heat spreading region Rth is indicated. The PCB spreading
Significance
Major improvements of the main heat path elements thermal properties estimation using the de-embedding technique can be observed in Fig. 8 data.
First, the de-embedding significantly enhances the accuracy of the main heat path thermal structures characterization. Fig. 8 light blue area represents the main thermal path estimation error when the effect of the heat propagation into the dome is not accounted for.
Junction-to-thermal pad Rth was used as a figure of merit to quantify the error. Direct
Conclusions
Structure function de-embedding technique presented in the paper enables multiple heat paths LEDs' DTCM generation based on finite element analysis. The de-embedding procedure enables a detailed analysis of both the main thermal heat path and the silicone dome effect separately, which significantly enhances inner thermal structures characterization accuracy. The increased accuracy of the LEDs' heat path elements thermal properties extraction leads to improvement of thermal failure modes
Acknowledgment
The contribution of European Union is acknowledged for supporting the study in the context of the ECSEL Joint Undertaking program #692465 (2016–2019). Additional intonation available on: www.DELPHI4LED.eu.
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