Fluid–solid coupling thermo-mechanical analysis of high power LED package during thermal shock testing

Abstract

The virtual design by numerical simulation to model various accelerated reliability testing conditions is adopted to validate and improve the reliability of the high power LED package. In this study, the reliability of the high power LED package during thermal shock testing is investigated by fluid–solid coupling thermo-mechanical modeling by considering nonlinear time and temperature dependent material properties. Through fluid–solid coupling transient thermal transfer analysis, it is found that the maximum thermal gradient exceeds 75 K during the rapid cooling process and 91 K during the rapid heating process of the thermal shock testing which is ignored in the traditional isothermal assumption. The calculation results indicate that the equivalent plastic strain range of the bonding wire within the LED package with consideration of the temperature gradient is much higher than that with the isothermal assumption. The assumption of the isothermal condition is not appropriate which will lead to overestimation of the predicted lifetime. The viscoelastic behaviors of the silicone have significant influences on the lifetime prediction of the bonding wire and silicone with low elastic modulus and coefficient of thermal expansion (CTE) can significantly enhance the reliability of the bonding wire under the thermal shock loading. The results in this study could provide a guideline on design for reliability in the high power LED packaging.

Introduction

Solid state lighting in terms of high power LEDs will become the fourth generation illumination source due to its superior characteristics such as high efficiency, low power consumption, high reliability and long life [1], [2]. Currently, the high power LEDs have found the applications in general illumination, automotive front lighting, backlighting for LCD displays and city improvement engineering [3], [4], [5]. The theoretical viewpoint is that LEDs can work as long as 100,000 h under the perfect operating conditions. However, the material degradation and structural defects due to the electrical, thermal, mechanical and chemical stresses will lead to the lumen degradation, color variation or even early catastrophic failure of the LED modules [6], [7], [8], [9]. Therefore, the reliability of the high power LED package is becoming an urgent issue for the emerging illumination applications.

Due to large mismatch of CTE between different materials used in the LED package, significant thermo-mechanical stresses will be induced under thermal loading which might lead to failures such as cracks in the materials, delaminations at the different interfaces, and facture failures of the boding wire [8], [9], [10]. Therefore, thermal shock testing from 233 K to 398 K was adopted to evaluate the capability of the LED package to withstand the thermal loading by many vendors.

The LED package structure, in particular the bonding wire, suffers significant thermo-mechanical stresses and strains during the thermal shock testing. The deep insights of the potential failure modes of the bonding wire may be obtained from the accurate nonlinear finite element simulation. An approach which combines temperature cycle failure data and finite element analysis results to estimate bonding wire fatigue parameters and mean time to failure was developed by Chidambaram [11]. Thermo-mechanical modeling was conducted to simulate the thermal cycling induced stresses and strains in the bonding wire of the chip on broad (COB) package by Pang et al. [12]. Different finite element techniques including contact algorithm, 2D approach, and 3D global local modeling were employed to study the thermo-mechanical behavior of the bonding wire embedded in the electronic package by van Driel et al. [13].

In addition, considering that significant thermal gradient will be induced to the LED package during thermal shock testing, the uniform temperature assumption may not be appropriate. The thermal gradient will induce additional stresses and strains which can significantly shorten the fatigue lifetime of the bonding wire. The influences of thermal gradient on the reliability performance of electronic package have been considered by some studies. The thermo-mechanical responses of a ceramic ball grid array (CBGA) package under convective condition with different assumptions such as isothermal, constant heat transfer coefficient and local heat transfer coefficient were studied by Hong and Yuan [14]. An integrated CFD thermo-mechanical analysis was performed to evaluate the transient effects of thermal shock on the reliability of the PBGA package by Mercado et al. [15]. In their study, the sequentially coupled heat transfer and thermal stress analysis was performed by applying the heat transfer coefficients boundary condition which was obtained from the CFD calculation. And it was demonstrated that the assumption of uniform temperature distribution was not sufficient for determining the critical stresses leading to die cracking. However, few works have been reported on the fluid–solid coupling thermo-mechanical analysis of the electronic package reliability during thermal shock testing which can avoid conducting heat transfer simulation by setting local time dependent heat transfer coefficients.

In this paper, a fluid–solid coupling thermo-mechanical modeling by using nonlinear time and temperature dependent material properties was established to investigate the reliability of the high power LED package during thermal shock testing. The viscoelastic behaviors of the silicone were characterized by dynamical mechanical analysis (DMA) method and expressed by Prony series based on the Maxwell model. The temperature dependent CTE of the silicone were determined by the thermal mechanical analysis (TMA) testing. The sharply temperature change and the significant thermal gradient were obtained by transient thermal transfer analysis with the fluid–solid coupling method. The stress and strain behaviors of the LED package, especially the bonding wire, were analyzed through thermo-mechanical calculation results. The effects of thermal gradient within the LED package during the thermal shock testing were evaluated comparing to the results from the thermo-mechanical analysis with isothermal assumption. The effects of silicone viscoelastic behaviors on the stress and strain responses of the bonding wire were also estimated by comparing to the results from the thermo-mechanical analysis with only temperature dependent elastic material property. The effects of the elastic modulus and CTE of the silicone on the reliability performance of boding wire were also analyzed.