Finite element-based lifetime modelling of SAC solder joints in LED applications

Abstract

The integrity of the solder joint is critical to ensure long-term reliability of power-LEDs under harsh environmental conditions. Many studies have focused on prediction of the lifetime for different lead-free solder joints in temperature cycling tests especially Ball Grid Arrays (BGA), flip chip, SMD, etc [1]. However, physics-of-failure (PoF) based solder fatigue modelling is an on-going struggle for microelectronics and microsystems industry since it heavily depends on the geometry of the solder solder (i.e. package type and therefore, correct implementation of the thermo-mechanical response) and, precise knowledge of valid process dependent material properties, above all for the solder material. Further, a comparison of lifetime data between different package types depends on a stringent definition of a suitable failure criterion and evaluation of the local failure parameter. This makes it challenging to establish a reliable and universal lifetime model for a specific solder material across package types. In LEDs L2 bonding by solder attach, due to thermal management requirements the use of Insulated Material Substrates (IMS) is compulsory. The combination of Metal-Core-PCB boards and the ceramic substrate in LEDs introduces a complexity factor caused by the large E-modulus and CTE mismatch which must be considered.In this paper, a model is developed to predict the lifetime of SAC305 solder joint in a LED on board setup using accelerated tests and finite element modelling where crack length is the failure criterion and the accumulated creep strain is the computational failure parameter to quantify damage. Moreover, influence of geometrical factors on reliability of LED is investigated. Moreover, it was observed that process uncertainties such as void could largely impact the lifetime which leads us to dive further in robustness evaluation within a physics of failure reliability paradigm.