Transient thermal analysis for accelerated reliability testing of LEDs

Abstract

Reliability of LED light sources is essential for many general and automotive lighting application where exchange of LED modules would be expensive. Reliability testing to predict the life-time of the devices is required. In this paper transient thermal testing is applied and further developed to monitor the structural integrity of the LED modules during accelerated stress testing. The temperature shock test is one important test because thermo-mechanical stress is a dominating root cause for structural failures as delamination in the package or cracking of solder joints between package and board. New flip chip LEDs (WLP-LEDs) are assembled directly on printed circuit boards (PCB). The initial assembly quality is tested using X-Ray, optical inspection and transient thermal analysis (TTA). In combination the methods deliver essential data to ensure initial assembly quality. Afterwards, the LED modules are exposed to temperature shock tests (−40°C to 125°C) and the TTA is performed repeatedly. By TTA the structural integrity is resolved and cracks or delamination can be identified. Failure modes can be separated and the root cause is investigated by support of transient finite element analysis. One test board is measured In-situ during temperature shock testing. Due to differences in thermo-mechanical stress under hot and cold conditions additional information can be obtained.