Transient Thermal Analysis as a Highly Sensitive Test Method for the Reliability Investigation of High Power LEDs During Temperature Cycle Tests

Abstract

AbstractÀLight emitting diodes (LEDs) are today standard and mature light sources. They have several key advantages such as small size, low energy consumption, and long lifetime. However, high reliability of the LED system is required to achieve long lifetime of the light source. Thermomechanical stress due to temperature cycling causes failure of electronic systems. The electronic component itself or the interconnect device, for example, the printed circuit board (PCB), might fail. In many cases, the weakest link is found to be the solder interconnect between the package and the board. Cracking of the interconnect causes an open circuit and the system fails. In this paper the existing methods are compared in order to investigate LED interconnect failures during temperature cycle tests such as the simple light-on test, electric resistance measurement, and the shear test. This paper describes and introduces transient thermal analysis as a measurement method. This paper presents the first reliability data analysis with transient thermal analysis of ceramic high power LED packages on printed circuit boards, that is, insulated metal substrates, during the air-to-air thermal shock test (2408C to +1258C) and correlates it with cross sections. This work demonstrates the sensitivity of the thermal analysis to detect solder joint failures of the assembly. This paper compares the results with electric resistance measurements and light-on tests and shows that the resolution is significantly higher compared with the methods applied today. The sensitivity of the method enables the detection of a crack in a solder joint much earlier than the final failure of the joint, that is, the open circuit.

Transient thermal analysis as measurement method for IC package structural integrity

“…The LED packages A were soldered on Al-IMS boards. [13] Two test batches were assembled: one with SAC305 (Sn96.5/Ag3/Cu0.5) and another one with Innolot (Sn91, 175/Ag3, 5/Cu0, 7/Ni0, 125/Sb1, 5/Bi3). Every test group contained 40 LEDs.…”

“…The main results were published recently. [13,33] By normalization, the reference to an R th real value is lost. However, it can be reestablished by known good reference samples.…”

“…[12] Later, the method was improved and extended to investigate the reliability of the ceramic LED packages on printed circuit boards (PCB) during reliability tests. [13] Instead of the normal time derivation logarithmic time derivation was used and the method of normalization was improved. The predominant PCB for LED modules is insulated metal substrates (IMS) us-068105-2 ing aluminum as metal core (Al-IMS).…”