Failure mode and test method of sulfur induced light decay in lead-frame LED package

Mid-power LED in a lead-frame package is the most commonly used LED for LCD backlighting and retrofit LED light source such as light bulb and linear light tube replacement. The lead-frame package consists silver plated copper and white plastic housing, both for the purpose of increasing light reflection. The silver can gradually become darkening when reacting with certain organics gas or chemical compounds, especially at high temperature and under light radiation. This resulted in light decay of LED product. Sulfur and its oxides are common pollutants in the atmosphere, which will form black silver sulfide with the silver plating. The blackening phenomenon is usually more severe in the silver layer of leadframe around the heat source, the LED chip where temperature and light radiation are highest. Even under the condition without light, samples after sulfur exposure treatment for only dozens of minutes will indicate obvious blackening phenomenon surrounding the LED chip.This paper will focus on the sulfur induced LED packaging failure. The failure mechanism of silver darkening will be discussed. Due to the lack of reliability test standard in a sulfur containing environment, it is hard to run an accelerated reliability test for a given condition. In this paper, we will try to develop a test method to effectively evaluate the susceptibility of a LED package to sulfur and to estimate its lumen reduction rate. Finally, we will give some recommendations for metal coating layer, encapsulant material, and surface treatment to reduce the sulfur impact. With a proper design, right material selection, and controlled temperature conditions, lead-frame LED package can be reliability used in most lighting fixtures and display applications.