Reliability of SAC+ Solders for LED Packages

Abstract

Thermo-mechanical reliability is one major issues in solid state lighting industry which is currently still not solved satisfyingly for automotive applications. Mismatches in the coefficients of thermal expansion (CTE) between high-power LEDs packages and the printed circuit board, typically aluminum IMS (Al-IMS), in combination with high operation temperature and large temperature changes cause thermomechanical fatigue of the solder joints. The cracks in the solder joint results in an increase of the junction temperature and thus a reduction in lifetime and a decrease in light output. The LED package manufacturer optimize the packages for thermal performance and reliability. Improved SAC+ solders potentially increase reliability and reduce crack growth. A lager reliability study is performed including a total of 1800 LEDs, segmented by nine LED types and five SAC+ solder pastes. The effectiveness of the metallurgical modifications of different SAC+ solders is investigated. The LEDs are soldered on Al-IMS. To analyze the package design dependency different LED packages, e.g. ceramic and lead frame LEDs are included in the study. Quality inspections is performed by transient thermal analysis (TTA), scanning acoustic microscopy (SAM) and Xray. The initial thermal resistance of the LED modules and packages were separated by applying the dual thermal interface method. For most LED packages good agreement with the Rth in data sheets could be found but also incorrect data sheets are identified. The crack formation in the solder joints during temperature shock tests (-40 °C/125 °C, 30 minutes dwell time) is tracked by TTA measurement and SAM. First of all, it could be proven that in high stress applications the most important parameter is the silver content of the solder. Second, in the high stress application the high silver containing SAC308+SbBiNi (also called Innolot) performs best. However, for low stress lead frame packages the SAC105 performs still sufficient after the presently reached 500 cycles.