Design methodologies for reliability of SSL LED boards J. Jakovenko a,⇑ , J. Formánek a , X. Perpiñà b , X. Jorda b , M. Vellvehi b , R.J. Werkhoven c , M. Husák a , J.M.G. Kunen c , P. Bancken d , P.J. Bolt c , A. Gasse e a Czech Technical University in Prague, Faculty of Electrical Engineering, Technicka 2, Praha 6, Czech Republic b Centre Nacional de Microelectrònica, CNM-CSIC, Campus UAB, 08193 Bellaterra, Barcelona, Spain c TNO, P.O. Box 6235, 5600 HE Eindhoven, The Netherlands d Philips Lighting, LED Platform Development, Mathildelaan 1, 5611 BD Eindhoven, The Netherlands e CEA-Leti, MINATEC Campus, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France article info Article history: Received 31 August 2012 Received in revised form 1 February 2013 Accepted 23 February 2013 Available online 4 May 2013 abstract This work presents a comparison of various LED board technologies from thermal, mechanical and reli- ability point of view provided by an accurate 3-D modelling. LED boards are proposed as a possible tech- nology replacement of FR4 LED boards used in 400 lumen retrofit SSL lamps. Presented design methodology can be used for other high power SSL lamp designs. The performance of new LED board designs were evaluated by numerical modeling. Modeling methodology was proven by measurement on reference FR4 LED board. Thermal performance was compared by extracting of LED boards thermal resistances and thermal stress has been inspected considering the widest temperature operating range according to standards (40 to +125 °C). Thermo-mechanical and reliability analysis have been per- formed to study parameters of each LED board technology, using thermal boundary conditions extracted from the thermal simulation of a whole LED lamp. Elastic–plastic analysis with temperature dependent stress–strain material properties has been performed. The objective of the work is to optimize not only the thermal management by thermal simulation of LED boards, but also to find potential problems from mechanical failure point of view and to present a methodology to design SSL LED boards for reliability. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction SSL (Solid-state lighting) lamps are a fast emerging technology which is considered as the true alternative for the compact fluores- cent lamp (CFL) and incandescent lamp [1]. SSL technology uses semiconductor light-emitting diodes (LEDs) as light sources com- pared to incandescent electrical filaments in GLS or plasma in CFL lamps. The demand for more efficient lighting systems using LED technology has favoured such development. LEDs can have a lifetime up to 50,000 h. However, the lifetime strongly depends on LED die temperature [2]. Explicitly, generated heat and temper- ature changes of LED dies directly affect both short-term and long- term LED performance [3]. The effects in short-term performance have impact in colour shifting and light output reduction. The long-term performance caused by lumen degradation in time (age- ing process) reduce useful lifetime. LED operation at high temper- ature (above 125 °C) radically accelerates lumen degradation and reliability. The light output is then a function of temperature and time. When the light output decreases in time to an unacceptable level, the SSL lamp is considered to have failed. ASSIST (Alliance for Solid-State Illumination System) defines lifetime as the time in which light output has degraded to 70% of the initial light output [4]. Therefore, good thermal management of LED lamps is of para- mount importance and it is mandatory for the determining process of an optimal design for reliability of all other SSL lamp parts. Incandescent lamps emit mostly in the IR (Infra-Red) range, with only a small amount of visible light. On the contrary, SSL lamps generate all light in the visible spectrum, having negligible emission in the IR or UV (Ultra Violet) range. This is a main advan- tage above incandescent and fluorescent lamps. On the other hand, only 20–30% of the electrical power supplied to LED lamp is con- verted into visible light. The rest is transformed into the heat that must be conducted from the LED die through LED package to the underlying LED board or from the driver to heat sink and housing. This is a reason why thermal management is perhaps the most important aspect of successful SSL lamp system design. Heat dissipation and LED temperature were not a problem in the low power LED scenario. However, in today’s high power LED applications thermal management issues must be identified and taken into account [5] to ensure not only light maintenance and quality of LED lamps, but also their lifetime and reliability. SSL lamp materials and electronics can age at high operational temper- ature degrading the light output [6]. For this reason, leading man- ufacturers of high-power white LEDs keep junction temperatures below 120 °C [7,8]. The LED junction temperature depends on the 0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.02.017 ⇑ Corresponding author. E-mail address: jakovenk@fel.cvut.cz (J. Jakovenko). Microelectronics Reliability 53 (2013) 1076–1083 Contents lists available at SciVerse ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel