A polymer based miniature loop heat pipe with silicon substrate and temperature sensors for high brightness light-emitting diodes Huaiyu Ye a,b,c,⇑ , Robert Sokolovskij b , Henk W. van Zeijl b , Alexander W.J. Gielen c , Guoqi Zhang b a Materials Innovation Institute (M2i), Mekelweg 2, 2628 CD, Delft, The Netherlands b Delft Institute of Microsystems and Nanoelectronics (Dimes), Delft University of Technology, Mekelweg 6, 2628 CD, Delft, The Netherlands c Netherlands Organization for Applied Scientific Research (TNO), De Rondom 1, 5612 AP, Eindhoven, The Netherlands article info Article history: Received 16 October 2013 Received in revised form 20 January 2014 Accepted 20 February 2014 Available online xxxx Keywords: LED Thermal management Silicon substrate Temperature sensors Polymer loop heat pipe Color properties abstract Solid State Lighting (SSL) systems, powered by light-emitting diodes (LEDs), are revolutionizing the light- ing industry with energy saving and enhanced performance compared to traditional light sources. How- ever, around 70%–80% of the electric power will still be transferred to heat. As the elevated temperature negatively affects the maximum luminous output, efficiency, light quality, reliability and the lifetime of the SSL systems, thermal management is a key design aspect for LED products. In this work, an innovative thermal management with a package, a silicon substrate with temperature sensors and a polymer based loop heat pipe (LHP) was designed, manufactured and assembled. It can supply a low and relatively stable temperature to maintain higher optical power, more luminous flux and less color shift. In a word, the novel design can provide LEDs with the efficient thermal management and temperature monitoring with reduced weight, easy fabrication, less energy consumption and better light quality. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction In comparison to the incandescent lamp, Solid State Lighting (SSL) system, powered by light-emitting diodes (LEDs), consumes about one-tenth of the power [1,2]. Nowadays, LEDs are enabling novel applications with energy savings and environmental bene- fits. However, still around 70%–80% of the input power will transfer to heat [3]. As the elevated LED temperature directly affects the maximum light output, efficiency, quality, reliability and the life- time of the SSL systems, thermal management is a key design as- pect for LED products in terms of cost and performance [4]. The drive to obtain more light output is increasing, and consequently more heat needs to be dissipated from the LEDs. Current materials and novel thermal solutions all require heat sinks which are indis- pensable to dissipate heat to the environment. However, the bot- tleneck is always the low convection efficiency from solid surface to air [5]. Thus, more area is needed for better convection. To achieve more surface area with a solid, generally larger volume is required, resulting in bulky and heavy heat sinks, leading to unat- tractive LED products with cumbersome metal bulk. Besides, most highly thermally conductive materials are opaque and the heat sinks then completely prevent light transmission through them. Liquid cooling can enhance thermal management with simple structure, transparency and small weight [6] as it can replace the same volume of traditional metallic heat sinks. Furthermore, liquid can remove the heat rather than conduct it which performs as bet- ter ‘‘thermal conductivity’’ than solid [7,8]. Liquid cooling has al- ready been successfully applied in many semiconductor microelectronic applications [9] and will be easily used in LEDs. With exception of leakage and reliability, the driving force is major obstacle for setting the liquid cooling in the limited volume of LED product. Generally, small volume of liquid can be mainly driven by: micro-pump and heat pipe. Several micro-pumps have been intro- duced by Gravesen et al. [10], but they all require extra electric en- ergy which might deviate from the slogan of ‘‘high efficiency’’ of LEDs. Therefore, the heat pipe (HP) which is driven by the waste heat is more appealing. Especially, the loop heat pipe (LHP) possess all the main advantages of traditional HPs, but obtain special prop- erties to transfer heat for distances up to several meters at any ori- entation in the gravity field [11,12]. Polymer can be one of the best materials to archive easy fabrication, light weight and transpar- ency for thermal management of LEDs. Besides, the normal opera- tion temperature ranges of thermo plastics are quite close to SSL systems. However, poor thermal conductivity limited the applica- tions until the combinations of polymer and liquid cooling were first reported. In 2001, McDaniels and Peterson [13] presented ana- lytical modeling and estimated the thermal conductivity of a flat, flexible, polymer HP with a grooved wicking structure to be http://dx.doi.org/10.1016/j.microrel.2014.02.032 0026-2714/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Materials Innovation Institute (M2i), Mekelweg 2, 2628 CD, Delft, The Netherlands. Tel.: +31 0643810818. E-mail addresses: h.ye@m2i.nl, h.ye@tudelft.nl (H. Ye). Microelectronics Reliability xxx (2014) xxx–xxx Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel Please cite this article in press as: Ye H et al. A polymer based miniature loop heat pipe with silicon substrate and temperature sensors for high brightness light-emitting diodes. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.02.032