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56 The PCB Magazine • July 2016 by François Lechleiter, CIMULEC GROUP and Yves Jannot, UNIVERSITÉ DE LORRAINE, LEMTA CNRS Abstract Most of today's printed circuit board base materials are anisotropic and it is not possible to use a simple method to measure thermal conductivity along the different axes, especial- ly when a good accuracy is expected. Few base material suppliers' datasheets show X, Y and Z thermal conductivities. In most cases, a single value is given, moreover determined with a ge- neric methodology, and not necessarily adapt- ed to the reality of glass-reinforced composites with a strong anisotropy. After reminding the fundamentals in ther- mal science, this paper gives a short overview of the state-of the art in terms of thermal con- ductivity measurement on PCB base materi- als, and some typical values. It finally propos- es an innovative method called transient fin method, and associated test sample, to per- form reliable and consistent in-plane thermal conductivity measurement on anisotropic PCB base materials. Introduction Observed on a long enough time scale, elec- tronic systems behave like living species; they obey the laws of evolution. To be successful, they have to adapt to their environment, which means to market requirements, and thus to consumer expectations. Any system with a new, more powerful functionality naturally surpass- es the previous generation. As consumers, we normally prefer smaller, lighter, faster, more re- liable and cheaper electronic systems. This has led the electronic industry since its beginning. The PCB progressively becomes a little bit more than just the backbone of electronic sys- tems. At the beginning, the printed circuit board was providing essentially an electrical function, by interconnecting electrically components to- gether, and a mechanical function, by support- ing mechanically the components and holding them into a defined volume. Progressively, evo- lution towards microwave applications brought electromagnetic functionalities to the PCB. In addition, the constant increase of power den- sity made the PCB more and more capable of providing solutions for efficient thermal man- agement strategy. A Thermal Conductivity Measurement Method, Adapted to Composite Materials Used in the PCB Industry FEATURE

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