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58 The PCB Magazine • July 2016 Heat transfer is the exchange of thermal en- ergy between physical systems. It always occurs from a region of high temperature to another region of lower temperature. The fundamental modes of heat transfer are conduction, convec- tion and radiation. In a PCB, the predominant mode is conduction, for which thermal conduc- tivity of the material is a good indicator. There- fore, to accompany PCB evolution, base mate- rial suppliers improved the thermal conductiv- ity of their laminates and prepregs. Early PCBs were made of bakelite, having around 0.2 W.m - 1 .K -1 thermal conductivity. Nowadays, it is pos- sible to find dielectric materials compliant with printed circuit board manufacturing process and product specifications exhibiting a higher thermal conductivity, typically in the range of 2 W.m -1 .K -1 , when copper is almost 400 W.m -1 .K -1 . Nowadays, most of the critical parameters of PCBs have reached some limits. Improv- ing the overall performances of the electronic system often means to look for some percent. This is true for PCB thickness, via size, copper thickness, or etched features. This is also true for thermal behavior. Designers are looking for some Celsius for the system operating tempera- ture. In this context, it becomes more and more important to have reliable thermal models, and therefore to feed these models with consistent physical properties, including thermal conduc- tivity. Many methods exist to measure thermal conductivity. Most of them are adapted to iso- tropic and homogeneous materials, but there is not one single easy method to measure thermal conductivity of composite anisotropic materi- als, especially dielectrics used in the printed cir- cuit board industry. Therefore, this paper aims at proposing a method for consistent and reli- able thermal conductivity measurement, adapt- ed to anisotropic dielectric materials. Thermal Science Fundamentals To understand how to optimize thermal effi- ciency of a PCB, and why thermal conductivity of dielectric materials is important, this section gives an overview of some aspects of thermal science. Standard physic model says that heat propa- gates according to three modes: conduction, convection and radiation. Heat transfer always occur from hottest region to coolest region. The first law of thermodynamics says that the total quantity of energy in the universe re- mains constant. This is the principle of the con- servation of energy. The second law of thermo- dynamics states that the quality of this energy is degraded irreversibly. This is the principle of the degradation of energy, and systems powered by electricity respect these laws. Heat is generated all along the chain. At elec- tronic system level, the main contributors are often components. The printed circuit board, supporting and interconnecting them, is also very helpful to avoid their excessive heating. Indeed, excessive temperatures degrade overall system performance, from speed to long term reliability. Therefore, electronic system design- ers take great care of operating temperatures. Equations managing thermal exchanges can be easily found in the literature. Thermal Management of a PCB A multilayer PCB is usually a sandwich mix- ing copper and glass-reinforced composites. This is an environment where the predomi- nant heat transfer mode is conduction. Inside the material, convection and radiation can be neglected. To optimize thermal efficiency of a PCB, it is therefore important to maximize heat conduction. In physics, thermal conductivity is the prop- erty of a material to conduct heat. It is evalu- ated primarily in terms of Fourier's Law for heat conduction. Heat transfer occurs at a lower rate across materials of low thermal conductivity than across materials of high thermal conduc- tivity. This is why high thermal conductivity A THERMAL CONDUCTIVITY MEASUREMENT METHOD, ADAPTED TO COMPOSITE MATERIALS

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