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18 The PCB Magazine • March 2014 THERMALLY CONDUCTIVE SUBSTRATES & THERMAL MANAGEMENT continues sulator). But that's not necessarily a disadvan- tage. It's all about applications engineering and choosing the most appropriate material for the job. For a given application, the choice of dielec- tric is determined by the need to achieve a bal- ance between thermal conductivity, dielectric strength, reliability and of course unit cost. In general, the reinforced dielectrics have lower thermal conductivity, but higher breakdown voltage and slightly better thickness unifor- mity. The cost, of glass reinforced dielectrics, is generally lower than the non-reinforced grades, which demand extremely critical manufactur- ing control to ensure uniformity of filler dis- persion and the absence of traces of particulate foreign material which could lead to premature dielectric breakdown. Also, due to thickness distribution considerations, most manufactur- ers can only laminate non-reinforced materials in small panel sizes—increasing unit cost and potentially limiting applications. Typical characteristics of IMS laminates are: Aluminium from 0.5 mm to 3.0 mm thick, available in different grades to suit mechani- cal requirements, copper from 18 um to 410 um , and dielectric thickness from <50 µm to 150 µm. Printed circuit fabrication is by con- ventional print-and-etch techniques. The alu- minium is protected during etching by a peel- able film. Single-layer technology is suitable for most applications, but multilayer constructions are feasible by sequential lamination using ther- mally conductive prepregs and thin laminates constructed from thermally conductive pre- pregs. Watt per Metre-Kelvin (W/mK) The commonly quoted unit of thermal con- ductivity. Definition: a measure of the ability of a substance to conduct heat, determined by the rate of heat flow normally through an area in the substance divided by the area and by mi- nus the component of the temperature gradi- ent in the direction of flow: measured in watts per metre per Kelvin. Symbol λ, k sometimes shortened to just conductivity, is also known as the coefficient of heat conductivity. Because it's a coefficient, it needs to be considered in con- junction with thickness. The reciprocal is, ther- table 3: Variation of thermal impedance (°c.in²/w) with dielectric thickness, for different w/mK values of thermal conductivity. to reduce the thermal impedance by half, double the thermal conductivity or halve the thickness of the dielectric.