PCB007 Magazine

PCB-Mar2014

Issue link: https://iconnect007.uberflip.com/i/271756

Contents of this Issue

Navigation

Page 17 of 124

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.

Articles in this issue

Archives of this issue

view archives of PCB007 Magazine - PCB-Mar2014