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December 2017 • The PCB Magazine 15 UNDERSTANDING THERMAL MANAGEMENT AND MATERIALS TO BOOST POWER ELECTRONICS RELIABILITY nent, and the thermal conductivity of the ma- terial. It can be expressed as: Where: l = thickness, A = surface area, and λ = thermal conductivity of the material. Clearly, minimizing the thickness and max- imizing the surface area can both contribute toward lowering the thermal resistance, in ad- dition to choosing a material with the high- est practicable thermal conductivity. Design- ers can trade off each of these properties against the others, to achieve the desired performance, bearing in mind that material selection can be governed by factors such as cost, electrical prop- erties like isolation, mechanical properties like weight or corrosion resistance, or compliance issues like RoHS or REACH legislation. It can also be appreciated that parts with the highest thermal resistance have the greatest im- pact on the overall performance of the stack. Improving these can significantly enhance the effectiveness of the assembly in removing heat from the die. Enhancing Substrate Performance The industry-standard FR-4 fiber-glass/res- in laminate commonly used as the main sub- strate of an ordinary printed circuit board has relatively poor thermal conductivity compared to metals or ceramic-filled thermal materials. To enhance the thermal performance of a sub- strate directly beneath a power component, through-holes, or vias, may be drilled in the FR-4 and metal plated to improve heat transfer into a heatsink or baseplate. Alternatively, the FR-4 may be milled beneath the component, and a metallic "coin" inserted in the recess. This effec- tively reduces the thermal resistance contribut- ed by the low-conductivity FR-4, by reducing its thickness, and replaces the low-conductivity ma- terial with a metal of much higher conductivity. Although these approaches are effective in reducing the total thermal resistance of the sub- strate, disadvantages include the fact that the extra processes carried out to create the vias or insert a metal coin are time-consuming and so add to the cost of the assembly. An alternative is the insulated metal sub- strate (IMS). These were originally developed in the mid-1960s, and comprise an upper foil layer in which the electrical circuit is formed (usual- Figure 2: A simple model containing thermal resistances to aid steady-state analysis.