Dielectric materials are made by blending thermally efficient materials such as alumina and boron nitride with other ingredients, to provide a flexible yet resilient coating on the base plate. An important characteristic of the dielectric layer is the amount of electrical isolation it provides between the copper on the topside and the metallic base plate on the underside. This is known as its dielectric strength. A typical dielectric material may possess a dielectric strength of around 800 V/mil and be coated onto the base plate to a thickness of 8–12 mils (1 mil = 1 inch–3 = 25.4 µm).

Dielectric materials used on insulated metal circuit boards usually have a thermal conductivity figure in the region of 3W/mK. This is approximately 10 times the performance achieved by FR4 (flame retardant woven glass reinforced epoxy resin) PCB material.

A further key requirement of the dielectric layer is to be able to compensate for the different coefficients of thermal expansion of the copper track on the topside of the assembly and the aluminum base plate/heat spreader on the bottom side.

Going three-dimensional

Flat sheets of insulated metal circuit board comprising copper foil, a dielectric layer and an aluminum base plate have been available for several years. In the eyes of the forward-thinking LED module designer, the main problem has been that flat sheets of insulated metal circuit board limit them to 2D shapes.

To address this limitation, new dielectric materials are becoming available that have a low modulus, meaning that they are compliant with mechanical stress and strain. These materials not only accommodate the coefficient of expansion of the metal elements of the construction, but also enable parts to be formed into right angles, and even through 360˚. This enables designers to realize complex-shaped designs and ones that form a complete circle with either internal or external copper traces.

When designing with new, formable insulated metal circuit board materials it is possible to route the tracks around corners, which alleviates the need to use connectors and hard wiring. There are several benefits to this, including enhanced reliability resulting from having fewer junctions and interconnects. Despite the slightly higher cost of the new materials, the overall cost is reduced because fewer components are needed, and assembly time is reduced.

Strength and durability