TT electronics IRC Advanced Film Division has developed a substrate technology that offers better thermal performance and lower cost than conventional polymer-based insulated metal substrates (IMS). Designated Anotherm, IRC’s new material consists of a thermally conductive aluminum alloy substrate insulated by a thin, chemically grown, anodized dielectric layer with solderable screen-printed conductors applied to the anodized layer. Using Gerber files supplied by the customer, IRC builds custom circuit boards with single or multiple layers. The company supplies the boards in populated or unpopulated form.
“The Anotherm process utilizes low-cost manufacturing methods which result in a product that is compatible with traditional surface-mount technology assembly methods,” said Tom Morris, applications engineering manager for IRC’s Advanced Film Division. “Because of the high thermal conductivity that the base aluminum substrate offers, in many cases the Anotherm board eliminates the need for additional heatsinks.”
Existing insulated metal substrate technology often relies on a polymer or epoxy material that is mechanically applied to a bare aluminum substrate. This method requires extensive surface preparation, as well as a minimum thickness of approximately 75 microns. Moreover, traditional IMS technology is subject to delamination at high temperatures. IRC’s Anotherm process chemically converts the surface of the base aluminum itself to a rugged anodized insulating layer with a uniform thickness of only 35 microns, giving the Anotherm substrate its superior thermal properties, as well as eliminating the potential for delamination or peeling.
Anotherm’s thermal impedance is 0.2°C/W. The thermal impedance of a typical IMS would be 30% to 50% higher, according to Morris. Maximum continuous operating temperature for Anotherm is 400°C without a solder maskant and 175°C with a solder maskant.
However, with the better thermal performance, there is a tradeoff of high-voltage performance. Anotherm’s thin oxide layer has a dielectric strength of 250 Vac versus 1 kV or 2 kV for conventional IMS. Consequently, Anotherm would be suitable for applications such as low-voltage dc-dc converters but not offline power supplies. Other applications for the Anotherm substrate include high brightness LED assemblies, LED packages and power resistors for automotive applications.
Anotherm substrates use an additive process with just three steps. First, an aluminum oxide layer is grown on the aluminum substrate. Next, conductive material is screen printed onto the oxide layer and fired. Finally, a solder mask is screen printed onto the board and cured. In contrast, twice as many steps typically are required to manufacture IMS boards, which are built in a subtractive process. Because of their simpler assembly, Anotherm boards are one-third to one-fourth less costly than IMS.
Morris notes that Anotherm is not recommended for use with through-hole components because of the potential damage that can occur to the dielectric layer when through holes are made in the substrate. Similarly, the substrates do not lend themselves to use of vias. However, multilayer boards can still be manufactured with the aid of “crossovers”. (The additional layers are created by printing dielectric material on top of the first layer of dielectric and conductors.) Also, solderable edge clips enable the top side of the board to be connected with the bottom side.