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110 The PCB Magazine • December 2015 tend to be tougher and more robust, but can sag and distort when the density climbs above the glass transition temperature of the resin. With this recognition in the 1970s, there was a rise in interest in the use of metal more specifically steel as a substrate for the manufacture of circuits which were subject to high thermal loads includ- ing hybrid circuits. Early steel circuits were com- monly coated with porcelain enamel and then printed with conductive inks, which were fired in place creating a permanent bond between cir- cuits and the porcelain enamel coating. The practice of using porcelain enamel to coat kitchen cookware had been in use for many decades, so the inspiration may well have come at the dinner tables of more than one researcher of the time. While the first metal circuits were limited to one or two sides of the substrate, in- creases in integration at the level of the IC and increase use of multilayer circuits to intercon- nect them led some researchers to conclude that the marriage of multilayer circuits and metal sheets could produce real benefit. Early, simple solutions often involved the bonding metal sheet to the side opposite the components in a post-assembly operation, however interest soon turned to the idea of placing metal at the center of the structure and adding layers of circuitry to both sides and interconnecting them by plated through holes (Figure 2). The circuits of the day were commonly re- ferred to as metal core boards (MCB). (Note: other terms are also presently used, including: IMS—insulated metallic substrate; MCPCB— metal core printed circuit board; and MiB— metal in board. Metal core boards were of interest to the aerospace industry as a means of extracting heat from the electronic assembly through the board and into the racks, to which the circuit assemblies were mounted. One of the major challenges of assembling a metal core board is overcoming the risk of forming cold solder joints owing to the fact that a substantial ther- mal spreader lies just beneath the components and proximate to the plated through holes in the assembly. Given that, special processing considerations are a requirement, including substantial preheating before soldering. Metal cores in printed circuits have not been limited just to heat management however. In the early days of surface mount technology, copper clad Invar was called upon to provide a CTE matching base for mounting ceramic components owing to the industry's experienc- ing failures with ceramic components mounted directly to unrestrained resin-glass laminates such as FR4. Metal core technology held sway in many applications until surface mount technol- ogy using compliant lead frame constructions (e.g., PQFP, TSOP, etc.) which could ameliorate the CTE mismatch problem were adopted. While metal core boards have generally been relegated to niche status within the elec- Figure 2: Multilayer circuit board with a metal core for either thermal management, dimensional control or a combination of the two. e.i. Files INSULATED METAL BASE CIRCUITS—AN ENABLING TECHNOLOGY FOR POWER ELECTRONICS

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