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48 The PCB Magazine • December 2017 EVOLUTION OF THERMAL MANAGEMENT IN PCBS adverse reaction if the immersion gold bath is exposed to the aluminum. The work-around is to mask the exposed aluminum with a photo polymer and/or tape. Most fabricators do not want to take the risk of a scratch or pinhole in the protective material, and will avoid offering ENIG or ENEPIG as a viable surface finish for IMS product. Lead-free hot air leveling (HAL) is the most common finish, but the designer needs to verify their finish of choice with the fab house to be safe. The thermal conductivity of most laminates is reported as the thermal conductivity of the standalone dielectric. Some specification sheets report the thermal conductivity of the laminate (copper + dielectric + aluminum). This can be misleading. As an overall performance measure of the system, the designer may want to focus on the thermal impedance vs thermal conductivity. The thermal impedance (or thermal resistance) is the inverse of the thermal conductivity times the material thickness. Thus, the thinner the dielec - tric, the lower the thermal impedance. For the total system, or at a particular joint, the thermal impedance can be calculated (see equation 1). One can see that the benefit from going from a 5-mil dielectric with 3.2 W/mK thermal con- ductivity, to a 1.5-mil dielectric with a thermal conductivity of 2 W/mK will have a 2x improve- ment in thermal impedance of the dielectric. The thinner the dielectric, the lower the ther- mal impedance. Dielectric withstanding volt- age becomes a greater consideration as designs specify thinner dielectrics. To ensure quality and reliability, it is recommended that IMS laminate be 100% high pot tested by the laminate manu- facturer prior to shipping. This will ensure there are no pinholes or foreign material in the thin laminate that could cause a high pot failure. Traditionally, thermally conductive dielec- trics have been used in PCB designs demand- ing high current loads. Motor controls, power supplies, AC/DC converters and solid-state re- lays were the major industry applications. As discussed, the LED industry has been the driv- ing factor for the growth experienced over the last 10 years. Recently, we have seen a growing interest for thermally conductive dielectric solutions in full build multilayer PCBs. The thermal en- ergy output of components can be more effec- tively managed within the PCB versus use of expensive fans, heat pipes and liquid immer- sion cooling. In partnership with Advanced Mi- cro Devices Graphic Card Division, Lazer-Tech PCB and Cartel Electronics, 6-layer and 12-lay- er, HDI, impedance-controlled PCBs were built using 2.2 W/mK and 3.2 W/mK thermally con- ductive dielectric systems throughout the stack. A 16% reduction in the thermal resistance was realized with the PCBs made with the thermally conductive dielectric as compared to the PCBs made with FR-4. This improvement in thermal impedance allows tremendous opportunities in cost savings long term. Much more work is needed to fully appreciate the benefits of these materials in full build com- plex PCBs. As technologies continue to evolve the need for thermal management solutions in PCB design are only going to increase. PCB Reference 1. Fact or Fiction: LEDs don't produce heat, Tim Whitaker, LEDs Magazine. Gareth Parry is president of Draig Technologies Inc, technical consult- ing services for the electronics, PCB and design industries. Equation 1.