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PCB-Dec2017

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18 The PCB Magazine • December 2017 cesses can be another factor to consider when selecting an IMS baseplate material. As Table 1 shows, 5052 and 6061 aluminum alloys are highly suited to bending or CNC machining. Advances in Dielectric Materials The dielectric material may be a woven-glass- reinforced resin composite (prepreg) as in a con- ventional laminate construction, or a layer of unreinforced resin. Because the nature of the glass reinforcement tends to limit the thermal conductivity, non-reinforced dielectrics have the lowest thermal resistance. However, these de- mand critical control in manufacture to main- tain consistency of dielectric thickness, whereas glass fabric provides a natural mechanical spacer. Extra care is also needed, during manufacture, to ensure uniform distribution of the thermal filler particles and prevent inclusion of foreign mat - ter that may compromise the dielectric strength. Compared to a conventional FR-4 lami- nate, thermal resins are loaded with ceramic filler particles to a proportion as high as 70%, which significantly increases thermal conduc- tivity. At the same time, the resin is responsible for the adhesion of the circuit foil to the met- al baseplate and must withstand potentially se- vere thermal cycling. Even when loaded with ceramic particles, the dielectric has lower ther- mal conductivity than the metal baseplate and circuit foil layer; hence, improvements in the dielectric materials can have a significant im- pact on IMS thermal performance. The latest high-thermal-conductivity dielec- tric materials now provide several options for designers to stack multiple circuit foils, each separated by a thin, thermally conductive di- electric layer, and bonded to a copper or alumi- num metal base. This contrasts with historical constraints when designing with IMS designs, which has typically been compatible only with single-layer circuit designs, and hence allows space-saving designs or more complex circuitry within standard size enclosures. With these new materials, designers of au- tomotive systems can take advantage of oth- er important new options such as hybrid sub- strates featuring selective localized thermal en- hancement. As reliance on autonomous-driv- ing modes increases, moving towards fully self-driving vehicles in the future, we anticipate demand for logic and processing circuitry to be mounted on the same board as high-power con- verters or inverters in a single consolidated as- sembly. This could help overcome tight size and cost constraints, and is now achievable using today's highest performing dielectric materials such as Ventec's VT-5A2 as a thin core or as a glass-reinforced prepreg substrate. State-of-the- art manufacturing processes allow this material to encapsulate up to three-ounce copper foils, giving high power-handling capability. As a guide to the performance that can be expected from today's most advanced thermal dielectrics, thermal conductivity of 2.2W/m.K is eight times greater than ordinary FR-4, and maximum operating resilience to withstand electronic surface-mount assembly processes, including glass-transition and decomposition temperatures (Tg, Td) up to 190°C and 365°C, and extended time to delamination at peak lead- free reflow temperature (T300 > 15 minutes), as measured using standard IPC test methods. Conclusion Proper thermal management is critically im- portant in emerging power electronics applica- tions in the automotive, LED-lighting and re- newable-energy markets. Effective removal of heat is vital to prevent excessive operating tem- peratures and so maintain optimum reliability. Considering thermal management early in the development cycle is recommended, to meet cost and time targets as well as reliability. To- day's most advanced insulated metal substrates, featuring the latest thermal dielectric materi- als, give designers more choices in the quest to maximize performance, save space, and simpli- fy design and assembly. PCB References 1. Eurotech: Institute of Circuit Technology Northern Seminar 2016, Harrogate Mark Goodwin is COO of Ventec. UNDERSTANDING THERMAL MANAGEMENT AND MATERIALS TO BOOST POWER ELECTRONICS RELIABILITY

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