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38 DESIGN007 MAGAZINE I JANUARY 2022 at much higher voltages is required to ensure that PCBs are suitable for high power applica- tions. So, the chosen base material must show a CAF resistance performance at voltages of 1000 V or beyond. A material may be sufficient for high-speed digital application that operates at lower voltages, but as voltage levels increase for power distribution a higher voltage level will be needed for them as well. We have learned that the step from 100 V to 1000 V test voltage has led to the forma- tion of a failure mode that is not new but very rarely seen when using the 100 V CAF test. Copper migration through the resin has been observed when dielectric thickness is insuf- ficient. is mode has not been sufficiently investigated within the industry and requires more study. e root causes, factors affecting copper growth, as well as accelerating factors and the failure mechanism itself, are currently under extensive investigation. Furthermore, the conditions of an adequate accelerated life test for different high voltage devices are not yet determined. Test voltage levels range between 250 V up to 3000 V which is the current lab capability that is available. Also, the com- mon 1000 hours of test duration are extended to 2000 or 3000 hours. Reliability over an extended life cycle of an electric vehicle is the primary concern. For a combustion engine, car operation time was driving time. However, an electric vehicle is not only powered during a drive, but the system is also operating during the charging process. e operating time of the high voltage systems is increased and thus the 1000-hour ALT, which corresponds to 10,000 hours of operating time under accelerated conditions, must be extended for a longer period. As already mentioned for the 100 VDC CAF test conditions like voltage, the duration, fea- ture spacing and the construction of the TV are not standardized and differ depending on the OEM-specific requirements. Overall, there is still uncertainty about which test conditions and acceleration factors reflect real applica- tions. For the base material supplier, it is prob- lematic and indeed risky for the supplier to make a general statement about high voltage CAF resistance without appropriate testing of the material using a standardized test vehicle and test conditions. Current Carrying Capacity With the increase in voltage levels, higher current carrying capacities are needed to power electronic devices. Heavy copper is the common technology used to enable this with copper weights of 3 oz. up to 12 oz., which can make power electronic PCBs very heavy. Cop- per has excellent thermal conductivity which helps to dissipate the high amount of heat induced by power losses. Heavier copper has challenges in processing beginning with etch- ing of inner layer structures. e process can be slow and side wall features require a well- controlled etching process to ensure reliability. Furthermore, these structures must be filled properly during multilayer production, there- fore a prepreg with high resin content and suit- able flow behavior is required. Dynamic and static thermal loads during testing or operation of these devices can cause cracks at the heavy copper flanks and corners within the resin. Resin systems which show a higher resistance against cracking and crack propagation have an advan- tage when used within heavy copper boards. Today's commonly used semiconductors are silicon based. This semiconductor material is approaching its performance and efficiency limits.

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