As vehicles transition from being driven by mechanical power to being driven by electrical power, the electrical systems necessary for the vehicles bring several new challenges to manufacturers. Among them, the printed circuit board or the PCB is a critical component.
According to PCB Trace Technologies Inc, on the PCB level, few of the challenges requiring innovative solutions are miniaturization, low inductance, thermal management, and high current handling. PCB manufacturers are meeting the above requirements through newer technologies like chip embedding. They are embedding thin bare dies of power semiconductors within the PCB layers. This is a powerful alternative, fast replacing conventional power electronic modules.
The automotive industry is under tremendous legislative pressure to achieve CO2 reduction targets. Therefore, they are offering new solutions via hybrid and electrical drives for electrifying automotive applications. As a result of high power demands, there is increasing challenge for handling high currents and thermal management of dissipative power losses.
Within the vehicle, power semiconductors convert the power from batteries. For this, manufacturers use power modules made from substrate materials using ceramics or a PCB. The substrate helps handle high currents, manage heat dissipation, and allows operations at high switching frequencies. This is the optimum way it supports the electrical conversion of energy.
The cost advantage of PCBs over ceramics has led to the former achieving an increasing share in the power conversion applications within electric vehicles. Using ceramic substrates in power stages almost always requires an additional control board, with the related interconnection devices like cables, connectors, and plugs. On the other hand, using PCBs, it is possible to combine the power stage and the control board in one single substrate. For power electronic substrates in the automotive industry, PCB technology is advancing in various directions:
The automotive industry has been using heavy copper PCBs for some time, mostly in relay and fuse boxes. With the increase in electrical power in many applications, this technology is experiencing a revival. Using heavy copper layers as power lines has the additional advantage of reducing parasitic inductance, as it is possible to stack the conductors one above the other in multilayer boards. PCB manufacturers often realize up to four layers with 12 oz copper in the inner layers, leading to a potential carrying capacity of more than 1000 A. While the inner layers are 400 µm thick, manufacturers must keep the heavy copper in the outer layers below 150 µm. Failing to do so requires additional efforts for the solder mask process to provide an adequate electrical insulation.
Heavy copper technology has a disadvantage. It is not possible to etch fine-pitch structures along with heavy copper. Therefore, in most power electronics systems, it is customary to have a separated control board using regular copper thickness for assembly with surface mount technology, and a power stage with a heavy copper design. This requires an installation space with adequate area to host both boards, including any connectors interconnecting them.
PCB manufacturers have developed the power combi-board to achieve the requirements of both in one structure. They install heavy copper in the inner layers alongside the standard copper construction. A common outer layer using SMT compatible copper thickness serves to provide the electrical connection for the entire board.
However, the insulating layer between the heavy copper layers acts as a barrier for optimal heat transfer in the z-axis. As heavy copper PCB technology is useful for managing high currents, proper heat dissipation requires other technologies such as Insulated metal substrates and inlay technology.
Consisting mainly of a metal heat sink, an insulated metal substrate has a thin insulation layer separating the single copper layer on the top from the metal heat sink. This construction is very useful for simple designs hosting many heat generating components. However, for more complex designs, a single layer routing may not be adequate, and more than one layer may be necessary.
Most insulated metal substrate designs use aluminum as the heat sink. This reduces the weight, but introduces high CTE, thereby decreasing the reliability of the design. To improve the reliability, designers use copper as the heat sink material. This also helps to improve the thermal capacity of the board.
Heat must travel from a hot component to the heat sink in the shortest possible way, as this minimizes the thermal resistance. In most cases, heat travels in the z-axis, starting from the hot component in the assembled top side of the PCB, passing through the board before reaching the heat sink at the bottom of the board.
Rather than fixing a heat sink, PCB manufacturers now laminate a massive copper inlay within the PCB. This reduces the thermal resistance substantially. Apart from using the inlay as a sink for dissipating heat, it is also possible to use the inlay for carrying high currents, as its ohmic resistance is low.
However, conventional technologies encounter limitations when installing in restricted and confined spaces, especially when the power density is high. For saving space, PCB manufacturers require miniaturization, and they achieve this by installing some components inside the board, rather than mounting them on its outer surface.
For improving the heat dissipation from a hot component inside the PCB to the heat sink, manufacturers must use a power semiconductor with a lead frame. This acts as a heat spreader, reducing the thermal resistance significantly. A heavy copper layer at the top helps connect the contacts using micro vias filled with copper in place of bond wires that the conventional power modules use. This technology not only helps with heat dissipation, but also improves many electrical parameters like:
On-State Resistance: Chip embedding practically eliminates bond wires and the associated package resistance. However, the exact on-state resistance value depends on the generation of the semiconductor technology, its voltage class, and the type of the package.
Thermal Resistance: The lead frame provides excellent heat spreading, thereby improving the thermal resistance of the system significantly. Moreover, the thermal capacity of the lead frame also improves the robustness of the device and its thermal impedance.
Switching Performance: The top of the chip has an almost flat connection with the vias, thereby achieving a very low parasitic inductance value. This also leads to very short distances between the power semiconductor and the DC-link capacitors. The net effect of the above is enabling faster switching with substantially lower losses. This is especially true for modern fast-switching technologies employing SiC and GaN semiconductors.
Miniaturization: Present and future applications often need a reduction in form factor while requiring to provide additional functionality. Chip embedding helps to achieve saving valuable space at the PCB level.
Higher Reliability: Replacing ceramics or bond wires helps to substantially improve the reliability of the system. For instance, power cycling tests with a 120 K temperature difference on boards using embedded technology showed they were able to withstand more than 700,000 active cycles.
Cost Reduction: Considerable cost savings are possible through chip embedded technology. This comes from overall space savings, built-in insulation, lower EMC issues, smaller passive components, power components requiring lower chip surface area, optimized cooling, and the savings on cables and connectors.
Chip embedding technologies for PCBs enhance the performance of power electronic applications. This new technology has very low parasitic inductance, thereby supporting low-loss switching at high frequencies. This is highly desirable when using wide band-gap semiconductors and for the next generation of automotive drives using SiC and GaN devices.
The built-in insulation helps in assembling Smart Pack components directly on the heat sink. Depending on the requirement, the TIM, or thermal interface material, can be either electrically conducting or non-conducting.
Current sensing using shunts is a common practice. Manufacturers use shunts for measurements of phase currents in electrical motors in EVs. Shunts, being relatively large components, are good candidates for miniaturization efforts. By embedding a shunt as a Smart Pack component improves its heat dissipation dramatically. This increases the possibility of using shunts for measuring currents as high as 300 A. For improving the reliability, manufacturers replace solder joints to the shunt and the board circuit with micro-vias.
According to PCB Trace Technologies Inc, new PCB technologies are supporting electric vehicles in many ways. Not only are they minimizing form factors, they are increasing the system performance and reliability by reducing the cost on the system level. Embedding a power electronic device within the PCB is helping to replace the conventional power module, improve the system performance, and its reliability significantly. This is useful not only for the low voltage applications, but also for high current usage, as well as for high voltage applications with wide band-gap devices.