The trend toward higher power densities in electronic applications places ever greater demands on heat management: It is becoming a critical factor in ensuring performance, reliability and longevity – especially in high-power applications such as industrial drives, automotive systems and power supplies.
MOSFETs are traditionally cooled via the solder connection between the bottom side of the package and the printed circuit board (bottom-side cooling). The heat generated by the semiconductor chip is conducted through the drain pad connection to the printed circuit board and then from there to a heat sink or cooling plane, typically achieved through an array of thermal vias. This method has several limitations, particularly a comparatively high heat resistance. This results from multiple interfaces – from chip to package, from package to printed circuit board and from there to heat sink – and depends significantly on the PCB material used (e.g., FR4), as well as on the number and placement of the thermal vias. Furthermore, heat dissipation is impacted by the PCB area and layout.
A way to improve heat dissipation is to cool both the top and bottom of the package. In this case, the heat is dissipated both via the bottom of the package to the PCB and via the top of the package to a heat sink. However, this solution also presents several disadvantages:
- The complexity of mechanical integration and the soldering process increases, as additional heat sinks or thermal pads are needed.
- The additional cooling measures result in higher costs.
- There are also increased space requirements.
- Further, it creates higher mechanical stress due to the mismatch in thermal expansion between the two sides of the component.
Advantages of top-side cooling
Top-cooled MOSFETs are designed with the drain exposed on the top of the package. This allows the drain to directly contact a heat sink or cooling plate, thus significantly reducing heat resistance.
This approach presents several key advantages: Firstly, overall heat resistance is lower, as the heat path is shorter and offers higher thermal conductivity, as thermal vias are not required to connect the heat sink, for example (see Fig. 1). Secondly, due to more efficient cooling performance, developers can increase the power output without exceeding thermal limits, thereby boosting power density. Thirdly, top-side cooling simplifies the printed circuit board design, as the bottom side can be used fully for electrical connections and is not needed for complex thermal via structures.
Moreover, lower operating temperatures help to extend the lifespan of the components and improve overall system reliability. Yet another advantage is that the heat sink can be mechanically decoupled from the printed circuit board. This helps to reduce stress on the solder joints and to prevent deformation or cracks during temperature cycling.
Comparison of proven advantages
The top-cooled PowerPAK-8x8LR package from Vishay uses power clips instead of wire bonds to minimize both electrical and thermal resistance. Additionally, it is equipped with gull-wing connections for mechanical relief (strain relief) (see Fig. 2). The exposed drain clip on the top side of the package provides a heat path with low heat resistance and forms the basis for the top-side cooling property. This allows the component to achieve greater performance when a heat sink is mounted on the top. Since the heat is dissipated directly to the heat sink, the printed circuit board is no longer the primary heat path. This eliminates the need for thermal vias in the area of the MOSFETs on the printed circuit board. When combined with the miniaturization of the other components, it results in a lower copper content of the printed circuit board and less costs.
The direct comparison of various cooling concepts clearly shows the advantages of top-side cooling (see Fig. 3). Three cooling concepts were compared in a test setup: A conventional MOSFET package with top-side cooling (heat sink and airflow), the same package with an additional cooling connection to the PCB by means of a thermal pad (17 W/mK) and the top-side-cooled PowerPAK 8x8LR package with optimized top-side cooling. While there are no discernible differences between the cooling concepts at low currents, the top-side cooling approach shows a clear advantage at higher currents. At 110 A, the standard package with only top-side cooling reaches roughly 115 °C, with additional PCB cooling around 105 °C. The PowerPAK 8x8LR with top-side cooling, on the other hand, only reaches just over 80 °C. The results prove: The effective dissipation of heat losses via the exposed drain pad on the top side is much more efficient than conventional heat dissipation via the printed circuit board. As such, the performance limit of the system can be increased without the need for any additional thermal measures.
To ensure the integrity of the connection to the printed circuit board and high reliability at printed circuit board level, the package was also subjected to rigorous stress testing. A maximum junction temperature of up to 175 °C results in a longer lifespan than components with lower temperature limits.
Future-proof cooling concept
Top-side cooling is especially advantageous in devices such as high-power modules, automotive inverters and server power supplies, where space and thermal constraints are critical. Through the advancement of package technologies, MOSFETs with top-side cooling are becoming increasingly popular and cost effective, making them suitable for a wider range of applications. In addition to the 8 mm x 8 mm PowerPAK 8x8LR (see Fig. 4), Vishay offers alternatives in various sizes to meet different power requirements, such as the 5 mm x 7 mm PowerPAK SO-10LR and the 10 mm x 15 mm PowerPAK 10x15LR, both of which will be available later on this year.
While bottom-side cooling and double-side cooling still have their place in power electronics, top-side cooling has become an attractive alternative due to its clear advantages in terms of performance, design flexibility and long-term reliability.
For more information and a direct ordering option, please visit our e-commerce platform at www.rutronik24.com.
Subscribe to our newsletter and stay updated.