When designing a PCB with high-speed components like a 600MHz ARM processor and DDR3 RAM, it’s important to consider which layer to place these components on. The PCB will be housed in a plastic enclosure that isn’t very airtight, so airflow might be limited.
Intuitively, it seems like placing the components on the top layer would allow heat to dissipate upwards through convection. Conversely, placing them on the bottom layer might trap heat, as the warm air wouldn’t have an easy path to escape, potentially causing the components to overheat.
Does this airflow effect actually exist, and if so, does it significantly impact the operating temperature of the IC packages? I’d appreciate any insights or experiences you might have on this issue.
Yes, the convection effect you described does play a role in heat dissipation, even with passive cooling. For example, some manufacturers of passively convection-cooled PSUs recommend installing them on vertical surfaces to enhance airflow.
In the context of an IC mounted on a horizontal PCB, placing the components on the top layer can indeed facilitate better heat dissipation through natural convection. However, it’s important to note that the majority of heat is conducted away by the copper planes of the PCB and can be managed with additional cooling solutions, such as heat sinks or thermal vias.
Even if an IC is mounted on the bottom layer, effective thermal management can still be achieved through these methods. Therefore, while placing high-speed components on the top layer can be beneficial for natural convection, the overall heat management strategy and use of thermal solutions will have a significant impact on maintaining optimal operating temperatures.
Convection is a circulating of heat in a medium (air in this case), and whether you heat it from below as in the case when the hot component is on the top side of the board, or heat it from the top as in the case when the hot component is on the underside of the board, either way, you still have a flow. What makes a bigger difference is the amount of space in which the circulation takes place. A bigger space is usually better when you have hot devices.
If the design is so close to overheating that it matters whether the hot devices are on the top (presumably in a larger space) or on the bottom (an assumed smaller space), it might be time to consider an active cooling solution. It may also be worth asking whether a user might put the box down in the upside-down orientation, and whether that may cause failure to operate properly.
When heat really is a problem, having hot devices on the bottom of the board in a much smaller space than those on the top, you can still fit a copper plate to the inside of the base of the casework and conduct the heat into it using thermally conductive pads or similar. This gets the heat away more easily than say having to fit a heatsink to components on the top of the PCB. So it might not be as bad a problem to deal with.
The IC package itself doesn’t have an inherent orientation of “up” or “down” (though the pins will all be on one side). The key factor is how you manage heat dissipation. Convection naturally drives hot air upward, so it’s important to arrange your components to allow this airflow as freely as possible.
The impact on temperature can be significant, but you’d need to model the airflow and heat dissipation to get precise answers. Datasheet temperature rise figures can provide a baseline, but remember they’re based on specific conditions.
Designing a PCB is always a balancing act between optimal routing, component placement, thermal management, and other factors. Reviewing reference designs can give you a good sense of how to position components effectively, and it’s generally wise not to stray too far from established layouts.
Effective thermal management is crucial in electronic design, especially when dealing with high-speed components. It’s important to calculate the power dissipation for each component and the PCB as a whole, considering all possible operating scenarios and incorporating safety margins. This ensures that the cooling solutions, whether passive or active, are adequate.
The choice between placing components on the top or bottom layer should factor in airflow and how heat dissipates in the enclosure. Generally, placing components on the top layer can aid natural convection, but this should not be the sole strategy for thermal management. Utilizing copper pours, heatsinks, thermal vias, or even more advanced cooling solutions like Peltier elements or liquid cooling can significantly enhance heat dissipation.
Component datasheets are valuable resources for understanding power dissipation and the best methods to manage heat effectively. Ultimately, ensuring your design remains robust under various orientations and conditions is essential for reliable operation.