Reducing Thermal Pad Footprint on QFN ICs

I’m currently designing a PCB with tight space constraints, and I have a QFN IC with a large thermal pad, even though it doesn’t require extensive heat dissipation. To optimize space, I’m considering reducing the footprint of the thermal pad to accommodate some vias and traces. I plan to cover it with solder resist to ensure integrity.
While this solution seems reasonable to me, I’m curious why it’s not a more common practice. I’d like to know any potential drawbacks or risks of this approach.

Solder mask by definition is ONLY applied to prevent conductive layers from being unintentionally soldered during the manufacturing process.

Solder mask was not developed for long term electrical isolation and as such it can flake off or include pin holes which in turn can allow electrical issues under extreme conditions.

You might be able to get by with this method although it does come with risk. I recommend avoiding this approach and instead find an alternate solution.

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Hello George,
Even if the IC doesn’t require extensive heat dissipation under normal operating conditions, reducing the thermal pad’s size may impact its ability to dissipate heat effectively. This could potentially lead to increased operating temperatures, affecting the overall performance and reliability of the IC & Quality of Signals.

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I wouldn’t recommend this approach. While small designs may not generate significant heat, exposing a pad so close to your traces carries risks. In high-volume production, some boards might experience issues where part of the trace becomes exposed or the via mask gets scraped off. Over time, wear and tear from regular use could expose components, potentially leading to unexpected behavior.
In general, it’s not considered good design practice. It’s advisable to use another layer for routing instead of relying on this method.

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This approach might technically function, but keep in mind that placing solder mask over the traces beneath the pad will increase the height, which can prevent the chip from making full thermal contact with the pad. Whether this lack of thermal contact is critical depends on your application.

Implementing this method could potentially raise concerns with your assembly house due to the unconventional approach and the impact on assembly processes

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To ensure compliance with IPC standards, it’s important not to use solder mask as an insulator. Based on past experiences, I would recommend exploring other options, using solder mask in this way can significantly reduce yield, and production house design for manufacturing (DFM) checks often reject such designs due to the anticipated drop in production pass yield.