Does "heat" affect signal integrity?

Question

I have a PCB that is having signal integrity issues. This is a four-layer board, the top and bottom layer is signals and power and the inner two layers is a solid ground plane with no breaks in it.

My SMPS seems to causing the PCB FR4 material to heat up to around 65 degrees Celsius (when it is loaded.) If I touch anywhere on the PCB it causes one of the data lines to invert its signal. Sometimes even binding a THT electrolytic capacitor to lay directly on the board causes issues.

When the PCB is not loaded, I have no signal integrity, even if I touch the PCB or lay the capacitir on the board.

Does heat (in this case at 65 degrees Celsius) cause signal integrity issues? How does it cause it and what can I do to solve this issue?

N.B This PCB is heatsinked through an aluminum enclosure.

Answer 1

The temperature of the PCB material (If it's below the glass transition temperature of the material, typically above 160 C, and below the maximum ambient operating temperature of the chips involved, typically 85 C or higher) is not likely to affect signal integrity directly. If you want to test whether temperature is the actual culprit you could run the circuit with a light load on the power converter but with hot air around the board (either from an air gun or a thermal chamber) to raise the temperature to 65 C. 

However, a hot PCB indicates the DC-DC converter is handling higher power, and therefore higher currents. There are a few ways this could affect the data signals, including

1. Higher currents in the converter produce higher magnetic fields which are more likely to interfere with data signals.

2. If the inductor in the converter is saturating due to high current it will both produce even greater magnetic fields around the inductor, and likely produce more energy in higher harmonics of the converter's switching frequency, which is more likely to produce harmful interference.

3. If the converter is failing to produce sufficient power for the circuit, it is likely to produce high ripple voltage on its output. If this output is powering the data-transmitting circuit, that could cause failures.

4. At high power, the converter might produce high ripple voltage or sag on its input power rail. This could cause failure even in circuits that aren't powered by the converter. (Saturation or partial saturation of the converter's inductor could make this much worse)

Frankly, direct electromagnetic interference with data signals at the rates of SPI and I2C interfaces is fairly unlikely. A failure mechanism related to power integrity is much more common in my experience.