MB: part the heats up..

The temps you get really depend on where the sensors are. 30-40c for a motherboard temp is perfectly normal. Motherboards generally do not overheat or has issues with heating. You could increase airflow to lower this maybe a couple degrees but you don't really have an issue with temps.

The voltage regulators are the part of the motherboard that generally heats. It is the heatsinks around the CPU. The type of CPU cooling affects the voltage regulator temps. A AIO radiator that moves no air in the area of the CPU socket will cause the voltage regulators to run hotter.

Correct undervolting can reduce temps. but how much isn't really clear. A little .05v dip won't do much at all.

You are asking about a patient? You've come to this hospital but we can't see the patient. It's nice that you want to address this issue. We first however need to know some specifics about what's under the hood. You can use https://www.ccleaner.com/speccy/download. C+P the summary page or offer a screenshot. . When you are replying look above the text box at all the wonderful tools. See the icon that looks like a Polaroid pic? Click it. A box pops up. Paste a example.png
example.jpg or a example.bmp and click okay. I used https://s5.postimg.org/56wxtovnr/2018.png. I also have a Vivo Titan case and a Seasonic G Series 650W 80+ Gold PSU.

What's going to be important is the probably just the motherboard. Take a look to the left and above the CPU socket, where you installed the CPU. is just an example. There's a little strip of black and red next to the place where a 8 pin CPU supplemental power connector would be attached. That's a tiny heatsink. It's a passive heatsink that helps to wick heat away from the motherboard and at that point internal case fans and efficient airflow become important. Without correct airflow the motherboard and its passive coolers won't work too well. Now, take a look at this board See those heatsinks? Bigger, beefier and more of them. That motherboard should be cooler thanks to the extra heatsinks and their sizes.

People know they aren't going to OC so they purchase a budget board. The companies pumping those out know this. These users won't OC so we don't need to worry about cooling too much. They do JUST enough. This other board is for Intel processors designed to be OC'd(those with a K in the Sku like 8700K). The user will be pushing their CPU pretty hard so we need some extra cooling. Their board costs $110 and allows overclocking. Even if one won't be overclocking those board come with some other advantages. Those advantages extend to electrical needs as well but that's not the issue at the moment.