Question about PC Case Fan

Or you could just get a fan controller and set the exhausts to a lower speed than the intakes.

Of those two options, though, I'd go for the 1st. With the second option, you're just pushing the rising heat in your case back down into it.

You never use the top or back as intakes, you're just pushing hot air back in.

Personally, I just make it front intake and back/top exhaust.

Take a look at this thorough test: https://www.hardwaremax.net/guides/hardware/305-der-perfekte-airflow?showall=&start=2

intakes have to bring colder air in, exhausts have to take hotter air out. So what dantenemo says.

I personally have two big fans in front, and two big fans at back. If you want fans at the top, make them exhaust fans as well, but I think they're a waste of time.

I'd use your first configuration so that your fans are assisting natural convection, not fighting against it. Having the interior of the case at a higher pressure than the environment only stops dust from entering on air flowing in through leaks/gaps in the case, which is minimal compared to the air your fans are pulling in. If you're not you're using a filter on your intake fans, the internal pressure of your case is irrelevant and you're better off using a thermally sensible design configuration than worrying if air leaks into or out of the case gaps (differential pressures across case leakage gaps are much lower than those across the fans, and the apertures are typically small enough to be partially blocked by the dust passing through, so they become less significant over time). If the flow path in the case is relatively free, a lot of entrained dust stays in suspension and goes straight out the exhaust, so if you are using filters, you are going to need to clean them far more often than you would need to blow out the case internals without them.

To reduce dust, you need to minimize air velocity, which means using the biggest fans you can (greater surface area means lower velocity and noise for the same air volume) and running them as slowly as you can to move the air required by the heat exchangers. Most decent motherboards have thermistors for temperature measurement and software to manage fan speeds. You should use this so that your fans aren't running faster than necessary, making noise and moving dust that they don't need to. You could also further reduce dust entrainment by using bellmouth fan entries on intake fans to increase the effective induction diameter, reducing the entry velocity and the mass of dust particles that can be suspended.

As an aside, I think that the case pressure theory about dust is misguided - it is right but not for the reason that people think. Maintaining a positive pressure differential between the internals and untreated air is important in industry for excluding corrosive gases like hydrogen sulfide from sensitive copper components, and extremely small particles like smoke from inhabited spaces like fire escape stairwells, but for solids suspended in fluid, the velocity of the fluid is what is important. The main reason that dust deposition is reduced inside a case with "more" intake fans (more fan surface area) than exhaust fans (area) is that the flow volume into the case always equals the flow volume out of the case, so the reduced exhaust area means a higher exhaust velocity and any dust particle that the intake fans can suspend can also be maintained in suspension by the exhaust fans, assuming it did not settle due to static electricity attraction or a low velocity area before it got to them. In the opposite scenario, there will dust particles with masses that can be suspended by the higher air velocity through the intake fan but not the exhaust fan, so they must settle within the case.

Regardless of the setup, get Noctua or Be Quiet fans. They move a lot of air without making much noise.

Or just buy a laptop.

i have 2 fans on the front who take air in. they are also covered with a fine filter to prevent dust accumulating too much. one on the back who blows the air out. that is enough for me. you want a steady and parallel air flow. if you have air colliding each other you create air whirls, so i wouldnt recommend air flow from 2 different directions.
i also dont need to clean my pc so often. i only clean every month the filter on the front and that is enough. in 2 years not much dust has build up inside my pc.

Negative air pressure in such a small object as a PC means nothing.

You want to keep it like this:
Front : Intake
Top : Exhaust
Back : Exhaust

You don't want to be using the back or top as an intake. If you're concerned about negative pressure (there's really no need to), make the exhaust fans spin slower or disable one of the exhaust fans so you have 2 in and 2 out.

Good Question and Good Read, Thank you for the thread. I've always placed a standing fan behind the old PC I am using now to ventilate as I feel the airflow is hot. Always thought the fan at the back of the case is taking in air from the outside instead of pushing it out. Probably the positioning of the case. I have removed the side panel of the case so now the fan is facing the motherboard. Just ordered a new PC recently and I've been thinking how I should ventilate it so this helped alot. Cheers, Cruse~