GPU and motherboard back2back style: all the options

It is becoming increasingly clear that the new trend to shave those annoying extra liters and achieve maximum petiteness is to place the GPU back to back with the motherboard by way of a pcie riser, and let both the CPU and the GPU breathe straight from the sides, in an open air configuration.

gone are 5.25" ODDs
gone are 3.5" HDDs
mostly gone is the need for 2.5" drives
the holy grail of SFX-L PSUs is almost here (silverstone 700W with 120mm fan and the smartest fan control in a SFX yet)
soon to be gone is the need of SLI for 4K
beastly short GPUs are coming thanks to HBM
an increasing number of CPUs (no OC) are totally ok being cooled by a simple Noctua L9i

So far we have seen 3 cases with this "back2back" style:
- DAN A4-SFX [~ 7.25 liters]
- MSI Nightblade MI/MI2 (only sold with CPU and GPU) [pretty big, mentioned just for the style]
- Hutzy XS (flex atx psu) [~ 4 liters]
- the MI-6 by Firewolfy [~ 6.2 liters]

Now, since an increasingly large number of users will be ok with short (<19cm) cards in the near future, the "Hutzy style" is the next logical step for this style of case. 2 years from now with beastly HBM2 itx-short nvidia cards the dan A4 will feel too long for a lot of users.

Basically, all hail the HutzyXS but with an SFX (-L? or not?) psu instead of the flex atx (= small fan, noisy).

This is the state of the art.

This thread is meant to discuss about the general concept of back2back pcie-riser style cases and explore every new case of this kind on the horizon.

Couple of side notes, in no particular order
- this style of case feels like it should have been around for years (it's not like pcie risers weren't available), but only recently m.2 SSDs have become ubiquitous as boot drives and the interest for ODDs, 3.5" and 2.5" drives is fading into oblivion (as far as a gaming rig is concerned), making sub-5L builds interesting
- SFX-L in these kind of smaller-no-matter-what builds should be seen as a stop gap solution until a true SFX with zero issues is available; at 99mm wide, the Hutzy is a hair thinner than a 100mm SFX psu, that could be placed above or below the mobo+gpu section. SFX-L would make the case too wide
- another piece of the equation are CPU coolers, and in this regard it would be nice to have L9i-style coolers with official 120mm fans that assume ultra-low-profile RAMs, instead of ghettoing fans to coolers with cable ties as shown in some threads

smaller said:

- another piece of the equation are CPU coolers, and in this regard ti would be nice to have L9i-style coolers with official 120mm mm fans that assume ultra-low-profile RAMs, instead of ghettoing fans to coolers with cable ties as shown in some threads

Click to expand...

There is also the problem with CPU socket allocation. Some mobos has it do close to the PCIe slot or something that the assumption of ultra-low-profile RAM might no be enough. Though there might be a way to manufacture it so a simple rotation solves the issue... I don't know...

Instead of custom designed cases someone should start a "custom ITX" cpu cooler thread (with some serious compromises like the size of the RAM assumption) lol

I have been toying with the idea of scratch-building a case such as this but instead of using a long riser-cable use couple of angled cards and/or a short cable with a custom riser-PCB that would reverse the graphics card's orientation 180°.
The motherboard would be mounted on tabs only above and below the GPU thus removing the space of a motherboard tray.
Then both motherboard and GPU would be back-to-back, both facing outwards, a few millimetres would be saved and you would get around the issue of interference on PCIe riser cables.
Shielding between motherboard and GPU could be done with a regular paper-metal-paper laminate or similar which could safely touch either or both cards without issues.
You may want to route a DV cable though.

smaller said:

mostly gone is the need for 2.5" drives

Click to expand...

I am a bit concerned about cooling M.2 drives. From what I have read, M.2 drives are able to get pretty hot compared to other drives. Most mITX boards I have seen have had the M.2 slot on the back - where with a back2back config they would get very little airflow.
The only boards I have seen with the M.2. slot on the top have also used SoDimms and/or smaller server-style cooling mounts because there would be not space for it otherwise.

The issue with FlexATX PSUs is a lack of market for enthusiast models. If you build it, they will come: Build a gaming case with a FlexATX requirement, people will build machines using FlexATX PSUs, FlexATX PSU manufacturers will try to take advantage of the market.

And with the size of FlexATX PSUs, 500w would be the upper limit, and even then, a small fan would be more than enough to exhaust the heat.

Findecanor said:

I have been toying with the idea of scratch-building a case such as this but instead of using a long riser-cable use couple of angled cards and/or a short cable with a custom riser-PCB that would reverse the graphics card's orientation 180°.
The motherboard would be mounted on tabs only above and below the GPU thus removing the space of a motherboard tray.
Then both motherboard and GPU would be back-to-back, both facing outwards, a few millimetres would be saved and you would get around the issue of interference on PCIe riser cables.
Shielding between motherboard and GPU could be done with a regular paper-metal-paper laminate or similar which could safely touch either or both cards without issues.
You may want to route a DV cable though.


I am a bit concerned about cooling M.2 drives. From what I have read, M.2 drives are able to get pretty hot compared to other drives. Most mITX boards I have seen have had the M.2 slot on the back - where with a back2back config they would get very little airflow.
The only boards I have seen with the M.2. slot on the top have also used SoDimms and/or smaller server-style cooling mounts because there would be not space for it otherwise.

Click to expand...

M.2 Sata3 are decent in temps. I've been using M.2 SSDs for about 2-3 years and never seen it get hot. PCIe speed SSDs do seem to get hot tho

KazeoHin said:

The issue with FlexATX PSUs is a lack of market for enthusiast models. If you build it, they will come: Build a gaming case with a FlexATX requirement, people will build machines using FlexATX PSUs, FlexATX PSU manufacturers will try to take advantage of the market.

And with the size of FlexATX PSUs, 500w would be the upper limit, and even then, a small fan would be more than enough to exhaust the heat.

Click to expand...

Didn't the evga come with a Flex PSU? Iirc, the fan noise seemed to be the biggest complaint.

smaller said:

- SFX-L in these kind of smaller-no-matter-what builds should be seen as a stop gap solution until a true SFX with zero issues is available; at 99mm wide, the Hutzy is a hair thinner than a 100mm SFX psu, that could be placed above or below the mobo+gpu section. SFX-L would make the case too wide

Click to expand...

SFX and SFX-L are both 125mm wide, it's the length that's 100 or 130mm. TFX might be a good compromise form factor - it's ~80mm wide like flex, but with the 63.5mm height of SFX, which allows for an 80mm fan. It's a neglected market segment though, with few options available and none over 400W AFAIK.

A problem with this kind of layout is the available PCIe flex risers either 1) aren't shielded properly, 2) are bulky and inflexible, or 3) very expensive. The Dan A4's dimensions probably wouldn't be possible if he weren't using the 3M shielded riser, but even in quantity it adds a lot to the cost of the case.

smaller said:

Basically, all hail the HutzyXS but with an SFX (-L? or not?) psu instead of the flex atx (= small fan, noisy).

Click to expand...

Actually, something like this in on this forum, Firewolfys MI 6. It uses the b2b-layout too, has support for a short gpu and for 65mm coolers. That said, a SFX power supply really eats up space quickly. It's 6,2l compared to the HutzyXS' 4l.

Hanakuso said:

Didn't the evga come with a Flex PSU? Iirc, the fan noise seemed to be the biggest complaint.

Click to expand...

They came with a 1U server PSU. Note that this is all relative. Some people didn't notice at all. Some probably lost sleep over it. My understanding was that it was "not noisy" until under load, during which time you may be doing something that has sound that can cover up the noise, such as gaming with a headset on.

Before addressing a couple of posts here, I'd like to expand on the subject of "assumptions".
I have a feeling that when you go below 8L
you're mostly in it for the sake of it. It's a challenge. Nobody is that space limited.
The challenge being "how low can you go using
- a standard and mainstream mini-ITX motherboard
- a standard and mainstream ITX-sized air cooled dual slot full height GPU
- a standard and mainstream SFX psu
- no external power brick

with everything being completely modular and upgradable in the years to come.
As you see I didn't mention being able to
- use any (or even most) RAM modules
- use any (or even most) GPU
- use 3.5" or 2.5" drives
- use a slim ODD

These rules are as arbitrary as anybody else's, but this is where i feel the line should be drawn for this style of case. There's plenty of options for people wanting to compromise less.

Couple of things we could start assuming
- VLP (very low profile) ram modules - so it's easier to use a 120mm or 140mm slim fan, provided we have a way to mount it off-center and don't cross the borders of the mITX motherboard, and also don't interfere with wifi mini-pcie daughterboards or MaximusImpact-style daughterboards while at that)
- 2.5" drives considered expendable - by this I mean to accept they could or could not be left out, that said most of the time there's a way to strap a 2.5" somewhere anyway, but the possibility of a no_2.5" case should be considered (about the heat build up of m.2 pcie blade SSDs, the only real problem with this style of case are motherboards with the m.2 port on the back)
- SFX and not SFX-L - if we look at the current Hutzy XS and DAN A4 designs, we see that this style of case has a thickness in the ballpark of 95-115mm; SFX has two dimensions that can fit into that, SFX-L has only one, so only SFX could be easily added above/below or on the front the current Hutzy design, simple as that

Now on to addressing a couple of posts

kooki
What I'm thinking about is a cooler L9i-style tall enough to rise a 120/140mm SLIM fan just above motherboard capacitors and VLP rams, and with a way to slide the fan on the X and Y axis in order to mount it OFF CENTER (related to the CPU socket); it doesn't make super sense in terms of focusing pressure on the fins of the cooler, but we're dealing with an open air situation anyway and it would double as a case fan for that half of the case. A mITX is 170x170mm, I wanna see a 140x140mm 15mm-slim fan hovering over it, off center the cpu socket, the only off limit zone are the I/O connectors. All within the height constraints and not being shy about nearly touching the (perforated) side panel of the case.

Findecanor
Apple have used blade-style (just not with m.2 connector) pcie 4x SSDs for years. They don't constitute an heat issue per se.
We just need to avoid the relatively few motherboards with m.2 on the back.
That said, if we can find a way to strap an Intel 750 2.5" ssd somewhere, the better.

KazeoHin (and similar reasoning about making FlexATX "cool" I've read in the Hutzy thread)
We must be realistic about the fact this is all very niche, more of a challenge, and we probably have not enough leverage to push for enthusiast grade FlexATX PSUs.

Hanakuso
I definitely wouldn't scale back to a SATA m.2 just out of the fear that the far superior pcie4x m.2 could thermal throttle a little (while still being better than the SATA blade).

Necere
As I said above, only SFX is narrow enough to be just strapped above/below/front of the "100mm thick mobo+gpu sandwich" at the core of this style of case. DAN A4 solves this by putting it parallel to the motherboard but that's a big and long 7.25L case.
As for your concerns about the added cost of a quality shielded pcie riser, well this is all pretty niche and it doesn't come cheap. As for the quality, if big companies (MSI, Lian Li, etc.) are using risers in mainstream production models, apparently there are pcie risers that work properly and consistently.

Aircoookie
Great, I've checked out the MI6 and added it to the OP. It's 6.2L but he "wastes" some space for 2.5" drives. But more importantly, in his thread this other case was mentioned

Mini ITX semi Passiv Gaming PC mit 4,7l Volumen und integriertem Netzteil

wow
this is everything I've talked about so far.
We just need to put an SFX on the front in place of that FlexATX psu.
And make it in a gorgeous minimalistic design like DAN A4.

Nice case, but yeah the tiny fans in the flex PSUs drove me to the sfx/l for the MI-6 since I was trying to make it real quiet. And I couldn't get a TFX with power and full sets of connectors.

I can't get comfortable removing those 2.5s, at least for me. I keep thinking about how cheap HDDs for storage for videos, pics, programs that don't need to be on a ssd, and backups of course.

Basically one could do this
- 1TB m.2 SSD inside the machine
- 8TB NAS (2 x 8TB raid1 for good measure) hidden in some closet for cheap data storage and backups

Hopefully connected via gigabit, but even WiFi in this day and age is getting crazy fast and definitely fast enough to browse a NAS comfortably.

If we look at how long these case projects take from conception to being in production, chances are 1TB m.2 SSDs won't be that pricey by the time one builds inside such a case. Lots of people already keep everything on the boot SSD and use an HDD for backup.

smaller said:

- SFX and not SFX-L - if we look at the current Hutzy XS and DAN A4 designs, we see that this style of case has a thickness in the ballpark of 95-115mm; SFX has two dimensions that can fit into that, SFX-L has only one, so only SFX could be easily added above/below or on the front the current Hutzy design, simple as that
[...]
As I said above, only SFX is narrow enough to be just strapped above/below/front of the "100mm thick mobo+gpu sandwich" at the core of this style of case. DAN A4 solves this by putting it parallel to the motherboard but that's a big and long 7.25L case.

Click to expand...

Again, an SFX PSU is 125mm wide, not 100mm. Maybe an image will help get the point across:



That's SFX on the left, FlexATX on the right. Unless you don't mind the cables sticking out the sides of the case, you're not getting SFX into a 100mm wide case without orienting it parallel to the motherboard like the A4.

smaller said:

As for your concerns about the added cost of a quality shielded pcie riser, well this is all pretty niche and it doesn't come cheap. As for the quality, if big companies (MSI, Lian Li, etc.) are using risers in mainstream production models, apparently there are pcie risers that work properly and consistently.

Click to expand...

I didn't say you can't get quality. There are three things that are of concern to us with the available flexible PCIe risers:

1. Thin and flexible, to enable placing the GPU behind the motherboard without added bulk
2. Proper shielding
3. Inexpensive

The point is, at it stands you can only get two of these three properties in one riser. The risers that Lian Li uses are shielded and relatively inexpensive, but they're thick and inflexible, making them unsuitable for running behind the motherboard. The Dan A4 is only really possible with the thin and flexible 3M riser, but that costs $50-$70 by itself, resulting in a prohibitively expensive total price for the case. If this is a custom one off case for yourself you might not care about that, but a production case is going to have a tough time.

Ok, now I see what you mean, I was disregarding on which side the connectors are, my fault.
That's a big problem. One that could even bring me to raise the ideal thickness for this kind of case from 100mm to 130mm (and after that propose to split it asimmetrically between the GPU compartment and the motherboard compartment, in order to allow for taller CPU coolers).

But I've also come up with two bizarre solutions without altering the 100mm thickness and I figured I could share them for a laugh.

The basic shape is the 190 (L) x 180 (H) x 100 (W) box from that german language forum, which is close to the minimum possible volume for this kind of case.

To that box, one could add (on top or at the bottom) one of these "PSU tumors":

1) "the mohawk"
65mm (or tight fit anyway) box shaped "tumor" on top of the case, ~125mm tall and ~125mm long to fit an SFX (upright with the connectors facing to the front of the case) plus cable clearance. (it should be mentioned that in the recent Corsair SF600 the connectors are more recessed than in Silverstone counterparts, so probably we could get away with even less than 125mm long since the PSU it self is 100mm and 25mm for cable clearance feels like a lot but a sqarish "mohawk" maybe is desirable for the looks; on the other hand SFX-L support would require a 140mm long "mohawk", on top of a 190mm long case).
Bonus: wifi AC + bluetooth antennas could be hidden in the corners of the "mohawk"

2) "the foot"
A perfectly round puck at the bottom of the case, big enough to fit the SFX (non L) plus cable clearance, so a circle around a 125x125mm square, 65mm tall, psu fan facing downward and four 15mm feet to rise it and make it breathe.
Bonus 1: put Power, Reset, activity led and internal header USB 3.0 ports on the puck and leave the main box completely clean
Bonus 2: the foot could be detachable and be offered also in a FlexATX variant ("the slipper", basically like the Hutzy case) and an SFX-L variant ("big foot").


As for the pcie riser concerns, in a world where Caselabs cases exist and DAN A4 itself is becoming a thing that exists (at about 250€/250$), probably even just having 2 out of 3 of your bullet points could be fine.

smaller said:

Ok, now I see what you mean, I was disregarding on which side the connectors are, my fault.
That's a big problem. One that could even bring me to raise the ideal thickness for this kind of case from 100mm to 130mm (and after that propose to split it asimmetrically between the GPU compartment and the motherboard compartment, in order to allow for taller CPU coolers).

But I've also come up with two bizarre solutions without altering the 100mm thickness and I figured I could share them for a laugh.

The basic shape is the 190 (L) x 180 (H) x 100 (W) box from that german language forum, which is close to the minimum possible volume for this kind of case.

To that box, one could add (on top or at the bottom) one of these "PSU tumors":

1) "the mohawk"
65mm (or tight fit anyway) box shaped "tumor" on top of the case, ~125mm tall and ~125mm long to fit an SFX (upright with the connectors facing to the front of the case) plus cable clearance. (it should be mentioned that in the recent Corsair SF600 the connectors are more recessed than in Silverstone counterparts, so probably we could get away with even less than 125mm long since the PSU it self is 100mm and 25mm for cable clearance feels like a lot but a sqarish "mohawk" maybe is desirable for the looks; on the other hand SFX-L support would require a 140mm long "mohawk", on top of a 190mm long case).
Bonus: wifi AC + bluetooth antennas could be hidden in the corners of the "mohawk"

2) "the foot"
A perfectly round puck at the bottom of the case, big enough to fit the SFX (non L) plus cable clearance, so a circle around a 125x125mm square, 65mm tall, psu fan facing downward and four 15mm feet to rise it and make it breathe.
Bonus 1: put Power, Reset, activity led and internal header USB 3.0 ports on the puck and leave the main box completely clean
Bonus 2: the foot could be detachable and be offered also in a FlexATX variant ("the slipper", basically like the Hutzy case) and an SFX-L variant ("big foot").

Click to expand...

Here's what I get based on your descriptions for the "mohawk" and "foot" designs:



The mohawk looks silly, IMO. The foot maybe a bit less so, but it's not like either of these designs save any real volume over just making the whole case wide enough for the PSU.

smaller said:

As for the pcie riser concerns, in a world where Caselabs cases exist and DAN A4 itself is becoming a thing that exists (at about 250€/250$), probably even just having 2 out of 3 of your bullet points could be fine.

Click to expand...

The difference is, I think, that Caselabs buyers go into it with the expectation that they'll be spending several thousand dollars on GPUs and watercooling parts, so $300-$500 on just the case doesn't seem that excessive. In contrast, for the tiny cases like we're discussing you either can't fit or can't cool/power anywhere close to the same level of hardware, so $250 is a much higher proportion of the cost, and therefore a harder sell. You also get, physically, a lot more case when you buy a Caselabs.

The A4 is also unproven as yet - it remains to be seen if (enough) people are willing to pay the price to have one. I could say that the success of the M1 has shown that yes, there is enough interest for a premium mini case (at least at the ~$200 price point), but at the same time, the M1 is able to pack in at least some of that hardware that people spending a lot on a show off build want (e.g., custom water cooling). The same can't be said for the A4, or any case below about 10L, really. The Dune Case recently failed its Kickstarter, and that was a small (9.4L), seemingly well-engineered case with a price point under $200. Did it fail because it looked like a Mac Pro rip off, and people weren't into that? Hard to say, but it shows that simply packing a lot into a small volume isn't necessarily enough by itself.

Now that I see them I kinda like the foot one (maybe making the circle even bigger so the base of the case is fully inside the circle and SFX-L is supported) but as I said they were meant for a laugh. (still if someone came up with some kind of Philippe Starck shiny monolith design with the round base that would be cool)

So we're back to square one, and to accepting that realistically the case must be at least 130mm wide to house an SFX/SFX-L perpendicular to the motherboard.
This bumps the volume from ~4L of FlexATX solutions to ~6L.
Basically what Firewolfy MI-6 is doing plus shaving a couple centimeters here and there.

- SFX-L on the bottom with fan side down and feet for breathing
- perforated sides
- perforated top (not possible if SFX-L on top)
- mounting holes to strap two 2.5" (one U.2 SSD and one HDD) somewhere with minimum wasted space (if we're doing ~6L, no sense in leaving 2.5" drives out)

Splitting the 130mm wide chamber asimmetrically (50+80 instead of 65+65) or very asimmetrically with the GPU nearly touching the side panel is an option that should be explored and would allow taller CPU coolers.

(as for the Dune case, it was probably a bit too shady for people to jump in on the project)

Even splitting 50+80 would probably be enough to use a 65mm noctua L9x65 instead of a 37mm L9i


(credits kwyjibo for the photo)

this way we'd have better cooling than 90% of those small Lian Li (and others) cases with the SFX psu floating above the motherboard and forcing you to use a L9i.
That's exactly were the back2back style would show its advantages.

Variation on my crazy idea above: GPU mounted vertically on a custom PCB behind motherboard. There would need to be only a very short riser cable from the MB to the custom PCB which would use an angled PCIe socket.
Internal DV cable from top of GPU to back of case. SFX PSU below sandwich taking in air from side, exhausting at the back.

In effect, it would be like an upright DAN A4-SFX but with the desk footprint as small (or smaller) than MI-6. If you would use a short GPU then you would get more space for storage and the PSU could be mounted with fan inwards to pull air over the SSD/HD to cool it.

Because the back of the motherboard plate would be unobstructed by a riser cable, you would be able to have a cutout plate for easier mounting of a cooler.
If you would make the custom riser card larger, it could also have the front-facing I/O ports mounted on it. If it is large enough, it could even be the motherboard plate or at least a part of it holding the front and back together.

smaller said:

Even splitting 50+80 would probably be enough to use a 65mm noctua L9x65 instead of a 37mm L9i
(credits kwyjibo for the photo)

this way we'd have better cooling than 90% of those small Lian Li (and others) cases with the SFX psu floating above the motherboard and forcing you to use a L9i.
That's exactly were the back2back style would show its advantages.

Click to expand...

This is an interesting concept. Keeping the gpu side to a minimum (just allowing for the size of two slots, and maybe a back plate) may even allow for some larger coolers that support low profile 140mm fans, ie. Samuel 17, AXP100 with the bracket, or the Raijintek Pallas (which maybe pushing it at 68mm). There may even be a super low profile AIO water cooler available one day that could fit such dimensions (considering the H55 pump is about 28mm and the Silverstone TD03-Slim radiator and fan totaling 37mm, total of 65mm in height). This design however is not all that much smaller than the DAN A4-SFX so I would personally lean towards DANs solution for greater GPU flexibility even though CPU cooler height is limited to 48mm. I wonder in such a layout, whether having the PSU fan facing the motherboard and GPU would be very effective at pulling some of the heat away from these components.

I also think before throwing out the idea of the Hutzy XS because of the 40mm fan limitation of a Flex ATX, perhaps some investigation should be done into finding the quietest 40mm solution. The first fan that comes to mind is the Noctua NF-A4x10 FLX with the low noise adapter which a number of reviews are suggesting as being quiet for a 40mm fan.

Maybe flexATX PSUs are not that bad after all...they're the only way to go sub-4L..not sure how safe it is to mod them with a different fan, though...

As for the asymmetrical design, it's safe to say we need at least 45mm in the GPU chamber (dual slot GPUs are 40-41mm at the bracket, itx-sized ones rarely exceed that because they are meant for cramped systems) and 75mm in the motherboard chamber in order to be able to use a L9x65 (10mm for PCB+socket+CPU plus 65mm for the cooler).

So a bare minimum of 120mm internal (plus side panels thickness) to be able to use a L9x65...

...think I'll be fine with L9i and symmetrical.

So we're back to all hail the Hutzy and let's hope the FlexATX isn't too loud.

thought i would chime in. you guys are forgetting the cryorig c7, its an inbetween cooler thats 47mm tall,

Good.

Another bizarre idea for a laugh:
- GPU chamber constant at 50mm
- motherboard chamber available in two sizes, maybe called "115x" (50mm) and "lga 2011" (80mm)

The motherboard chamber "cover" would come in the form of a clip-on "shoebox top" (a box minus a side) and you could order one size or both while placing your order (and even stand alone afterwards).

This is possible because there are no case fans so the only really constant part is the PCB sandwich plus PSU, so the side covers could be clip on.

The same flexbility could be used to allow both SFX and FlexATX, depending on the cover.

As long as the overall rigidity of the case isn't affected.

The problem with that would be that you'd never have a nice box, it'd always be strange combinations of modules not really fitting together. You might be able to make it look good, but that's a hard design task.
It makes much more sense to have one version with support for FlexATX and smaller coolers and one version with support for SFX-L and larger coolers. Would probably be cheaper, too.