I don't think those are CFM rates.

The fans for those tiny heatsinks back then were more like 2-5 CFM.

I, for one, bought five of these 40x40x20 heatsinks for my 486 / AM5x86 CPUs. Paired with a $5 Zephyr Silentium PC 40mm fan, things are cool and dead silent. Better safe than sorry, I always say 😁

Personally I add at least a heatsink on anything 286~386 and everything 486+ has active cooling.
All those cpus get hot and with all those years in their back it is better to be kept cool IMHO so they last longer

Anything above 25Mhz needs a heatsink.
Never hurts to have one, though I'm not sure what the adhesive they used for the early chips was.

Anything above 25Mhz needs a heatsink.

Not really. Yes, even if taken in context.

I don't know if even back then there were different core stepping that had different realistic working temperature/TDP like happened on some Pentium4 where the same cpu could have had a 10-20W lower TDP depending if you're lucky to have a specific core stepping, but if I watch at the Intel 486 Overdrive 66Mhz that's basically a DX2-66 it came with a glued nice heatsink on it and it sure need it because it get hot quite soon, not to mention the 486 Overdrive 100Mhz that imho would have needed a fan by default getting scary hot on the surface. But I suppose they knew these cpu would have been mostly inside closed cases and I imagine the skin finger is not a scientific test to say how much their circuits would suffer those temps; anyway imho for my own cpu I think to consider a heatsink required from the 50/66Mhz and above. But still what would we say about the 386? The Am386DX-40 in the PGA package is a great cpu that I remember was running very very hot on the surface. Sometimes I wonder if the QFP package version is safe too because it get hot on the -plastic- surface that I don't think is the perfect heat material and I imagine the internal temperature would be even higher.

The Serpent Rider wrote on 2020-07-04, 12:33:

Anything above 25Mhz needs a heatsink.

Not really. Yes, even if taken in context.

Yeah, that's why I was trying to take a more scientific approach by looking at the datasheet. It seems to be a combination of the ambient temperature + how much airflow your case has due to the power supply and case fans.

cyclone3d wrote on 2020-07-04, 04:11:

I don't think those are CFM rates.

The fans for those tiny heatsinks back then were more like 2-5 CFM.

I was thinking they were intended to be read the other way around, i.e., the CFM provided by your case/power supply fans determine whether you need a heatsink or not, i.e., the CFM of the cpu fan is irrelevant.

jakethompson1 wrote on 2020-07-04, 17:54:

cyclone3d wrote on 2020-07-04, 04:11:

I don't think those are CFM rates.

The fans for those tiny heatsinks back then were more like 2-5 CFM.

I was thinking they were intended to be read the other way around, i.e., the CFM provided by your case/power supply fans determine whether you need a heatsink or not, i.e., the CFM of the cpu fan is irrelevant.

Find me a case that has 200CFM being blown through it... much less 1000CFM.

Back then you were lucky to have 35CFM being blown through the case.

For 200CFM, you would need probably 8-12 80mm fans. Back then all you normally had was a single 80mm power supply fan.

cyclone3d wrote on 2020-07-04, 21:00:

Find me a case that has 200CFM being blown through it... much less 1000CFM.

Back then you were lucky top have 35CFM being blown through the case.

For 200CFM, you would need probably 10-15 80mm fans minimum. Back then all you normally had was a single 80mm power supply fan.

Agreed; can you make any sense out of page 15-3 of the Intel 486DX2 data sheet? It seems like it's saying that without a heatsink, it's only ok to use a 486DX2 with <200CFM airflow if it's below freezing in the room. Obviously it can't be that bad. It does say "PRELIMINARY" so perhaps those numbers were revised later?

Edit: Hey! It looks like it might be linear rather than cubic feet. I think I mentally made that unit "conversion" automatically

I've got a AMD DX2 -80 which despite being listed as a 2x multiplier part (and I was anticipating running a 40MHz system bus) is actually a 3x multiplier, so using a 25MHz system bus is needed to get it to identify as a DX2-80 in DOS (and even then it's actually 75MHz).

Anyway I've put a Socket 370 heatsink and fan on it, if I run the CPU at 33MHz system bus (so the CPU is identified as an AMD DX4-100 during POST) will this heatsink and fan combo be sufficient?

I run my dx2-80 passively with some 3m thermal tape and a heatsink that had a seized fan sitting stop a k6-2 300

Almoststew1990 wrote on 2020-07-11, 16:57:

I've got a AMD DX2 -80 which despite being listed as a 2x multiplier part (and I was anticipating running a 40MHz system bus) is actually a 3x multiplier, so using a 25MHz system bus is needed to get it to identify as a DX2-80 in DOS (and even then it's actually 75MHz).

Anyway I've put a Socket 370 heatsink and fan on it, if I run the CPU at 33MHz system bus (so the CPU is identified as an AMD DX4-100 during POST) will this heatsink and fan combo be sufficient?

An AMD 486 DX2 80 should run at 2x40MHz . If yours is running at 3 times base clock, you board's multiplier settings not being set correctly are likely the issue . It is not uncommon for clocked doubled 486s to actually work with undocumentrd 3x multipliers . That said, if you are running yours stable at 3x33MHz, that is great. A socket 370 CPU and fan, if making proper contact, should keep your 486 cool enough .

hwh wrote on 2020-07-04, 08:29:

Anything above 25Mhz needs a heatsink.
Never hurts to have one, though I'm not sure what the adhesive they used for the early chips was.

I always heard it was 50mhz. I know I ran my 486sx-33 naked back in the day.

For what it's worth when I got a PC back in late 1994 with an Intel 486 DX2-66 it came without any heatsink, and I used that PC regularly for over 3 years. I still have the CPU and it still works.

Nowadays I would absolutely put a heatsink on it though.

Almoststew1990 wrote on 2020-07-11, 16:57:

I've got a AMD DX2 -80 which despite being listed as a 2x multiplier part (and I was anticipating running a 40MHz system bus) is actually a 3x multiplier, so using a 25MHz system bus is needed to get it to identify as a DX2-80 in DOS (and even then it's actually 75MHz).

Anyway I've put a Socket 370 heatsink and fan on it, if I run the CPU at 33MHz system bus (so the CPU is identified as an AMD DX4-100 during POST) will this heatsink and fan combo be sufficient?

Where are you getting that is 3x?
According to page 24 of this datasheet the Ta values are super forgiving even with no airflow and no heatsink: https://en.wikichip.org/w/images/b/b6/AMD_Am4 … %2C_1995%29.pdf

From the fact that my system bus is jumper ed to 25MHz and this gives a CPU speed of 75MHz. Same for 33MHz bus - gives a CPU speed of 100MHz

Weird, could your DX2-80 be a phony? 😜

jakethompson1 wrote on 2020-07-04, 03:54:

I've got 486DX2 parts on the way (fun!) and thinking what to do (or not) about a heat sink as I see different opinions on the is […]
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I've got 486DX2 parts on the way (fun!) and thinking what to do (or not) about a heat sink as I see different opinions on the issue.

I decided to go to the source and found the 486DX2 data sheet (https://archive.org/details/bitsavers_intelda … e/n219/mode/2up). I don't quite understand the table on page 15-3 but it seems that at room temperature the CPU needs 400 CFM of airflow to be safe without a heatsink.

On the other hand on page 15-2 it seems the table shows the most optimistic situation of a heat sink and 1000 CFM airflow, is 2.5 degrees Celsius per watt thermal resistance.

I was checking out adhesive sinks at DigiKey at around 44mmx44mm size and came up on this one: https://www.ohmite.com/assets/docs/sink_bgah.pdf

Nice thing is it only costs $6 and includes thermal tape.

It appears its performance at 200 CM is better than Intel's ratings at 1000 CFM ... are heatsinks really that much better than back then or this one just needs more vertical clearance?

I found a similar table for the Am5x86 (https://en.wikichip.org/w/images/f/f0/Am5x86_ … %2C_1996%29.pdf) on page 65. It seems this heatsink wouldn't even need a fan to outperform what they list for a heatsink+fan.

Table 15-3 is showing maximum ambient temperatures in degrees Celsius vs Airflow in Linear Feet per Minute (LFM). The maximum ambient temperature should be treated like an absolute maximum that you never intend the CPU to operate at. So with Ta at 18.9C/66F, no heatsink, and 400 LFM of air hitting the chip case, the CPU would be at maximum operating temperature. If ambient temperature goes up just a degree or so, your chip is toast.

Table 15-2 basically shows the specs of their reference cooling solution and the thermal resistance specs of the chip itself. You can plug these numbers into the formulas and get the numbers in table 15-3. When referring to the 1000 LFM thermal resistance performance of the reference heatsink compared to the Ohmite sink, I assume you're referring to the 45x45x12.5 Ohmite model. This heatsink is much taller and has much more surface area compared to the reference sink, so the increased performance is no surprise. If you can find a datasheet for a heatsink of similar dimensions and designs to the reference sink, you'll probably find the numbers to be similar. I don't know if heatsinks have gotten better, I just think that the reference design was "good enough" for Intel's engineers.
Keep in mind that the Ohmite heatsink is intended to operate with (usually ducted) airflow traveling laterally along its fins, not top-down like traditional heatsink+fan combos of the day.

On to the CFM vs LFM issue:
To convert to CFM, you have to multiply LFM by the cross-sectional area of the airflow.
So, for example, let's assume you've built a duct around this Ohmite heat sink, the cross-sectional area of the airflow is 68mm x 12.5mm. In feet that's 0.223 ft x 0.041 ft. If you want 200 LFM, then you need a fan that can push 1.83 CFM.

200LFM * (0.223ft x 0.041ft) = 1.83 CFM

This is easily achievable with most small 40mm fans. The hardest part will be building the duct. If you don't use a duct, you will have to figure out what the airflow looks like so you can calculate the cross-section. For instance, in a traditional, non-ducted, downward pointing fan situation, the cross-section could be estimated as 40mm x 40mm. In this case you would need to push 3.43 CFM to achieve the same result.

200LFM * (0.131)^2 = 3.43 CFM

Keep in mind that because of the Ohmite fin design, the air may not flow around all the fins in this scenario, raising the heatsink's thermal resistance. So you may have to push even more air to achieve the same thermal resistance.

Now, because the heatsink's performance appears to be much better than the reference design, you can probably get away with no fan. You can put the datapoints from the Ohmite datasheet into a spreadsheet and extrapolate that curve to see what the thermal resistance is at 0 LFM. The result may not exactly reflect reality, but it may be close enough to determine if it's safe.

The reason the Am5x86 chips require less cooling is they run at a lower voltage, so they dissipate less heat. The Intel DX2 is a 5V part and the Am DX5 is a 3.3V part. In fact, I compared datasheets, and the Am5x86DX5 133 allegedly has better thermal characteristics than the Intel DX and DX2 chips.

As others have commented, I would err on the side of too much cooling with these old chips. It's only a matter of time before the gold scrappers have made your chip impossible to replace.