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I’m refurbishing my middle school computer, a Macintosh Performa 635CD which has the Motorola 68LC040 processor. It is a 33 MHz chip. This era didn’t use any sort of heat spreader or heat sink, and there’s a fan attached to the computer chassis that sits a couple inches or so above the chip. I know it’s not necessary, but if it won’t hurt it I’d like to put a heat sink on it just because this machine means very much to me and I’m doing a series of upgrades including even an SSD. Anyway, I have never done this before so I’d like to ask a couple of honest questions:

  1. Will it hurt the chip? I notice modern CPU’s have what looks like a thin metal plate that people put thermal paste onto when installing a heat sink, and this chip, just like the old 486’s does not. Is it ok to apply the compound directly to that beautiful dark grey chip surface?

  2. Applying paste. Assuming it’s ok to directly apply, then I’m open to any advice on how for an old chip like this because I’ve looked at probably a dozen YouTube videos and most prefer the so called “pea method”, while others do that and demonstrate that it makes parts of the CPU hotter, because it doesn’t spread over the entire chip, yet spreading it with a card or something is dangerous, because it will sort of roll over the edge of the chip onto the board and cause damage. There are chipset heat sinks with thermal paste pads preinstalled where you just peel off the plastic or wax paper and then plop it right down on the CPU, but everything I’ve read about those says they’re junk.

  • This is unlikely to improve longevity. Keeping the case fan in good working order, maybe upgrading it, and recapping the motherboard will be more effective. – Brian H Dec 28 '19 at 22:23
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    @BrianH Well, while the existing solution is within the limits set by Motorola, it's still at the upper end. Lowering gate temperature is known to increase longevity. But of course you're right about keeping air flow good as it helps the whole machine. So cleaning is for all old machines the most important step – Raffzahn Dec 28 '19 at 22:29
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    I'd just opt for a CPU fan if being extra cautious. The ceramic chips probably benefit more from forced convection than covering with a "foreign" material. – Brian H Dec 28 '19 at 22:43
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    "yet spreading it with a card or something is dangerous because it will sort of roll over the edge of the chip onto the board and cause damage" Only if you apply too much. Don't apply too much. If you do, it won't work as good. – Mast Dec 29 '19 at 10:55
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    Any method that doesn't leave air bubbles is okay. Spreading the paste before applying the radiator runs at risk of trapping the bubbles, alongside with methods that involve putting a # or a circular dab of paste (with hollow middle). The pea method is safest as it will push all the air to the sides. A cross or an X will work too providing you don't squeeze the paste onto the pins. But generally if the chip can run without a radiator, it's really hard to make things worse, and while the pea method may not provide maximum cooling, it's probably the safest and definitely good enough. – SF. Dec 31 '19 at 16:14
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I’m refurbishing my middle school computer, a Macintosh Performa 635CD

Cool. Eventually one the best 68k machine of all.

Will it hurt the chip?

If applied correctly not. Any increased cooling is welcome. Keep in mind that any heat spreader is at first an additional insulation, so its relative dissapation has to be higher than that.

It it is always a good idea to keep temperature down to increase longevity of chips. While the 040 isn't as sensitive as faster CPUs. Its 6..10 Watt dissapation at 33 MHz may seam low, it does need cooling in every situation (well, maybe except mounted in a fridge :). For basic 6 Watt at room temperature (25 C) Motorola recommends an airflow of 250 LFM or better. That's why Apple placed a fan right above the CPU. These 250 LFM are just enough to keep it within design limit, not optimal. So adding a heat sink will improve cooling considerable due the larger surface contact between airflow and heat sink.

Now, removed heat is a function of temperature difference, surface size and coolant (air) flow. By just adding a heat sink you improve only surface size. It might be a good idea to add on the air flow as well. So why not adding a CPU fan as well?

Some old PC might be a good source for both.

Applying paste. Assuming it’s ok to directly apply,

Yes, and Motorola recommends it for not glued on heat sinks.

I’ve looked at probably a dozen youtube vids

Don't let them scare you. Everything modern plays in a complete different league than a 040. If at all, you need to look at videos explaining the use with 486 and early Pentium - and I mean early real early.

yet spreading it with a card or something is dangerous because it will sort of roll over the edge of the chip onto the board and cause damage.

Spreading with a straight object is not problem - and it's up to you to do it without making a mess.

I usually put a little drop on, slightly out of center and used a razor blade to distribute it a bit. After that I put on the heat sink and turned it a bit sideways to really spread it.

The whole point is to use as little as possible. The whole schmeer isn't meant to be anywhere between the heat sink and the packaging at all. It's only the lesser of two evils meant to fill only those areas were both parts don't align perfect as it's thermal conductivity is better than air.

There are chipset heatsinks with thermal paste pads [...] but everything I’ve read about those says they’re junk.

No, they are good. In fact, most(default) cooler use them nowadays. They did win the race because of simplicity in setup. But as with all parts of the heat dissapation system, they are meant for a certain setup. Just picking a random one and using it may not be the best choice.

It may be a good idea to have a read of the 68040 User Manual section 11.8 Thermal Characteristics and 11.9 Thermal Management. They describe in great detail the various parameters. Section 11.9 not only discus various levels of cooling, but as well give tables for quick reference.

Looking at the tables adding a heat sink plus forced air (a reasonable sized fan) whill give quite great results and keep the gate temperature down.


Additional Musings:

As said, I belive the 630 to be one the greatest 68k Mac of all (*1). It can bring you quite close to the top performance possible back then while still being a quite compact desktop computer. I got a 630 DOS on my workbench fo all things classic Mac and haven't found anything better. Depending on the mainboard it can hold up to 132 MiB of RAM - that's 4 MiB (the soldered on ones) more than the top of the line 840 AV (*2). Under classic OS 132 MiB is like infinite memory. At least I never managed to fill it.

The 630 can as well be fitted with a full 68040 to add an FPU and set to run at 40 MHz(!), again on par with the 840AV. Given, the disk system is a bit slower due using IDE, but as you already mentioned, it as well allows the use of a more modern Flash drive, flipping chart :)

I would suggest to have a look into which board is used in your machine (while trying to max out RAM) to see how to turn the clockspeed to 40 MHz. Usually all 040 will go along. While this is as well possible without, it should go hand in hand with adding a heat sink and CPU fan.


*1 - The best from today's view, might be the the LC 475 and its direct siblings. In RAM size and performance it's a 630 in a pizza case, missing the CD drive, but adding higher graphics resolution. Unlike the 630 it can still fit the IIe Card allowing it to double as a IIe as well.

*2 - Quadra 950 could do 256 MiB

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    You’ve given me alot to look over, I appreciate that very much. I know the chip doesn’t require it, I’m just trying to give every little luxury to this machine within my capabilities. – Mr.Macintosh Dec 28 '19 at 22:45
  • I’m lucky that the caps look reasonably good. I have zero soldering or electrical experience, so recapping is something I won’t be able to do unless I find a local computer shop that would do it for me. 🙁 I’m concerned about one of the bigger caps in the power supply but everything looks clean and flattoped. – Mr.Macintosh Dec 28 '19 at 22:55
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    My (modern) heat sink compound came with a self adhesive mask and a plastic card (an old membership card would work the same)- if you're worried about a mess, masking off the parts you want to mask, spreading a very thin coat, then removing it would be a way to get a more precise spread. – Journeyman Geek Dec 29 '19 at 6:35
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    As at least one object of the exercise is to get the most life possible, it is worth noting that the life of a typical semiconductor device is reduced for every continuous increase in die temperature. The usual rule of thumb is that life is halved for each 10C rise although in my experience it is not quite that high. The failure rate is, however, accelerated by increasing die temperature so adding a heat sink (which will reduce the effect) is a very good idea indeed. – Peter Smith Dec 29 '19 at 14:13
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  1. No, it won't hurt it. It's ceramic and you can stick a heatsink on it and it'll work. It might discolour it a little.

  2. The advice you've received is correct. Thermal paste is usually electrically conductive, and it can cause short circuits and potentially cause damage that way, if it leaks over the edge. Given that you don't need a heatsink in the first place, I'd recommend being conservative in applying thermal paste.

I have to say though, it really is pointless. A 68040 @ 33 MHz puts out less than 5 W of heat.

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    Or use an old-school, non-conductive thermal paste... as said, this CPU is designed to be usable without any heatsinking at all, a non-conductive paste ... or even the real old school solution, plain silicone grease - should be plenty thermal and pasty enough. – rackandboneman Dec 29 '19 at 4:31
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    +1 for the final sentence. These old CPUs don't need heatsinks or fans. – Jack Aidley Dec 29 '19 at 12:19
  • I don’t have anywhere to power an additional fan, just the one built into the chassis so that’ll have to do. Finding the right size was a challenge but I ultimately ordered the correct size...a custom heatsink from AlphaNova Tech that should fit the chip exactly. Thanks everyone! – Mr.Macintosh Dec 29 '19 at 13:13
  • @Jack Aidley. You’re correct of course. As stated earlier though, this machine means very much to me (emotional attachment to a machine is something hard to explain to those who don’t get it), but the idea is to offer this machine every little luxury I can give it, trying to give it the best I can. So here we are at the processor. It may not quite need a heatsink, but there’s no harm in being able to drop even a single degree off running time right? If anything, the emperor needed new clothes :) – Mr.Macintosh Dec 29 '19 at 13:17
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    Also: An improperly attached, conductive heatsink coming loose WILL shorten the lifetime of the machine dramatically indeed! – rackandboneman Dec 29 '19 at 22:07

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