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MacBook Pro: The Thermal Paste Question
Pages: 1, 2, 3

The Result

To test out the result of replacing the Thermal Paste, Greg and I ran both of our laptops at max CPU for a while. It's actually quite easy to do. You just need to fire up two terminal windows and execute the following in each:

$ yes > /dev/null

There are probably better ways to load up the CPU, but this did the trick for both of our machines. More importantly, we were treating both machines in the same way so that we could make an apple-to-apple comparison, so to speak.

After heating up the systems, we used a fancy Fluke IR thermometer to take readings from all over both systems. All of the readings told us the same thing: my modified laptop was running a degree or two cooler than Greg's unmodified version. Here are some pretty typical readings taken from exactly the same location on the bottom of each machine:

Originaltempsmall Modifiedtempsmall

Greg's MacBook Pro with the original unmodified thermal paste is on the left with a reading of 116 degrees Fahrenheit. My modified one is on the right with a reading of 114. Here's another reading from the top, centered on the keyboard:

Toptemporiginal Toptempmodified

On the left is Greg's laptop, reading 97F. On the right is mine, checking in at 95F. We took many more readings; some places were hotter than the pictures above, up to 121F. But the message was pretty much the same: all that work seems to have given my laptop a slight advantage--two degrees of advantage, to be exact. With that small of a result, you can actually argue that there are any number of other causes. For example, one flaw in our methodology is that we didn't compare the laptop temperatures beforehand to see if there was a similar two-degree delta.

No matter how you argue it, however, this wasn't the finding I hoped for. It didn't match the glowing reports on the web. Maybe other people's thermal paste skills are better than mine. Or maybe there's another reason. You see, there's a bit more to this story that I haven't told yet. It's time to admit a mistake I made.

An Unanticipated Finding

What I left out of the description above is that I actually disassembled and reassembled my MacBook Pro twice. Yes, twice. When I fired up my laptop the first time, I was greeted by the fans whooshing at full speed. Now, it was nothing like hearing my G5 running full out, but it made a fair amount of noise, and you could really feel the air moving through the case.

How loud was it? It was loud enough to be too loud while hacking code in bed. It was almost too loud when I sat on the couch on a fairly quiet day. But it wasn't too loud for other situations. For example, I couldn't hear it at all when sitting at the local cafe enjoying a double-shot Americano. I can tell you this because I actually waited a day to disassemble the MacBook Pro again to see what was wrong. I figured there was a slight chance the temperature control hardware had to reset itself, or that the Arctic Silver was conducting heat so much better into the pipes that the calibration for running the fans was off.

These weren't likely possibilities, mind you, but I'd taken the experiment this far; I wanted to explore all the possibilities. Not to mention, I really needed to do some other things for a while. And, my MacBook Pro was cool; actually, it was cold to the touch. I compiled a fresh install of Subversion, which is known to strain the system, and left the laptop on my lap while wearing shorts. I felt only the barest amount of warmth from the laptop. The keyboard area was actually cool to the touch. It was the most amazing thing.

For a day I left my system like this. Except for the noise, it was enjoyable. My MacBook Pro was once again a true laptop.

But after a day of listening to whooshing, as well as resetting the PRAM and performing other voodoo tricks to see if the PMU and SMC units would notice the more advanced thermal paste in action, I realized it was time to open things up again and find out what had happened.

The answer was simple: the connection from the motherboard to the heat pipe sensor didn't make it through the reassembly. This meant that, at boot time, the system couldn't take a reading of the heat pipe's temperature. This caused the system to enter into an "Oh crap, we don't know what's going on so crank up the fans to save the ship" mode. It's the same kind of thing that happens when you take the inside door off of a G5. The system's temperature management system responds to a failure in its setup with full-throttle fans.

Once I corrected the problem and carefully reassembled my MacBook Pro, the noise went away. And with it went the enjoyable coolness, which was replaced by a familiar warmth. It was this second reassembly that was used to take the temperature readings above.

So, maybe the reports of cooler laptops from people who replaced their thermal paste are really reports of laptops whose temperature control units are simply responding to a disconnected temperature sensor. Or maybe they really did do a better job than I did. I can neither prove nor disprove any of those statements. I can only report on my findings. However, since several internet reports of cooler laptops are accompanied by reports of louder fan noise, it's a strong possibility in my book.

The Ultimate Cool Solution

If heat is more of an issue than noise, you could open up your MacBook Pro and disconnect the thermal sensor. This will recreate the issue I had after my first reassembly and cause your laptop's fans to kick into high gear all of the time. It's actually easy to do. You don't have to remove the logic board or a lot of connections. You just need to pop the top of your case, remove the keyboard cable, and then disconnect the sensor where it connects to the top of your logic board. It's circled in the following picture:


Once disconnected, cover the end with Kapton tape so that it can't short anything else out out, and then tape it down out of the way.

Of course, I can't guarantee your results. You may blow your fans up, though probably not. And you'll notice that I haven't told you how to crack your machine. There are lots of guides you can find online with a quick Google search, but I'm leaving that to you as a further admonition to open your MacBook Pro at your own risk.

Parting Thoughts

With all of that work, my MacBook Pro was transformed from a slightly too hot yet amazing workhorse of a machine to, unfortunately, an amazing workhorse of a machine that runs a bit too hot. How much is a bit too hot? Well, from a little experimentation, the bottom of the case gets about 10-15 degrees Fahrenheit too hot for comfortable use on a lap. I arrived at these numbers by sleeping my MacBook, letting it cool, then measuring its temperature every few minutes as it warmed back up. The threshold between "just warm" and "too hot" is amazingly small, really. A change of five degrees in the outside case temperature is all it takes.

As far as the thermal paste issue is concerned, my opinion at this point is that although the factory's gooey application may look horrible, it's probably a perfectly acceptable practice given the factory and service center's desire to ensure proper contact between the chips and heat pipe with a minimum of fuss. After all, I can tell you that it took much longer to carefully apply the Arctic Silver as instructed than it would have to just glob on the stuff. And if the end result is the same, why not?

I have another concern, besides comfort, when it comes to heat coming off of the MacBook Pro. That concern is battery life. Lithium-ion batteries hate heat. Apple even has a web page dedicated to discussing how heat affects laptop batteries. It advises keeping your Mac as close to room temperature as possible, and certainly below 95 degrees Fahrenheit. Somehow, I don't think that a laptop that regularly runs with an outside case temperature of 115-120F qualifies as a good environment for lithium-ion batteries. I'll have to do some more testing with my fancy thermometer to determine how much the batteries heat up.

Now, there is hope for the future based on my experience with maxing out my fans. It's obvious that the cooling hardware in the MacBook Pro is first-class. When the system running full out, I can load the CPU and GPU to the hilt and the machine stays cool, almost cold, to the touch. The problem with heat in the current MacBook Pros seems to be a software one; the system firmware designers are apparently optimizing for quiet over heat. A smallish change to the balance between noise and heat will probably cool the laptop quite a bit without needing to run the fans all out, and it's something that I would welcome. Unfortunately, this is something that probably only Apple can fix with another SMC firmware update.

As a final thought, if you are considering breaking open your shiny MacBook Pro and modifying its thermal paste, you probably shouldn't. You run a decent chance of turning your machine into a brick and even if you do the job perfectly, you're not going to get a big change. I gave myself 10-20 percent odds of hurting my machine. As it turns out, I did just fine and my machine survived with no harm done, but it might not have happened that way. You might not be so lucky.

James Duncan Davidson is a freelance author, software developer, and consultant focusing on Mac OS X, Java, XML, and open source technologies. He currently resides in San Francisco, California.

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