It has taken a few days to look at the data I've collected and tried to figure out what is going on. Why did I not see a notable increase in performance via reduced temperatures or faster clock speeds? Why did the quad core version have its thermals mostly sorted, albeit they are still high? Why do I have a 8-10 ˚C delta between half of my cores? What is my power consumption of the CPU more than 1.5 times that of the rated TDP? Did Apple fsck up or did Intel? Yes, Intel.

First Things First, Lapping

Comparative analysis is a great way to learn about something. You can see how some is or isn't supposed to work/perform/behave and then make corrections or perturbations and see what the results are. In a way, this isn't dissimilar to a phrase from the security community: trust but verify. As such, cleaning off the non-fluid stock TIM and looking at the cold plate mounting on the CPU die is the first thing to verify.

I spent way too many hours sanding and lapping the copper slug on the heat sink. Over 2 dozen mounts later, I have consistent contact and performance. The process I used was initially a bit flawed due to sleep depravation and I probably removed a little too much copper and spent 3-4 times longer than I should've on this process. However, the copper slug is already too thin and I highly doubt the difference is significantly tangible. Instead of showing all the mounts, I've selected a few key points to demonstrate the process.

I used wet sanding with 600 grit and 1500 grit along with localized polishing with felt wheels and aluminum compound in a 99% isopropyl solute. In order to check how well the cold plate and die mounted together, I used dry erase marker as a solid film. It's opacity/visible darkness is proportional to how thick the film is, but it is thinner and easier to work with, plus a lot less costly, than using thermal grease. I think mounted the heat sink back onto the PCB for 30-300 seconds, more time when more contact area was made, and then removed and photographed the results.

Anyhow, here is the initial contact.

About midway through the process:

Amount of MX-2 finally applied:

Final mating conditions denoting very good contact for by-hand finishin, imho:

Initial Results

I don't have graphs for you right now, mostly because I need to incorporate other sensor data that I've not had a chance to process. However I do have global frequency data, temperature data, and thermal resistance of the cooling solution. Remember, the TIM is a practical part of the cooling solution. A couple of additional notes before continuing:

• Peak/maximums are during the first 10 minutes of the thermal load, excluding thermal resistance
• Minimums are during the last 10 minutes of the thermal load, excluding thermal resistance
• Averages are from the entire 60 minute load
• [Maximum] Sustained are from the middle ~40 minutes of the load, which should be consistency of intermittent moderate/heavy workloads.
• Standard deviation is from the middle ~40 minutes of the load, which again should indicate consistent performance.

Stock

	Frequency	Temperature	Power	Therm.Resistance
Max. Peak	4300 MHz	100.3 ˚C	115.78 W	2.0960 ˚C/W
Max. Sustained	3800 MHz	100.4 ˚C	81.67 W	1.1959 ˚C/W
Average	3792 MHz	96.6 ˚C	69.71 W	1.0714 ˚C/W
Minimum	3700 MHz	65.1 ˚C	62.78 W	0.4722 ˚C/W
Standard Deviation	34.66 MHz	1.1 ˚C	2.14 W	0.0208 ˚C/W
Time to EQM.	40 minutes

Lapping + MX-2

	Frequency	Temperature	Power	Thermal Resistance
Max. Peak	4300 MHz	100.3 ˚C	113.96 W	1.0723 ˚C/W
Max. Sustained	3900 MHz	100.5 ˚C	83.47 W	1.1916 ˚C/W
Average	3818 MHz	96.1 ˚C	70.76 W	1.0496 ˚C/W
Minimum	3800 MHz	63.3 ˚C	67.63 W	0.4432 ˚C/W
Standard Deviation	28.89 MHz	0.9 ˚C	1.90 W	0.0198 ˚C/W
Time to EQM.	25 minutes

Notes

The maximum and minimum frequencies went up 100 MHz and the standard deviation went down 17%, thus meaning that there is greater frequency stability. Average temperatures went down 0.5 ˚C, which I know is from 3 cores running at cooler temperatures, but I do not have data analyzed to show this yet. In addition to the minor frequency bump of 0.686%, which I'd normally write off as statistical noise, average and sustained power consumption went up ~1.15% and ~2.2%. Average thermal resistance, which is the temperature delta compared to ambient by unit of power, decreased by 2.18%

Yes, I ignored peak power and thermal resistance. This is where not having sufficient temporal resolution is a factor, i.e. sampling rate isn't fast enough. However, this tells me something very significant: With the two averaged together, the corresponding TDP is ~1.77x that of PL1 when it should be only 1.25x. Ummm...

If you do just a little bit of digging online, you'll note that the i7-8700K has a slightly different Turbo Boost, per-core, frequency behavior, but these differences are only 100 MHz, or a single clock multiplier. However, the 'K' is an unlocked processor and it has a 95W TDP rating. A couple hundred MHz does not account for a 30W TDP difference. Especially when power testing the two chips against each other produced a ~5 W power consumption difference.

So, did Apple design a poor cooling solution? Eh, probably. Is it that bad? Probably not. For the high end i7-8700B processor, it appears that Apple designed a cooling solution to Intel's spec, but Intel's spec is wrong.

This is substantiated by another metric I observed. Remember how I said that the mini is a heavily grounded computer? Grounding means that various conductors are attached at both ends if they leave the PCB. The heat sink is grounded at the cold plate, mid way on the heat pipes, and in 8 places/screws on the fin array. 4 of those fin fasteners are straight through the PCB. The cold plate fasteners are straight through the PCB. This means that all of the heat that passes into these anchor points get passed to the ground plane(s) which are copper and distribute this heat to the rest of the logic/motherboard. Under the stock setup it took ~40 minutes for other parts of the PCB to reach thermal equilibrium. Under the lapped setup it took ~20 minutes for the same parts to reach equilibrium. More even, less localized heating means that heat is being distributed better in the system.

Where Do I Go From Here?

I've reached out to a couple of people who I've seen that have reviewed/tested the thermal performance of the Mac mini. If they agree, they'll be running the same thermal load and collecting metrics for me to compare all three processor configurations under Apple's design.

I will be waiting for replacement heat sinks to appear on iFixit or the like. Even though I don't have "industry contacts" I might attempt to get one in this manner too. I need to open up the mini again in a few days and take additional measurements and photos of the RAM RF cage and cold place clearance. From that information, I am highly considering using pure Indium on a 145 copper block machined to surround the heat pipes and better fill the volume of space occupied by the stock cold plate.