TDP, Thermal Design Power

Very recently, the enthusiast community has dug into Intel's TDP ratings and performance. Even since Multicore Enhancement, aka MCE, the performance of Intel's CPUs has varied depending on the motherboard manufacturer. The frequency at which a CPU operates is dependent upon a number of factors and even some limitations like T-Junction, which is 100 ˚C for all three CPUs in this configuration.

If you want to dive into the discussion of Intel's TDP ratings and how they fluctuate, you can read a number of articles such as this on over on Anandtech.com. More or less, TDP is allowed to be fairly high for some time frame before throttling down for thermal reasons inside the die. This upper limit is called the PL2 state, which is 125% the rating of the listed, marketed TDP, which is called PL1. For all three CPUs in the Mac mini 2018, the PL1 TDP is 65 W, thus PL2 is 81.25 W.

TIM & Cooling methods

If you've been around the enthusiast community for some time, you'll know that there are a number of factors that affect the thermal performance of a CPU. The one that is easily modified by reasonably skilled individuals is the TIM, or thermal interface material. This ranges across many types of materials from a variety of thermal pads, to "eh" bulk compounds, to higher end boutique pastes, to exotic Galinstan alloys.

The reason why people change out their TIM is because it is the simplest way to increase the cooling capacity of the cooler being used. Each part of a cooling interface is quite literally stacked up with each other part. In other words, if you change one component, you change the performance of it all. If you remove components, you reduce the amount of material thermal energy has to pass through, making the whole system more efficient. As such, more advanced modifications involve de-lidding a CPU, which is removing the integrated heat spreader. Sometimes the TIM is replaced between the die and IHS, or sometimes the IHS is removed all together and "direct die" cooling is employed.

In essence, mobile/embedded coolers are factory direct die cooling, though they are usually used for lower TDP applications. Apple though, in the case of the Mac mini, is using desktop TDP parts in an application similar to a mobile cooling solution, which is why cooling performance falls short.

Mounting Pressure

Regardless of which method you use, one factor always has a significant effect - contact pressure.

From at least Intel's 5th generation to its existing 8th generation processors, the maximum applied compressive load is 15 pounds without a backplate and 25 pounds with a backplate.

• 5th gen doc, page 8
• 6th gen doc, page 125
• 8th gen doc, page 140 (last page).

Also, yes, what I listed is for 'H' and 'U' series CPUs, which are exposed die, are mobile chips. Even though the ones in the Mac mini are 'B' series CPUs, the dies are on the same FCBGA1440 package, so they are subject to the same specification.

There are 2 leaf springs on the back of the cooling solution in the Mac mini. Using a calibrated scale, it was a rather round number of 1 kilogram that was required to compress either spring in to the same flatness as it was installed. This means that 2 kgf was being applied as the clamping load.

The die area of the i7-8700B is 149.6 mm^2. Now, Intel's documentation is a little sparse on the mounting and clamping loads, etc., so my interpretation is as follows: Even though the values of 15 & 25 have the units of lbf or "pounds of force," this is not equivalent to the clamping load, or how much weight you press down on the die with. Yes, those units are the same, but hear me out: If Apple's implementation is sub-par to where they don't have the clamping load correct, ~4.4 lbf instead of 15-25 lbf, then increasing the clamping load would increase the cooling performance of solution they designed. But it doesn't.

While I was observing the lapped heat sink and new TIM run, there wasn't a significant difference in long-term performance even though I applied a 0.016" cardboard shim to the mounting plates before placing the springs on top. Thus the 2 kgf load should be considered over the surface area of the die, 149.6 mm^2. Converting that to psi, you get ~19.05 psi of pressure applied to the die. The "pounds" in psi is a force, or lbf, and 19 is very squarely in the range of 15-25; especially considering the mounting plates for the leaf springs are, functionally, a partial backplate.

Overall Thermals

As prooved by the MacBook thermal throttling issues earlier in 2018, Apple is starting to release products that are improperly designed or configured to reach the performance of their CPU. Unfortunately, the Mac mini is also one of these products, though the i9 MBP uses an i9-8950HK. 

If you took that CPU, which has a TDP of 45 W, and used it instead, you'd have a reduction of about 5% in performance compared to the i7-8700B - assuming it didn't thermal throttle. Currently, my testing shows the i7-8700B as only able to sustain 3.8 GHz vs its 4.3 GHz limit, which is a reduction of 12% in performance compared to its own spec.

As for the specs regarding the temperature readouts - aka Digital Temperature Sensors (DTS) - inside the die, Intel states that they have a +/- 5 ˚C rating (Section 5.1.5.2.1), which means that there *can* be a 10˚ separation of die temps while they are all equal and constant. Since the CPU throttles a core once said core hits "100 ˚C", effective performance between cores can vary significantly when you run the CPU at a global temperature of 95 ˚C or higher. Current dogma for the enthusiast community, though, is to keep temps below 85 ˚C. On average or via core maxima, this is the temperature goal I'm aiming for, if it is possible.