Microsoft Hololens 2 (2020) Teardown Part 1
In a competitive industry, knowing the other players is important. With the recent corporate deaths in the industry, Microsoft’s HoloLens 2 is arguably the top contender in currently-purchasable AR headsets right now. While it does has the known issues including small field of view, low transparency, and comparable heavy weight for most of the use cases, and the display quality would need some improvement, HoloLens 2 is one of the best-integrated AR products right now, backed by several billion dollars of engineering expertise and Microsoft’s mighty name.
We recently acquired a HoloLens 2 to assess its display quality and to evaluate its internal construction. Despite being very heavy, the HoloLens 2 is pretty comfortable to wear; part of the innovation was counterweighting the display unit at the front by putting the band adjustment, batteries, and compute in the back of the headset.
This first part of the teardown covers the back assembly, which contains the band adjustment and compute, but no optics, displays, or display drivers. Some of the images are galleries; click to go to the next image.
To disassemble the back, the adjustment knob has to come out first. After staring confusedly at it for half an hour, we concluded the way to remove the knob was to “pull real hard”. Fortunately, this worked, revealing one of the nicest knobs we’d ever seen:
The knob is made from anodized aluminum, with an internal ratcheting mechanism - I wonder how many engineers worked on it? Removing the knob reveals the other half of the band adjustment mechanism, a pair of gears which drive the two halves of the band:
With the knob out of the way, the back cover comes off quite easily, revealing the delicious innards:
At the bottom, we can see the battery. The battery is 4400mAh (16.7Wh), about what you’d find in a premium smartphone, and is cleverly built out of four 1100mAh cells from DYNAPACK to better conform to the shape of the headset. At the top, we see the the rather neat thermal solution for the compute - what at first glance appears to be a heatspreader is actually a tiny vapor chamber (?!):
The vapor chamber transfers heat to a piece of graphene tape on the back housing of the device. Also visible on the back housing is the monstrous flex which carries signals from the USB-C port, the power button and state-of-charge LEDs, and what appears to be a small thermal sensor.
Microsoft’s thermal solution is pretty clever. One of the biggest challenges with designing mobile electronics is getting heat through the usually-plastic housing of the device; polycarbonate is not a very good thermal conductor. HoloLens 2 seems to do it by using advanced techniques to spread the heat over a large area, then letting it conduct through the thin plastic back housing.
Moving back to the main assembly, we note the two antennas cleverly arranged on the sides, with tiny matching networks carefully hidden on the antennas:
Removing the antennas and heatspreader reveals the main circuit board:
The micro-coax cables on the right carry display data to the displays at the front. Undoing the ZIF connectors and removing a few screws frees the board:
Oof. Imagine being the guy laying out the board and being told “please leave an octagonal hole in the center of your board so we can put a knob through it”.
The board is a little facetious. We immediately notice the large black chip on the right, labeled ‘QUALCOMM PM845’. However, this is not a Snapdragon 845; rather, it is an integrated PMIC for Snapdragon 845-based mobile devices. The PMIC integrates supply generation for not only the SoC blocks, but also DDR. In addition, it manages USB-PD handshaking for charging. The real Snapdragon 845 is carefully hidden behind the Hynix package on the back; the package is a 32Gbit (4GB) LPDDR4X which is stacked on top of the 845. The chip below it is a THGAF8G9T43BAIR, a 64GB UFS flash memory which provides storage for the device.
With the board removed, we can see the rest of the rear assembly. The flex at the top center-left carries power to the display engines at the front. The micro-coax bundle on the right carries data from the computer to the displays; the connector used is very similar to what you’d find in a laptop display.
Conclusions
A lot of thought went into making the compute unit fit the form factor of the headset. The compute board is relatively small; a lot of the area is taken up by connectors and, of course, that hole in the center. Despite the level of integration, the device still uses off-the-shelf connectors and batteries; whereas e.g. Apple commissions custom-shaped packs for their laptops, HoloLens 2 still uses a 4P of off-the-shelf standard cells. This is almost certainly a manufacturing volume issue; HoloLens 2 is not exactly a mass-market product and its hard to justify custom connectors and cells at these quantities. It’s also a little unclear why the Microsoft uses discrete connectors and cables versus a large rigid-flex that spans the headset - we’d believe that for the display data, signal integrity over such a long flex would have been an issue, but running the power to the display over a bundle of wires seems strange. Maybe it was a mechanical thing?
The thermals are very well-done, although a bit over-the-top; we do wonder how much performance is gained from using the vapor chamber over the usual piece of copper tape. On the other hand, the vapor chamber is inexpensive and, for all we know, was the difference between an overheated Snapdragon and one that was merely hot.
The cost of the rear assembly is going to be dominated by the 845 (we don’t know how much Microsoft pays for the chip, but its not cheap). The flash and RAM are comparably cheap, at roughly $40 for the pair. It’s also interesting to think about assembly; the whole electronics package is a complex three-dimensional structure with relatively low ship volume, so we’re almost certainly looking at manual assembly over high levels of automation.
Next up: a display quality analysis (HoloLens 2 uses an absurd double-LBS system to achieve its marketed ‘1080p’ resolution) followed by hopefully a look into the display engine components.
In General, Teardowns, Electronics, AR