Instrumental report says there wasn't sufficient clearance between the battery and its enclosure, and that 'aggressive design' led to the battery being squeezed.

Ever since the late Samsung Galaxy Note 7 was withdrawn from sale, there has been no shortage of theories β€” both legitimate and dubious β€” about exactly what caused the phone's battery to go boom. Samsung originally blamed a battery cell issue in cells from one supplier, however the second recall and subsequent cancellation of the phone would seem to discount that.

Now a report from engineering firm Instrumental claims to have solved the mystery. The cause of the catastrophic Note 7 battery failures, the firm's CEO says, basically has to do with the battery being too big. In her report, Anna Shedletsky concludes that there wasn't enough space around the battery to allow for various manufacturing tolerances, and the slight expansion of the battery through use.

What's interesting is that there is evidence in the design of an intellectual tension between safety and pushing the boundaries. Samsung engineers designed out all of the margin in the thickness of the battery, which is the direction where you get the most capacity gain for each unit of volume. But, the battery also sits within a CNC-machined pocket -- a costly choice likely made to protect it from being poked by other internal components. Looking at the design, Samsung engineers were clearly trying to balance the risk of a super-aggressive manufacturing process to maximize capacity, while attempting to protect it internally.

High-res images posted by Instrumental show less than 0.1mm between that "machined pocket" and the top of the battery. Other dimensions are similarly constrained.

Note 7 close-up

While this is all based on a sample size of one, Instrumental's full report is well worth a read if you're at all interested in how a modern smartphone is put together, and the challenges involved in cramming ever more technology into a smaller space. Interestingly, Shedletsky says that even if the Note 7 hadn't suffered an early demise, the shortage of space around the battery would have eventually caused the phones to break apart through battery swelling.

One thing's for sure: With a new Samsung flagship expected early next year, the Korean company is sure to be paying extra attention to engineering tolerances and battery size.