Aggressive design caused Samsung Galaxy Note 7 battery explosions
instrumental.ai> 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.
Seems like really vague speculation. Any evidence this is not pseudo-engineering nonsense?
Hi, I'm Anna Shedletsky, the author of the post linked above. I spent six years as a product design engineer at Apple, designing very similar products and making similar design decisions that Samsung engineers needed to make.
Typical smart phone / tablet type products use a stamped metal frame ($) to stiffen the product and support the display. A few phones use die-cast magnesium frames ($$). A CNC'ed pocket ($$$$) is a significant cost adder for this product (larger starting billet, longer cycle time, and thousands of CNC machines required) -- so this choice was clearly made for a reason.
Additionally, the fact that the battery is surrounded on five sides by a machined pocket cuts into battery volume -- most products just rely on an air gap (0.5-1mm) between the battery pouch and the next nearest component that could poke it during assembly and usage. Take a look at some other product teardowns on iFixit, and you'll see this (example from iPhone 7: https://d3nevzfk7ii3be.cloudfront.net/igi/fJI2bhqYXg4CMNPi.h...)
They added significant cost to the product and sacrificed a little battery capacity to have that pocket. I hypothesize that it was for battery safety given the aggressive design.
What about forging? Rough out the shape from AlZnMgCu1,5 (EN AW-7075) alloy, heat to 450°C, and slam it between a pair of dies.
Compared to cast or machined parts, you get a 20% higher strength-to-weight ratio from the forging. The cycle time is decent I think, perhaps 3 per second with a 0.06 second contact time in the dies.
Tolerances are poor with forging.
It's not called out specifically in the prose of article, but I think the dimensions overlaid on the image show quite a bit of intentional space left for tolerance around the sides of the battery, and they mention the walls formed by the aluminum carrier around the battery pack. This is pretty clearly intentional "walling off" of the battery from the rest of the PCB, and (I assume) is more protection than other devices were designed with.
If my assumption is correct, though, I find it strange that article footnote seems to contradict this by essentially saying the engineers didn't give enough space around the 'front' and 'back' of the battery. Did they protect the PCB and battery from each other, and fail to protect the battery from the front and back of the phone?
The images show very tight clearances around the battery (0.1mm is the thickness of a human hair) in plan view (XY). The footnote explains discusses the lack of "ceiling space" for the battery to expand into, which mostly happens in Z (the thickness of the phone). In Z, the design is line-to-line.
To a machinist, a tenth of a millimeter is a yawning gulf.