In a post written on the company's blog, chief executive Anna Shedletsky said that after acquiring a Note7 and taking it apart — with a fire extinguisher kept nearby — she and chief technology officer Sam Weiss had found that the design of the Note7 compressed the battery even during normal operation.
The Note7 was released in August and the first reports of explosions were recorded in September. Samsung then issued a worldwide recall but after replacement devices were also reported to be combusting it shut down sales and then stopped production of the Note7 altogether.
The Instrumental researchers found the battery was so tightly packed inside the device's body that any pressure from battery expansion, or stress on the body itself, could squeeze together layers inside the battery with explosive results.
"Any battery engineer will tell you that it’s necessary to leave some percentage of ceiling above the battery, 10% is a rough rule-of-thumb, and over time the battery will expand into that space."
A high resolution image from an Instrumental station shows the tight XY clearances to the battery. It's also possible to see the pocket formed by aluminum walls on all sides, which prevents the rough edges of the PCB from touching the battery pouch. Courtesy: Instrumental
She said the two-month-old Note7 unit had no ceiling. "The battery and adhesive was 5.2mm thick, resting in a 5.2mm deep pocket. There should have been a 0.5mm ceiling. This is what mechanical engineers call line-to-line – and since it breaks such a basic rule, it must have been intentional. It is even possible that our unit was under pressure when we opened it."
Shedletsky said a smaller battery using standard manufacturing parameters would have solved the explosion issue and the swell issue.
"But, a smaller battery would have reduced the system’s battery life below the level of its predecessor, the Note 5, as well as its biggest competitor, the iPhone 7 Plus," she wrote.
"Either way, it’s now clear to us that there was no competitive salvageable design."
She said the design and validation process for a new product was challenging for everyone. "In this case, Samsung took a deliberate step towards danger, and their existing test infrastructure and design validation process failed them. They shipped a dangerous product.
"That this is possible at one of the top consumer electronic companies in the world is humbling – and demonstrates the need for better tools. Instrumental is building them."
Both the Instrumental staff have previously worked for Apple. Shedletsky was the Apple Watch System Product Design lead and manager from October 2012 to February 2015. Prior to that she worked as an iPod Product Design engineer from July 2009 to October 2012. Weiss was a product design engineer for Apple Watch from July 2012 to June 2015.