About 700 Samsung researchers spent months evaluating the devices and found that the original and replacement phones both had different flaws that caused the devices to overheat and occasionally explode.
In the first batch of Note 7s, the battery was too big for the case it was in. So the batteries' electrodes bent in some cases and caused the phone to overheat.
Then Samsung issued a recall and replaced the batteries it had been using. However, those batteries also had an issue.
The replacement batteries had an "abnormal weld spot" which basically allowed parts of the battery to touch areas it wasn't supposed to. Some devices lacked insulation in batteries, as well. Those flaws also caused Note 7s to overheat and explode.
If you're a picture person and would like to see what we're talking about – Samsung has diagrams of faulty versus normal batteries here. Here's one example:
How Samsung will prevent similar incidents
Samsung has already recalled its Note 7s, and 96 percent of the 3 million devices sold have been returned.
Future devices will need to go through extra battery safety tests.
"Since the Note7 recall, we’ve re-assessed every step of the smartphone manufacturing process and developed this 8-Point Battery Safety Check," Samsung said.
Basically, batteries will be put through "extreme testing," then inspected by X-ray.
The company says it wants people's trust, so it's released details regarding the 8-Point Battery Safety Check here.