Hello Philipp,
Saturday, August 5, 2017, 8:41:42 PM, you wrote:
PS> Yes, you're probably right.
PS> Though I wonder when and how they programmed the firmware. Before or PS> after soldering?
Most likely before, unless they have some debug header exposed. From [1]:
When the hardware and software nears production readiness, it is common practice to preprogram flash memory devices prior to starting high-volume PCB manufacturing flows for two principal reasons. First, firmware loaded onto the device can be used to perform basic booting and testing of the PCB during manufacturing to check system/module functionality. Second, loading the final firmware, operating system (OS), and application code on the flash device prior to manufacturing maintains a high-volume manufacturing beat rate. To support these usage models, multiple vendors provide systems for loading firmware and data into flash memory devices prior to the PCB solder flow process.
Modern flash chips don't have issues retaining programmed bits during reflow soldering as long as the correct temperature profile is observed [2].
[1]: http://www.electronicdesign.com/memory/understanding-onboard-flash-programmi... [2]: http://dataioinfo.com/LiveImages/26/20/DocumentURL.pdf