In the case of OTP microcontrollers, until before the assembly process, any data that is written can be erased, so that testing of the PROM part can be done. However, once assembly is done, the contents of the PROM can no longer be erased, so that past the assembly process, PROM write tests cannot be performed. Therefore, the screening after a customer wrote progrem is needed to eliminate PROM defects caused during the assembly process.
On the other hand, in the case of flash microcontrollers, erasure by writing to the flash memory part is possible even past the assembly process. In other words, defects caused during the assembly process are already eliminated in the manufacturing process. Therefore, screening after the customer wrote the program is not required.