AN886 Datasheet, PDF (8/9 Pages) STMicroelectronics – A customer who develops an MCU-based application needs various levels of flexibility in order

SELECTING BETWEEN ROM, FASTROM AND OTP FOR A MICROCONTROLLER
capsulated, it is not possible to test 100% of the user program area. For this reason the final
test is limited to a reserved number of bytes which are programmed and then verified.
OTP rejects rates due to programmability failures
As the programmability of the OTP cannot be fully tested once the die has been encapsulated,
a customer is likely to find a programming reject rate below 1% when programming is per-
formed using qualified programming tools. This rate should be considered as normal for
OTPs and should not generate customer quality returns. If higher failure rates are observed,
it is recommended to check the programming equipment.
Recommended screening conditions for OTP (data retention failures)
Programmability failures can be fully screened because gang programmers check the con-
tent of OTP devices after programming. But gang programmers cannot perform data reten-
tion tests.
High temperature aging is recommended for screening microcontrollers that use OTP devices
and performing data retention tests. The well-known retention failure mechanism corresponds
to charge losses during the component life time. The customer is likely to find a data retention
failure rate below 0.5% when programming is performed using qualified programming
tools. Therefore, it is strongly recommended to screen these potential failures as described
hereafter:
Figure 5. Recommended Screening Conditions for OTP
PROGRAM
PROGRAM
VERIFY
Programmability
Failures
HIGH
TEMPERATURE
AGING
Nitrogen atmosphere
150 ø C / 24 h
Normal atmosphere
125 ø C / 48 h
PROGRAM
VERIFY
Data Retention
Failures
MOUNT
VR02100F
Screening devices that are subject to data retention failures will allow customers to obtain
OTP devices with a similar quality level compared to ROM devices.
In order to further lower the reject levels for programmability and data retention, STMicroelec-
tronics is continuously pursuing technology improvements in areas as soft die mounting, low
stress mold compounds and passivation layer enhancements.
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