8/9

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

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.

VR02100F

PROGRAM

VERIFY

HIGH

AGING

PROGRAM

MOUNT

Nitrogen atmosphere

Normal atmosphere

TEMPERATURE

VERIFY

150 ø C / 24 h

125 ø C / 48 h

PROGRAM

Programmability

Failures

Data Retention

Failures