Toshiba TX49 TMPR4937, Руководство пользователя

Страница: 38 / 553

3 General Safety Precautions and Usage Considerations
3-16
3.5.10 Tightening torque
(1) Make sure the screws are tightened with fastening torques not exceeding the torque values stipulated in
individual datasheets and databooks for the devices used.
(2) Do not allow a power screwdriver (electrical or air-driven) to touch devices.
3.5.11 Repeated device mounting and usage
Do not remount or re-use devices which fall into the categories listed below; these devices may cause significant
problems relating to performance and reliability.
(1) Devices which have been removed from the board after soldering
(2) Devices which have been inserted in the wrong orientation or which have had reverse current applied
(3) Devices which have undergone lead forming more than once
3.6 Protecting Devices in the Field
3.6.1 Temperature
Semiconductor devices are generally more sensitive to temperature than are other electronic components. The
various electrical characteristics of a semiconductor device are dependent on the ambient temperature at which
the device is used. It is therefore necessary to understand the temperature characteristics of a device and to
incorporate device derating into circuit design. Note also that if a device is used above its maximum temperature
rating, device deterioration is more rapid and it will reach the end of its usable life sooner than expected.
3.6.2 Humidity
Resin-molded devices are sometimes improperly sealed. When these devices are used for an extended period of
time in a high-humidity environment, moisture can penetrate into the device and cause chip degradation or
malfunction. Furthermore, when devices are mounted on a regular printed circuit board, the impedance between
wiring components can decrease under high-humidity conditions. In systems which require a high signal-source
impedance, circuit board leakage or leakage between device lead pins can cause malfunctions. The application
of a moisture-proof treatment to the device surface should be considered in this case. On the other hand,
operation under low-humidity conditions can damage a device due to the occurrence of electrostatic discharge.
Unless damp-proofing measures have been specifically taken, use devices only in environments with appropriate
ambient moisture levels (i.e. within a relative humidity range of 40% to 60%).
3.6.3 Corrosive gases
Corrosive gases can cause chemical reactions in devices, degrading device characteristics.
For example, sulphur-bearing corrosive gases emanating from rubber placed near a device (accompanied by
condensation under high-humidity conditions) can corrode a device’s leads. The resulting chemical reaction
between leads forms foreign particles which can cause electrical leakage.
3.6.4 Radioactive and cosmic rays
Most industrial and consumer semiconductor devices are not designed with protection against radioactive and
cosmic rays. Devices used in aerospace equipment or in radioactive environments must therefore be shielded.