All MOS devices have insulated gates that are subject to voltage breakdown. For instance the gate oxide for Motorola CMOS devices is about 900 A thick and breaks down at a gate-source potential of about 100 V. To guard against such a breakdown from static discharge or other voltage transient, the protection networks shown in figure are used on each input to the CMOS device.
Static damaged devices behave in different ways, depending on the severity of the damage. The most severely damaged inputs are the easiest to detect because the input has been completely destroyed and is either shorted to VDD, shorted to Vgs or open-circuited. The effect is that the device no longer responds to signals present at damaged input. Less severe cases are more difficult to detect because they indicate as intermittent failures or as degraded performance. Another effect of static damage is that the inputs generally have increased leakage currents.
Although the input protection network provides a great deal of protection, CMOS devices are not immune to large static voltage discharges that can be generated during handling. For example, static voltage generated by a person walking over a waxed floor have been measured in the 4-15 kV range (depending upon humidity, surface conditions etc.).
Important precautions for protecting/safe guarding Cmos devices
1. Do not exceed the maximum ratings specified in the data sheet.
2. All unused device inputs should be connected to VDD or Vss
3. All low-impedance equipment (pulse generators, etc) should be connected to CMOS inputs only after the device is energized. Similarly, this type of equipment should be disconnected before power is switched off.
4. Circuit boards containing CMOS devices are merely extensions of the devices, and the same handling precautions apply. Contacting edge connectors wired directly to device inputs can cause damage. Plastic wrapping should be avoided. While external connections to a PC board are connected to an input of CMOS device, a resistor should be used in series with the input. This series resistor helps limit accidental damage if the PC board is removed and brought into contact with static generating materials. The limiting factor for the series resistor is the added delay. This is caused by the time constant formed by the series resistor and input capacitance. Note that the maximum input rise and fall times should not be exceeded. In figure two possible networks are shown using a series resistor to reduce ESD (electrostatic discharge) damage.
These networks are useful for the protection of
· digital inputs and outputs
· analog inputs and outputs
· 3-state outputs
· bidiSrectional (I/O) ports.
5. All CMOS devices should be stored or transported in materials that are antistatic. CMOS devices must not be inserted into conventional plastic ‘snow’, styro foam, or plastic trays, but should be left in their original container until ready for use.
6. All CMOS devices should be placed on a ground bench surface and operators should ground themselves prior to handling devices since a worker can be statically charged with respect to the bench surface. Wrist straps in contact with skin are strongly recommended.
7. Nylon or other static generating materials should not come in contact with CMOS devices.
8. If automatic handlers are being used, high levels of static electricity may be generated by the movement of the device, the belts or the boards. Reduce static build-up by using ionized air blowers or room humidifiers. All parts of the machines which come into contact with the top, bottom, or sides of IC packages must be grounded to metal or other conductive material.
9. Cold chambers employing C02 for cooling should be equipped with baffles, and the CMOS devices must be contained on or in conductive material.
10. When lead-straightening or hand-soldering is required, provide ground straps for the apparatus used and be sure that soldering ties are grounded.
11. The use of static detection meters for production line surveillance is highly recommended.
12. Equipment specifications should alert users to the presence of CMOS devices and require familiarization with this specification prior to performing any kind of maintenance or replacement of devices or modules.
13. Do not insert or remove CMOS devices from test sockets with power applied. Check all power supplies to be used for testing devices to be certain there are no voltage transient present.
14. Double check test equipment setup for proper polarity of VDD and VSS before conducting parametric or functional testing.
15. Do not recycle shipping rails or trays. Repeated use cause deterioration of their antistatic coating.