AN2434 Freescale Semiconductor / Motorola, AN2434 Datasheet - Page 21

• Manufacturer Part Number: AN2434
• Description: Input/Output (I/O) Pin Drivers
• Manufacturer: Freescale Semiconductor / Motorola

High Performance Embedded Systems Division
Reliability and Quality Assurance
6501 Wm. Cannon Drive West
Austin, Texas 78735-8598
Failure Analysis Technical Report on Electrical Overstress (EOS)
Typical Failure Mechanisms for EOS
The physical failure mechanism which results from an EOS event is greatly dependent on the total
energy applied to the pin. Both time and current play roles in determining the total energy.
Temperature can also be a factor since it is the act of melting metal and oxide which results in an EOS
failure.
There are really only two failure mechanisms normally associated with EOS, fused bond wires and
fuse die metalization (Figures 2 and 3). The average bond wire for a device is 1 ml in diameter and
approximately 60 mils long. A DC current of 1 Amp would be sufficient to fuse a bond wire of this
dimension. Higher current pulses of shorter duration could also fuse a bond wire. For example, a 5
Amp, 1 msec pulse would most likely fuse the average bond wire. In both these cases, the energy
dissipated is equivalent to the energy absorbed by the bond wire. Simple calculations show that when
the temperature necessary to absorb this energy exceeds the melting temperature of the gold bond
wire, the wire will fuse.
The second mechanism associated with and EOS event is fusing of die metalization. Typically, these
events are extremely high current spikes of short duration (<170 µsec). In this event, the heat
dissipated on the die is conducted way from metalization through the SiO
fairly good thermal conductor therefore the current needed to fuse the die metalization is fairly high,
typically around 10 Amps.
Since heat plays a big part in creating the EOS damage, it is important to note that in plastic
encapsulated devices, the actual fused metalization may not be visible due to carbonized plastic
residue left on the die. This carbonized plastic is the direct result of the locally absorbed energy and
it's resulting high temperature.
Figure 2. Optical micrograph of a fused
bond wire..
Figure 3. Optical micrograph of typical EOS
damage. Both fused and reflowed metal are
evident.
2
to the substrate. SiO
reflowed
reflowed
fused
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