MOSFET With Zener Diode Protection 

The layer of silicon dioxide between the gate and the channel is very thin and is subject to puncture by static electricity. When working with MOSFETs, special handling precautions should be taken as follows:

1. MOS devices should have their leads shorted together for shipment and storage. Conductive foam is often used for this purpose.

2. Avoid unnecessary handling, and pick up the device by the device by the case, not the leads.

3. All test equipment and tools should be connected to the earth's ground.

4. Workers handling MOSFET devices should have grounding straps attached to their wrists.

5. Never remove or insert MOSFET devices with the power on.

Many MOSFET devices are now available with built-in more Zener diode protection. Zener diodes are placed between the gate and the channel, as shown in Fig. 

The voltage rating of the more Zener diode is below the voltage that would cause punctures to the silicon dioxide insulator. If the voltage between the gate and the channel exceeds the voltage rating of the more Zener, the diode conducts.

thus protecting the MOSFET from damage. At normal operating voltages, more Zener diodes have no effect on the operation of the MOSFET

 

When a power MOSFET fails, the challenge is to determine the root cause of failure and implement a containment plan. However, this is more easily said than done. 

The complexity of semiconductor technologies and the advanced techniques required to analyze them means that failed components must be handled with extreme care in order to retain all possible failure signatures. 

Unfortunately, the urgencies of the manufacturing environment often lead to shortcuts as a failed component is rushed back manufacturer for failure analysis. If this is not carefully done, vital failure signatures can be lost, which adds further difficulties to the already complex task of failure analysis.

The end result is further delays or loss of opportunities to correctly identify the root cause of failure. Typically, failures are reported as In-Circuit Tester (ICT), Functional Tests, or Field Returns. 

The following steps help to ensure that the source of MOSFET failure will be identified while avoiding any further damage or loss of failure signatures

Follow ESD protection procedures throughout the hand of the suspect MOSFET

1. Resistive Test

Use a low-voltage laboratory-grade digital multimeter such as Keithley or HP to measure the resistance between gate and source, gate and drain, and

drain and source. Do not use a portable DMM. The 9 v battery supply can easily increase the extent of the damage or even destroy the evidence. It can also lead to

conflicting findings with repetitive tests. Thus a MOSFET with damage only in the gate-source region and exhibiting low gate-to-source resistance on the first measurement

can later exhibit high resistance and healthy functionality. Ina The result will be no fault found on the FA report.

2. De-soldering

After determining that the MOSFET has failed, the next step is to remove the suspect MOSFET from the PB board assembly. The objective here is to remove the part from the PC board assembly without causing any further physical damage and/or loss of evidence. 

Only uniform, controlled heating should be used for the removal of the MOSFET from the PC board assembly. Using standard soldering stations. can easily subject the part to non-uniform heating by individually heating up each solder joint. 

This approach is more likely to alter the failure signature. Do not use standard soldering stations. An advanced soldering station like the Metcal APR5000 is needed to generate a de-soldering temperature profile that is close to the reflow profile for the safe removal of the part.