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Posted on 27 June 2019

Device Failure due to Incorrect Mounting

Device Failure due to Incorrect Mounting

 

 

 

 

 

 

 

Power semiconductor device failure is often the result of incorrect mounting inside equipment. The most common case of incorrect mounting of power semiconductor devices (PSDs) occurs when one is attached to a heat sink. Damage to the semiconductor element may occur as a result of such mounting and also as a result of insufficient dissipation of heat from the device during the operation process. In practice, incorrect mounting of a heat sink can be determined during visual examination of a damaged PSD. Below are several examples of specific case of incorrect mounting of power semiconductor devices that may lead to device failure.

1. Incorrect preparation of contact surfaces of power semiconductor devices and heat sinks

Contact surface of a power semiconductor device

Such mounting does not guarantee secure heat (sometimes electrical) contact of power semiconductor device with the heat sink, and with increased mounting force can lead to damage of the semiconductor element. Covering contact surfaces of disc type devices with heat conducting pastes that lack electroconducting characteristics can also lead to device failure.

 

2. Heat conducting paste lacking electroconducting characteristics

Power semconductor device lacking thermal paste

Device failure of this nature may result from:

  • Presence of thin and spotty layers of insulating paste between the surfaces of the power semiconductor device case and heat sink
  • Mounting the electroconducting bar lead to electric contact
  • Repetitive resistance of contact equals or even exceeds the on-state voltage loss

The above errors may lead to additional power loss, inadmissible overheating, and thermal breakdown of the power semiconductor device. Due to the localization of the electric contact, overheated areas of the semiconductor element are localized as well, and device failure often results.

3. Incorrect selection of contact surface diameter of heat sink

Power semiconductor device contact surface diameter

Device failure may result if the clamping machine print is of a smaller diameter than the contact package base. Contact bases of disc type packages are made of soft material (oxygen-free copper), which is the reason why they do not transmit pressure loads to the associated parts of semiconductor element despite being relatively thick. In such cases, thermal and electrical contact areas between  semiconductor elements and package bases may be disrupted.

Loss of thermal contact leads to local overheating of the defined areas on the semiconductor element. However, even for a device operating in an overlapping single current pulse mode which does not require effective heat dissipation, incorrect mounting may lead to break-down due to insecure electrical contact. During high peak current pulses with high rise speed, electrical arc discharge may appear in the local areas of the power semiconductor device which do not have any electrical contact between the cathode metallization of the semiconductor element and the conducting cathode package base (or cathode layer). Gradual degradation of the semiconductor element surface occurs (pitting) and leads to a break-down and device failure.

4. Incorrect center element dimensions or axial offset of the power semiconductor device

Center alignment of power semiconductor devices

The above mentioned mounting problem leads to rapid increase in pressure at the centre of the semiconductor element. As a result, the element may be mechanically damaged, or (in the case of thyristors) short-circuiting of amplifying electrode by cathode base may occur, leading to device failure.

 

5. Uneven mounting force on the contact surfaces of the power semiconductor device

Uneven mounting force of semiconductor devices

Evenly distributing the force applied when mounting power semiconcuctor devices can help prevent device failure. An uneven distribution of mounting force on the contact surfaces of power semiconductor devices can lead to the destruction of the semiconductor element as a result of inadmissible increase in local pressure at hold-down, as well as overheating followed by a thermal breakdown due to insufficient heat dissipation of the element.

 

 

6. Mounting overforce that leads to mechanical damage of the semiconductor element

Excessive mounting force on a power semiconductor device

Device failure may also result from the application of excessive force during mounting. If excessive force is evenly distributed over the contact base surface of a power semiconductor device, the error can not be detected by visual examination. However, after decapsulation (for thyristors with subdivided amplifying electrode) a characteristic deep imprint of the amplifying electrode image can be seen on the cathode base of the package. Mounting overforce leads to destruction of semiconductor elements and to splitting of the semiconductor wafer which can be determined only by etching of the element with special etchants in standard conditions.

7. Determining Insufficient  Mounting Force using Infrared Imaging

Infrared imaging of a power semiconductor device

Application of insufficient force during mounting is another common cause of device failure. Insufficient mounting force, or its total absence, may lead to damage of operating thermal conditions and parametric failure of power semiconductor devices. During the operation process, the power semiconductor device may overheat as a result of insufficient mounting force. This can be detected using infrared imaging.

 

 

For more information, please read:

Device Failure due to Electrical and Thermal Conditions

Served Out Power Semiconductor Devices

 

 

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