Mounting Considerations For Power Semiconductors

PublishTime：2022-04-22

■INTRODUCTION:
●Current and power ratings of semiconductors are inseparably linked to their thermal environment. Except for lead−mounted parts used at low currents, a heat exchanger is required to prevent the junction temperature from exceeding its rated limit, thereby running the risk of a high failure rate. Furthermore, the semiconductor industry’s field history indicated that the failure rate of most silicon semiconductors decreases approximately by one−half for a decrease in junction temperature from 160°C to 135°C. Guidelines for designers of military power supplies impose a 110°C limit upon junction temperature. Proper mounting minimizes the temperature gradient between the semiconductor case and the heat exchanger.
●Most early life field failures of power semiconductors can be traced to faulty mounting procedures. With metal packaged devices, faulty mounting generally causes unnecessarily high junction temperature, resulting in reduced component lifetime, although mechanical damage has occurred on occasion from improperly mounting to a warped surface. With the widespread use of various plastic−packaged semiconductors, the prospect of mechanical damage is very significant. Mechanical damage can impair the case moisture resistance or crack the semiconductor die.
●Figure 1 shows an example of doing nearly everything wrong. A tab mount TO−220 package is shown being used as a replacement for a TO−213AA (TO−66) part which was socket mounted. To use the socket, the leads are bent − an operation which, if not properly done, can crack the package, break the internal bonding wires, or crack the die. The package is fastened with a sheet−metal screw through a 1/4″ hole containing a fiber−insulating sleeve. The force used to tighten the screw tends to pull the package into the hole, possibly causing enough distortion to crack the die. In addition, the contact area is small because of the area consumed by the large hole and the bowing of the package;the result is a much higher junction temperature than expected. If a rough heatsink surface and/or burrs around the hole were displayed in the illustration, most but not all, poor mounting practices would be covered.
●In many situations, the case of the semiconductor must be electrically isolated from its mounting surface. The isolation material is, to some extent, a thermal isolator as well, which raises junction operating temperatures. In addition, the possibility of arc−over problems is introduced if high voltages are present. Various regulating agencies also impose creepage distance specifications which further complicates design. Electrical isolation thus places additional demands upon the mounting procedure.
●Proper mounting procedures usually necessitate orderly attention to the following:
◆Preparing the mounting surface
◆Applying a thermal grease (if required)
◆Installing the insulator (if electrical isolation is desired)
◆Fastening the assembly
◆Connecting the terminals to the circuit

Brand of Product：	ON Semiconductor
Part#：
Product Category：	Power Semiconductors More
Application：	More
Data Type：	Application note & Design Guide More
Safety & Environment Standards：	More
Operating Temperature：	Please see the document for details
Qualification Grade：	More
Qualification Approval：	More
Package：	TO−220;TO−213AA;TO−66;DO–203AA;TO–210AA;TO–208AB;DO–203AB;TO–210AC;TO–208;DO–208AA;TO–204AA;TO–213AA;TO–126;TO–220AB
Language：	English Chinese Chinese and English Japanese
Date of Creation：	August, 2021
Version Number：	Rev. 5
Data Encoding：	AN1040/D
Data Size：	784 KB

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