AN1040

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.

Current and power ratings of semiconductors AN1040 are inseparably linked to their thermal environment. Except for leadmounted 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 onehalf for a decrease in junction temperature from 160°C to 135°C.(1) Guidelines for designers of military power supplies impose a 110°C limit upon junction temperature.(2) Proper mounting minimizes the temperature gradient between the semiconductor case and the heat exchanger.

Most early life field failures of AN1040 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 plasticpackaged 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 TO220 package is shown being used as a replacement for a TO213AA (TO66) 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. AN1040 is fastened with a sheetmetal screw through a 1/4, hole containing a fiberinsulating 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.