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Table of Contents
Torque Standards for Power Packages
Summary of data from sources
STMicroelectionics1) gives the following guidelines for various packages when using M3 hardware:
Package | Typical torque (N*m) | Maximum torque (N*m) |
---|---|---|
TO-220 | 0.55 | 0.70 |
TO-220SG | 0.55 | 0.70 |
Pentawatt | 0.55 | 0.70 |
Heptawatt | 0.55 | 0.70 |
TO-220FP | 0.55 | 0.70 |
ISOWATT220 | 0.55 | 0.70 |
TO-247 | 0.55 | 1.00 |
TO-218 | 0.55 | 1.00 |
The kind of insulator (mica, silicone, etc.) isn't specified for these data.
A different STMicroelectronics reference 2) provides empiric data for a number of packages over a range of torques using mica insulators. The data show marginal to no improvement in thermal resistance above 50Nxcm, which corroborates the 0.55 N*m typical figure given in the table.
However, the maximum torque is slightly contradicted here (emphasis added):
Suggested maximum torques for M3 screws are _80Nxcm_ for TO220, PENTAWATT, HEPTAWATT, MULTIWATT and CLIPWATT 19 H packages. Particularly for FLEXIWATT package, clipping system is strongly recommended. While suggested maximum torque for M3 screws is 70 Nxcm for TO220SG (single gauge).
The data given in the table above is the more conservative and therefore preferred: 0.55 N*m nominal for all packages listed with a maximum 0.7 N*m for everything except TO-247 and TO-218, which can be squeezed up to 1 N*m.
Infineon3) ran a test on TO-247 packages with torques ranging from 0.2 N*m to 2 N*m. No failures were reported. However details of the test (e.g., what they consider “standard thermal cycles”) are lacking. Infineon also recommends using the torque specified in datasheets. It's hasn't been confirmed that Infineon provides this specification for their devices, but it is known that these data are absent from a substantial number of other manufacturer's datasheets.
TI4).
Recommendations
TBD