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Torque Standards for Power Packages

The recommendations below are based on current data. As more sources are found, these recommendations may change.

Recommendations

The best data appears to be that provided in STMicroelectronics' TN1225¹⁾. Their data is reproduced verbatim below:

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

Use of M3 screws is assumed in the data above. However, using #4-40 screws (about 5% thinner than M3) with proper washers shouldn't change things significantly.

Discussion of data from sources

STMicroelectionics

STMicroelectionics gives the following guidelines for various packages when using M3 hardware²⁾:

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 provides empiric data for a number of packages over a range of torques using mica insulators.³⁾ The data show marginal to no improvement in thermal resistance above 50Nxcm, which corroborates the 0.55 N*m typical figure given in the previous 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.

Infineon

Infineon ran a test on TO-247 packages with torques ranging from 0.2 N*m to 2 N*m.⁴⁾ 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 parts, but it is known that these data are absent from a substantial number of other manufacturer's datasheets.

Texas Instruments

TI mentions 50N-cm as the recommended maximum for their multi-lead TO-247 packages.⁵⁾ How well this translates to other packages is unknown, but it does put the figure in the same ballpark as STMicroelectronics' recommendations.

¹⁾ , ²⁾

TN1225: Mounting instructions for THD (through hole device) packages. STMicroelectronics. Accessed June 2, 2022.

³⁾

ANAN260: Handling and mounting ICs in plastic power packages. STMicroelectronics. Accessed June 2, 2022.

⁴⁾

AN2017-42: TO-247-3 Advanced Isolation. Infineon. Accessed June 2, 2022.

⁵⁾

SNOA460: AN-1364 TO-247 Package. Texas Instruments. Accessed June 2, 2022.

Mithat Konar (the wiki)

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