How much torque should be used to screw down TO-220, TO-247, and  TO-264? - Page 1
Forum section: electronics / projects

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: April 13, 2025, 11:07:18 pm
content: Hello,I tried looking up this info some time ago, but couldn't find any info. I thought it would be easy to look up because there's probably a database out there of values and packages.What would be the torque you would apply to the screws (lb/in N/m) for TO-220, TO-247, and TO-264 packages?Thanks!

author: HwAoRrDk
author_rank: Super Contributor
timestamp: April 13, 2025, 11:44:57 pm
content: I suppose it depends on what you're mounting to - PCB, heatsink, etc.I found this app note, which recommends 0.8-1.1 Nm or 7-10 in-lb, but only in the context of mounting to a heatsink.https://www.aosmd.com/res/application_notes/package/AN101_TO220_Guidelines.pdfThis one recommends 0.5 Nm:https://www.infineon.com/dgdl/Infineon-Infineon-AN2006_12_Mounting_Considerations_TO220_FullPAK-AN-v2.1-en.pdf-ApplicationNotes-v02_01-EN.pdf?fileId=db3a304320896aa201208a8b700d0029And this one 15 in-lb:https://www.vishay.com/docs/72674/72674.pdf

author: incf
author_rank: Frequent Contributor
timestamp: April 13, 2025, 11:53:27 pm
content: I'd speculate that they don't need to be very tight at all. 10-20% of the max rated torque of the fastener is probably sufficient to get good thermal contact and not risk the screw backing out. (especially if thread locking adhesive is used)In general what may matter is not breaking or bending whatever it is being attached to - like a PCB.

author: Zoli
author_rank: Frequent Contributor
timestamp: April 14, 2025, 12:28:11 am
content: IMO: it depends on the mounting style: with or without insulating plastic washer.With insulating washer, I wouldn't go above 1Nm; usually, once past 1Nm, you can feel the washer deforming, which, in my books, is a big no-no.Without insulating washer, I would go for 1.5Nm, but not over 2Nm.

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2025, 01:08:33 am
content: 5 in-lbs dry torque for 4-40 and 8 in-lbs dry torque for 6-32 hardware. Use a conical compression washer if possible.The TO-220 specification is 8 in-lbs dry torque, but 4-40 hardware is only rated to 5 in-lbs. The other packages are tougher.

author: Smokey
author_rank: Super Contributor
timestamp: April 14, 2025, 02:02:51 am
content: Probably depends on what's on the interface. You want a target % compression on an insulator pad. Also don't crush the shoulder washer if it's plastic. Stuff like that.

author: 2N3055
author_rank: Super Contributor
timestamp: April 14, 2025, 06:10:40 am
content: M3 screw gets tightened to 0.5-0.6 Nm. Make sure thermal paste does not get on the threads, lubrication changes torque-actual screw tightness ratio.Make sure to not overtighten. Use torque wrench.Not only plastic insulating washers get damaged, on TO220 with soft interface, you can actually deform the tab so part where the silicon is (and the heat) can lift from the surface...David attached the old Motorola note on this. Study well.When possible use spring mounting. It works better long term.

author: Hydron
author_rank: Super Contributor
timestamp: April 14, 2025, 06:19:52 am
content: Tighten until it loosens, then back off a quarter turn

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2025, 06:22:51 am
content: Quote from: 2N3055 on April 14, 2025, 06:10:40 amThe recommendation from the application note seems high to me, however I have never tested it with a torque wrench and have always tightened by hand, even in a production environment.

author: 2N3055
author_rank: Super Contributor
timestamp: April 14, 2025, 07:55:04 am
content: Quote from: David Hess on April 14, 2025, 06:22:51 amDo not use data from Table 1. That is absolute minimum thermal resistance test data.Observe figures in Fig. 3 page 5. They max out at 6 IN-LBS, that is cca 0.67 Nm. As you can see, from 0.2-0.3 Nm it is already doing fine.I just checked my torque wrench, it is set to 0,45Nm. That works well.That app note explains a lot of things how thermal interface works. It is a good tutorial.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: April 15, 2025, 01:09:34 am
content: I'm asking about mounting to a heatsink, not a PCB. I'm not using washers for my current application, though I may do so in the future.Thanks again guys. Lots of good advice.