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The recommended way to mount a TO-220 package is to use a spring clip or bar pressing on the body of device. But the package was designed to be mounted using a bolt through the hole in the heatsink, and the manufacturers provide mounting instructions for doing so.

The mounting instructions frequently (Phillips, Alpha&Omega, Vishay, IR, Infineon, TI, ON semi etc) contain the suggestion that a rectangular washer be used, particularly when trying to mount fullpack (insulated) packages. For example, infineon, Mounting considerations for TO220 Full PAK.

... but the recomendation is as old as the package, and the same diagram seems to have been copied down from some ancient original source.

The main purpose of the washer is to protect the package: a secondary purpose is load spreading. This is not the insulated washer used for isolation beneath a standard TO-220 package.

Does anyone still use rectangular washers on TO220 packages? Does anyone supply TO-220 rectangular washers as a standard part? Have you ever seen a rectangular washer on the tab of a TO-220? If so, when?

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The use of the rectangular washer helps to spread the pressure from the bolt head evenly across the tab. The tabs are typically copper and can deform as the bolt is tightened.

The rectangular washer would typically be steel and less susceptible to deformation as the bolt is tightened.

Without the rectangular washer any deformation can cause the tab to get decreased surface contact with the heat sink surface which increases the case to heatsink thermal impedance.

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  • \$\begingroup\$ Even more important with Full-Pack (insulated) packages, because deformation can cause delamination of the plastic package. Have you ever seen it in use? Or actually specified it yourself? \$\endgroup\$ – david Feb 27 '15 at 2:50
  • \$\begingroup\$ @david - Yes. You are correct in bringing up the issue of delaminating the encapsulation off the tab. As far as my experience: I've seen products with the rectangular washers. I also recall seeing the rectangular part with a threaded hole and thus acting as a nut whereby the rectangular shape kept the "nut" from turning. As for what I've used and specified it has almost always been nylon bolts which can never get tight enough to cause tab deformation or delamination. \$\endgroup\$ – Michael Karas Feb 27 '15 at 5:19
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I haven't seen a rectangular metal washer used, but there are rectangular rubberish pads that are claimed to improve heat conductivity between the package and heatsink, while maintaining electrical insulation (if you use a nylon screw).

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  • \$\begingroup\$ Hitherto, I haven't seen a rectangular washer between the screw cap and the TO-220 body either. \$\endgroup\$ – Nick Alexeev Feb 26 '15 at 8:28

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