footprintheatsinkthrough-holeto220
I am using a TIP120 Darlington transistor in a through-hole (specifically TO-220) package, in a very simple application which does not generate much heat.
I remember reading that the metal tab at the top is only for connecting to an optional external heat sink.
Due to a design dimensional issue (unforeseen!), I need to reduce the height of this package in my case.
Can you cut the metal tab off without any effect on the functionality of the component?
As long as you can get good thermal and mechanical contact I don't see why it can't work. I think this version used a bit of heat shrink to hold it nice and tight.
Lead resistance is not a problem. Look at the IRFB7545PBF data sheet. Lead cross-section is ~0.38 mm x 1.14 mm (minimum). Cross-section area is ~ 0.5 mm\$^2\$. This is equivalent to 20 ga wire, and has a resistance of ~10 mΩ/ft. A lead length of 0.3 inch will give a total resistance of 0.25 mΩ. Two leads, of course, will have a total resistance of 0.5 mΩ (less than 10% of the stated 5.9 mΩ). Total power dissipation for the package is 26 W (67 x 67 x .0059), which is well within limits for a TO220.
Under this load, each lead will dissipate 1 W. For a long run in free air, this is enough to melt the lead, but that is not how a TO220 is used. The leads will be soldered into a pc board with very heavy traces (to avoid delamination of the board), and these traces will serve to conduct heat away from the leads.
And for what it's worth, the data-sheet specifies the 67 A figure for a case temperature of 100°C.
ETA - According to a FAQ note at irf.com, the TO220 package has a current limit of 75 A, so the original suspicion that a TO220 can't handle high currents is almost correct.
Best Answer
I have done it, but I would never ship anything like that out.
I suggest clamping the "waste" end of the TO-220 in a vise firmly and sawing it with a fine tooth hacksaw blade, not allowing anything to touch the epoxy.
Shearing would be much faster and neater, but I fear it would distort the metal and cause the part to become more unreliable than necessary.