How to make electrical connection to MOSFET tab?

How does one make a good electrical connection between a wire and the tab of a MOSFET? I tried soldering but the solder just won't stick to the tab. Would a simple nut and bolt suffice? if not, how can I improve the connection?

Which package?
You can normally solder to the tag, but it takes a fairly beefy iron (the tag is, after all, meant as part of the device’s heat-sink).
A solder tag, bolt and nut is an alternative.
Maybe a shake-proof washer too.

I have a 30W soldering iron and the package is TO-220... should it be possible to solder the tab with that iron?

Not sure - haven't used a low-power iron for a while. You could try cleaning the tab with a fine file or emery paper, and then tinning it before trying to attach the wire.

Thanks.. I'll try that and post back.

Em.. I got it to work.. eventually. How does one test a MOSFET? (without plugging it in to a working circuit?)

The "best" way to make connection to a tab is to solder or crimp a wire into a ring terminal or fork terminal (lug, etc.) [get those at hardware store, RadioShack] and then fasten that to the tab with a screw and nut.

Why do you need to make a connection to the tab? The tab is normally connected to one of the pins anyway, so you connect to the pin, not the tab.

I have FETs and centre-tap ("twin diode") rectifiers, in TO-220-like packages, that were manufactured with an unusable/stub centre pin.

Also, TO-3 pakages have two pins and the case.

dc42: Why do you need to make a connection to the tab? The tab is normally connected to one of the pins anyway, so you connect to the pin, not the tab.

In addition to the reasons that runaway_pancake gave, my application (coil-gun) needs to minimize resistance. So I thought that making a connection with the largest possible surface area would be best.. hence I chose the tab instead of the pin. (My MOSFET is in a TO-220 package)

Second, the wire I"m using is 22 guage (SWG), so it's almost as thick as the pin which might make for difficult soldering.

Lastly, I'm skeptical about the current carrying capacity of the pin but the source pin will be handling the same current so that's only a minor issue. Do you think I'd be better off trying to soldering it to the pin ? The current will be about 10 - 12 Amps.

If the application is a coil gun then the current will be pulsed, so connecting to the pin will be fine from a current rating point of view. If you do connect to the tab, attach the wire to a solder tag or crimp tag, then bolt the tag securely to the tab.