Connecting Wires to Transistors

[Prithul0218]

I want to connect some 14AWG wires to a TO-247 package transistor connected to a heatsink. The PCB is separated from the heatsink. I am afraid soldering the wires is not a good choice as the pin/solder might break over time due to mechanical vibrations.

So, what is the right way to connect wires to transistors mounted on heatsink?

[bd139]

This is used, for better or worse, in some commercial equipment. The main risk is the increased inductance and the probability of shorting and breaking from mechanical movement.

Some studies for you:

Racal 9915 counter uses this for power transistors mounted to case. I have two, the leads are bare, splayed apart and jump wires connected about 2-3cm to the board. Works fine after 42 years or so still and these are portable and have been bashed around a lot. It makes me feel a bit ugh looking at it but it clearly worked.

Tektronix 2235 uses a plastic connector connected to the MOSFET leads on the preregulator. The other end is soldered to the board. This was knocked by me during refurb which caused a problem with the connector or the mounting and subsequently the MOSFET exploded. This burned the connector. I snipped the connector off, spliced the leads to the wire and heat shrunk each wire. All has been good since.

For me:

1. Avoid wicking the solder in the wires.
2. Make sure the design can handle the inductance of the leads otherwise this can lead to oscillations and all sorts.
3. Do solder it (make sure it isn't just lap-soldered) and shrink it.
4. Make sure the leads on the transistors don't have to hold a lot of mass. They are more brittle than the wires.

I'm sure there will be differing opinions

[Ian.M]

14 AWG is fairly thick (1.6mm) compared to the transistor pins.   IMHO its imperative to anchor the wires to the heatsink close to the transistor. to prevent repeated strain on the pins.

[soldar]

The first thing to do is ask if you really need such thick wires.

And you can use some kind of terminal block which is mechanically fastened so that there is no mechanical stress on the semiconductor terminals. This might be as easy as using hot glue. 

[ciccio]

I agree with sodar.
14AWG is really thick, expecially when compared to the transistor's termimal cross section.
Such a tick wire is difficult to solder to the pins. It will take too much time for a good joint, you can overheat the transistor.
I've seen (in one of my previsuos jobs) tens of thousands of transistors soldered to wires, and it works, but the wires were never larger than 18 AWG,  the solder point was heat shrinked, the wires were tied tovhether and there was a mechanical stress relieve.
A larger wire is not needed and it will cause tto much mechanical stress on the transistor's case.
Best regards

[David Hess]

If the wires are stiff, then a terminal block like soldar suggests for strain relief is a good idea.  This also avoids the problem if attaching the leads directly to the wires.  For a terminal block, I would use a small printed circuit board attached to the heat sink were the transistor leads are which just has three trace patterns.  This also works well for leaded panel indicators and switches.

Tektronix 2235 uses a plastic connector connected to the MOSFET leads on the preregulator. The other end is soldered to the board. This was knocked by me during refurb which caused a problem with the connector or the mounting and subsequently the MOSFET exploded. This burned the connector. I snipped the connector off, spliced the leads to the wire and heat shrunk each wire. All has been good since.

I have seen some pretty good TO-220 sockets but even with them, long term reliability was questionable.

Didn't Tektronix later remove the socket and just use stranded wires soldered directly to the leads?  I do not have any service documentation about it if they did.

[bd139]

I don't know if they did or didn't but I hope they did!

[Raj]

use a metal tube ,solder it to transistor then crimp it to the wire. Insulate using heatshrink.