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I need to manually solder with a soldering iron an LFPAK56, Power-SO8 package MOSFET. I am reading that the maximum junction temperature for the device is 175C.

The soldering iron goes way over that temperature but of course the Tj temperature is define for when the MOSFET is in operation. Soldering maximum temperature must be much higher.

Wave soldering has a maximum temperature of about 260C for some 10 seconds. The MOSFET drain is quite a massive piece of metal and must be heated enough to permit solder melt. Is it even possible to solder such a device manually without overheating to destruction?

  • What is the maximum device temperature possible before a MOSFET starts degrading during manual soldering?
  • How long can it endure such a high temperature?
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    \$\begingroup\$ That information is usually in the data sheet. \$\endgroup\$
    – Kevin White
    Commented Jul 20, 2017 at 22:36

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If you look in the datasheet, you'll find a timed temperature profile (or a link to where that information is available on the manufacturer's web site) for machine soldering.

The temperatures will go above 175C, and for many seconds. The effects of this one time thermal abuse have been built into the specifications for the device, it has been designed to be machine soldered.

Unfortunately, hand soldering is much less controllable than a machine. However, several things are on your side. You can used leaded solder, which melts at a lower temperature than lead-free (the machine profile is certainly for lead-free). You can observe the tab, and stop heating moments after the solder flows.

As there's a lot of mass to heat, use the biggest soldering iron tip you can find. Use a controllable iron, and set the temperature to enough to melt the solder, but <= 260C. Practice on some dead devices. If you haven't any dead ones, practice on some live ones, and you'll soon get some dead ones. If you can do the job keeping the device within the time/temperature profile of the data sheet, then you're in with a good chance.

Where the device has an accessible tab, like a TO220, it's fairly easy. Where it's a power SO8, you can't heat the tab directly. With an etched board, the heat has to be applied from below, and the device reflowed on. When I don't have a board, I often 'dead-bug' the device, and solder heavy wires to the tab for the heatsink.

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Typical datasheets show an acceptable temperature profile suitable for high volume, high quality (reliability) production. In fact, while not powered, the component can withstand >> 400 °C before any permanent damage occurs to the circuit. The plastic on the package will degrade before the FET itself gets damaged.

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  • \$\begingroup\$ 400 celsius on the die sounds like a bit much--i'd start to worry about delamination of the metal layers from the silicon not too much higher than that. 400 celsius on the pins, though, if it's not for long enough to get the die to that temperature... they'll probably survive that. \$\endgroup\$
    – Hearth
    Commented Dec 17, 2021 at 3:25
  • \$\begingroup\$ It won't 'delaminate'; it's solidly held in place with silicon dioxide, and the package on top of that. Aluminum doesn't melt until over 600 °C. There is a small possibility that the different thermal coefficients of expansion (plastic, silicon, leadframe) will stress the device, but above 200 °C, the plastic softens and allows small stress relaxation. Consider that the devices are also rated for -40 °C, for a temperature difference of over 200 °C from the temperature where the package is molded. \$\endgroup\$
    – jp314
    Commented Dec 17, 2021 at 3:33
  • \$\begingroup\$ As someone working in the semiconductor manufacturing industry, I have been told by coworkers that trying to repeat an annealing step to diagnose a problem I found would cause the top metal to delaminate, so I'm incline to believe that there is a risk of that happening. \$\endgroup\$
    – Hearth
    Commented Dec 17, 2021 at 3:40

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