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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
    Jul 20, 2017 at 22:36

3 Answers 3

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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
    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
    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
    Dec 17, 2021 at 3:40
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Use 63% 37% solder! Use <= 250C for <= 10s. 260C maximum.

USE A LOT OF, GOOD QUALITY, SOLDERING PASTE. This heats up the soldering point only. With such, you must be able to solder even the case pin.

For the pin which is connected to the case, 275C may be OK for a shorter period. Do not go over 300C. In case you go 300C, use < 5s.

AFTER EACH ATEMPT, BLOW AT THE CASE AS WELL AS THE SOLDERING POINT. BLOW A LOT. THEN, WAIT, THEN, SOLDER AGAIN.

EVEN AFTER A SINGLE, LOW TEMPERATURE, SOLDERING, BLOW A LOT ALL SIDES OF THE COMPONENT AND WAIT.

ADDITIONAL INFORMATION :

In regards to Flux and Solder Paste : Yes, some people use the term " Solder Paste " to denote powder solder, mixed with EXTERNAL flux. They, also, use the term " Flux " to denote any flux, the internal flux, which, is built in the solder wire and the external flux, which, comes separately and is in the FORM of a paste. Paste is anything which is semi liquid and semi solid.

The problem with the term " flux " is the ambiguity : is this the internal or the external flux. Maybe, the term " flux paste " would be appropriate, yet, I would use the term " External Flux ".

So, yes, I meant EXTERNAL flux, when I said solder paste in the original post, yet, solder paste may be even better as long as the quality is good.

In terms of external flux, the most important feature is, as mentioned, to start to boil and heat up, mainly, the areas of the components to be soldered ( covered by solder ) and around. Also, the external flux keeps the molten solder at the soldered pin and track only ( mainly ).

Thus, to solder a case pin with 250C is possible, yet, slightly more difficult. Here, again, is how :

  1. Apply external flux to the iron only. Clean the iron with the wet sponge. This is for cleaning only.

  2. Apply a lot of external flux to the pin and the track. Ensure all sides are covered.

  3. Apply as much solder to the iron as possible to hold. Do not apply a lot of external flux to the iron in point 1., so the solder does not drip off of the iron.

  4. Apply external flux to the solder wire.

  5. Touch the solder point with the iron. Touch the so position iron tip and solder point with the solder wire, covered with external flux. Ensure the external flux starts to boil and the molten solder starts to spill out and cover the soldering areas.

Do not keep the soldering iron to the pin for more than 10s.

  1. After the iron is removed, blow VERY LIGHTLY at the solder point to solidify the hot, still, molten solder. This happens in for a second or two. Then, blow as strongly as possible towards the soldering point. Then, blow as strongly as possible towards the case for a long while. Many blows are necessary.

  2. Wait until the the case is at around room temperature. Touch the case with fingers to check. Solder another component while waiting.

Repeat all steps for the other side of the soldered point. Ensure the solders, covering, the old and the new solder sides become liquids and spill into one another. May need to touch the old side again. Again : do not keep the iron on the component for more than 10s.

Alternatively, 300C will do the job for sure. External solder helps a lot and not such huge amount is necessary.

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    \$\begingroup\$ Please stop shouting. \$\endgroup\$
    – Hearth
    Dec 17, 2021 at 3:18
  • \$\begingroup\$ Keep the soldering paste away from electronics. It usually contains a flux that is far too aggressive. It will eat the traces off the board. \$\endgroup\$
    – JRE
    Dec 17, 2021 at 11:38
  • \$\begingroup\$ No one shouts here. I don't know who initially said capitals are shouting. CAPITALS ARE FOR READABILITY! [Edited by a moderator.] \$\endgroup\$
    – StevenStanleyBayes
    Dec 18, 2021 at 5:59
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    \$\begingroup\$ You appear to be saying "solder paste" but talking about flux. They are very different things. But even so, flux will absolutely eat away at the board if it's anything more active than RMA, and even RMA flux will if you give it long enough. Acetone (easily available as nail polish remover) is probably the easiest way to get flux residue off; I don't recommend using laundry detergent because I have no clue what that will do to the rest of the stuff on the board. And I don't know about you, but I for one certainly parse all caps as shouting, and also as less readable than lowercase. \$\endgroup\$
    – Hearth
    Dec 18, 2021 at 6:14
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    \$\begingroup\$ You can use bold or cursive for emphasis. Don't use all caps. \$\endgroup\$
    – StarCat
    Dec 18, 2021 at 11:30

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