What is the maximum acceptable soldering temperature for a MOSFET?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

6. 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.

7. 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.