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Is the melting point of pure indium too low to safely use Indium to solder together a custom copper heatsink that will then be used ontop a motherboard mosfet?

In other words, can the mosfets get hot enough (without actually failing) during normal operation to melt the indium and thus cause the solder joint between the two pieces of copper fins to fail?

The Indium was chosen due to its thermal conductivity.

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  • \$\begingroup\$ That's just 150 C-ish. Most parts are not specified for junction temperatures above that, though some are. It's unlikely that you will see these temperatures external to the junction. That said, I've run the entire board and all the parts on it at 180 C for four hours at a stretch, in one in-situ FAB data-gathering test. But you know your situation better than we do. Vibration? Etc. And yes, mosfets can survive higher junction temperatures. The MCU I used as well as its internal EEROM all survived. \$\endgroup\$
    – jonk
    Mar 25 at 19:26
  • \$\begingroup\$ Is the thermal conductivity value a "nice to have" or a desperate "we can't use anything less"? It looks like SnCu solder has a decent thermal conductivity and it seems likely it is compatible with copper. \$\endgroup\$ Mar 25 at 19:40
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It is too low if the MOSFET is going to dissipate enough power and there is enough thermal resistance from it to the heatsink (and ambient) for that power to result in a temperature rise that exceeds 157°C.

Without additional information, that's the best anyone can do. Is it safe to use something that will melt at a certain temperature? Sure, as long as it doesn't get that hot.

However, it is not good design to use MOSFETs close to their max junction temperature, and furthermore, most FR4 circuit board composite is only rated to 140°C, though some higher temperature versions are ok up to 175°C. Regardless, on properly designed motherboard with sufficient heatsinking and cooling, it is very unlikely any MOSFET (or any other component for that matter) is going to ever get hot enough to melt indium solder.

That said, there is absolutely no reason to use indium solder. It has a thermal conductivity of ~81\$\frac{W}{mK}\$ compared to SAC305 (standard lead free solder) which is about 60\$\frac{W}{mK}\$. This is a largely moot point however the solder alloys with the metals it is joining and you can't really just look at the thermal conductivity of the solder at that point, but at the junction overall.

And the standard rule of thumb for the thermal resistance of two flat surfaces soldered together (with standard lead-free solder) is between 0.02\$\frac{°Ccm^2}{W}\$. Even a tiny 3x3mm PSON MOSFET package has a thermal pad of 0.32\$cm^2\$, so the solder joint only contributes 0.0625 \$\frac{°C}{W}\$ worth of thermal resistance. Using indium solder would lower that slightly most likely.

But this is a utterly negligable amount of the total thermal resistance. Just the junction-to-case resistance of any MOSFET absolutely dwarfs this value, as does the heatsink to ambient.

There is no meaningful benefit to using indium solder for heatsink attachment to a MOSFET. Just use regular solder.

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  • \$\begingroup\$ The custom copper heat sink attached to the mosfets is to obtain more height and surface area to take advantage of airflow. Solid copper weld/solder was used to create the heat sink. The heat sink also comes into contact with a factory heat sink that includes a heat pipe and various fixtures. Even after removal from the motherboard, my concern would be damage to any internals I am unaware of using copper welding temperatures. The surface area of the portions to be joined is approximately 2 square centimeters and 0.5 cm from the thermal tape that separates the mosfets from heat sinks. \$\endgroup\$
    – user280184
    Mar 26 at 6:14
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Melting point of indium 157°C is well below the typical 175°C storage junction temperature of even plastic MOSFETs.

They can get considerably hotter for a few seconds during soldering without failure (peak temperature ~300°C).

I have seen MOSFETs actually unsolder themselves and still be (more or less) okay.

Thermal conductivity should matter little (within reason) because solder is not normally used to bridge gaps. Indium is also very soft. And not cheap. It makes a nice cryogenic vacuum seal but I can't see using it for heat sinks.

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  • \$\begingroup\$ The Indium corporation sells pure indium and indium alloy based thermal interface materials. Presumably someone is buying them. (Not saying they are good for this this exact application) \$\endgroup\$
    – Matt
    Mar 25 at 19:59
  • \$\begingroup\$ @Matt Especially with the alloys I could see various applications. They sell 80Au20Sn eutectic gold/tin solder even. \$\endgroup\$ Mar 25 at 20:04
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Maximum junction temperature for MOSFETs is usually around 175C. As you probably know, the melting point of pure Indium is 157C. Will the heatsink get up to this temperature with the thermal resistance between junction and case, maybe maybe not but if it does you'll have the heatsink fail when the MOSFETs need it most and as others have mentioned, "maximum" is what is recommended, not what you could potentially see. Have you looked into thermal epoxies?

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