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Legs of upside-down TO220 FET will detach from body due to weight of heatsink?

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Flyback

Well-Known Member
Hi,
We have left our 60W Offline Flyback SMPS running for the weekend at 20W. I have just remembered that I forgot to solder the FET heatsink pin to the PCB. ..The TO220 FET hangs upside down in the enclosure, therefore, the heatsink will be weighing down on the legs of the TO220 FET which are soldered into the PCB…….do you think the FET legs may disconnect from the silicon inside the TO220 package, due to the weight of the heatsink? The heatsink is part number SK 469 35 STS BY Fischer.com…..

TO220 Heatsink (SK 469 35 STS)
https://www.fischerelektronik.de/we...ded heatsinks for PCB mounting/VA/SK46935STS/
 

unclejed613

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Most Helpful Member
depends on a few things, how much weight, how much vibration, how much thermal cycling. unless you have a big fan causing vibration, i wouldn't worry about it for over the weekend. as far as thermal cycling, it takes a long time for that to cause problems. i've seen stuff that's been running for 5-10 years and begins getting broken solder on the collector or drain lead. this is because the collector/drain heats up and the lead expands with heat, and contracts when powered off. the emitter/source and base/gate leads don't heat up as much, so the collector/drain lead pushes against the solder connection and eventually begins weakening the solder. so, if you've done a lot of repairs on older equipment, this is why it's usually the collector lead ending up with broken solder. it's kind of rare for the leads to break off inside the transistor, but it does occasionally happen. this is usually caused by vibration, or the PCB flexing.
 

shortbus=

Well-Known Member
Most Helpful Member
Hi,
We have left our 60W Offline Flyback SMPS running for the weekend at 20W. I have just remembered that I forgot to solder the FET heatsink pin to the PCB. ..The TO220 FET hangs upside down in the enclosure, therefore, the heatsink will be weighing down on the legs of the TO220 FET which are soldered into the PCB…….do you think the FET legs may disconnect from the silicon inside the TO220 package, due to the weight of the heatsink? The heatsink is part number SK 469 35 STS BY Fischer.com…..

TO220 Heatsink (SK 469 35 STS)
https://www.fischerelektronik.de/web_fischer/en_GB/heatsinks/A04/Extruded heatsinks for PCB mounting/VA/SK46935STS/
After all of the crap about buying things in country, you have talked about, you don't even do it. And will bet you that heatsink was made in China.
 

ronsimpson

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Most Helpful Member
do you think the FET legs may disconnect from the silicon inside the TO220 package
I have never seen a disconnection from mechanical stress. (hard to know)
I have seen many times when the legs broke on the outside from stress.
 

gophert

Well-Known Member
Most Helpful Member
Even tiny barbs on the leads will make them difficult to remove from the TO220 package. You can test the strength on any random TO220 package in your parts box. Any failure point will not be at the point where you show concern.
 

tomizett

Active Member
I agree with Jed that vibration is more likely to be a problem than static weight. Generally you want to avoid the stresses caused by thermal expansion, so in some circumstances a floating heatsink might be better than one that's solidly fixed down (?!).

I have seen one TO220 with an intermittent internal connection to a lead, but only one.
 

unclejed613

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Most Helpful Member
I have seen one TO220 with an intermittent internal connection to a lead, but only one.
i've seen all 3 leads on 78/7912 regulators on a particular series of Yamaha receivers break off at the point where they entered the case. Yamaha later added a bracket to keep the regulator PCB from bouncing or vibrating during shipping. the same models also had problems with a ribbon cable (the kind where they strip 1/8 inch off the end and the bare wires plug in to a socket) breaking off the wire ends, and added a holder for the cable. don't ever rely on transistor/regulator leads to act as a stabilizer/mounting point for a PCB.

when it comes to the thermal stress breakage of solder connections, it seems that the archaic practice of crimping transistor leads (to keep the transistor from dropping closer to the PCB between assembly and wave soldering) reduces the expansion/contraction stresses a lot because of the semi-circle of component lead providing some "give". there used to be special hand tools to use on the component leads before the components were inserted on the PCB. EDIT: oh, they still do make them https://www.olamefusa.com/e_forming_handtools.htm. lead forming like that can also reduce the effects of vibration. done right it can also make a circuit board layout with elevated components (like resistors and diodes that generate heat) look better.
 
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