Heatsink paste on SMPS FET

[eem2am]

Imagine you have designed a 50W offline flyback SMPS.

The switching transistor is a TO220 package , which must be screwed to a heatsink, and heatsink paste *must* be applied between heatsink and FET.

How can you know, that the assembly staff are properly applying the heatsink paste or not?

 

[enjunear]

Depending on the production quality controls at your facility, you might have some workmanship standard that calls out having a bead of heatsink compound around the interface between the two parts. If not, you could add it as a build note on your assembly drawings, so they must ensure adequate heatsink goop coverage, such that it forms a bead when it squishes out when the parts are bolted together. Your terminaology/wording should be more professional, but that's how I'd go about it from a direction/verification standpoint.

 

[eem2am]

thankyou,
-you have confirmed the way we have always done it...........by simply paying someone to look at the interface, and note that there is some paste squished out.

However, we have found that assembly staff are just making sure that there is a bit of paste round the outside, and they are not applying the paste properly on the inner bits........

the thing is, the paste is gooey and messy and unpleasant....staff do not like it, they suspect its carcinogenic, and keep handling of it to a minimum.......they simply press the heatsink on to a surface on which the paste is spread, ...and then make sure they get enough round the outside....but dont bother getting enough paste on the inner bits which we cannot see whan its all assembled.

...when they are running out and should get some more paste, they dont bother at first, and so loads of heatsinks are getting assembled with not enough paste......but the staff do make sure that there is enough paste round the outside so that they look properly pasted.

....some assembly staff have told us that they simply dont believe that the paste is necessary, and think just screwing metal to metal is sufficient.

I can't see how we can possibly check that all FET/heatsinks are getting properly pasted....any ideas?

 

[alexan_e]

Maybe have the heatsink already applied with paste from a different person and then the man that does the assembly will just screw the transistor to the heatsink,
this could also be reversed and have the first person apply paste to the transistors but i think that the first way is easier since the heatsink will be treated and transfered to another section only one time
while the transistor would have to be soldered first, then go for paste cover and then return for assembly.

Because this will be a 2 stage operation it will be easy to check if the person that applies paste does a good job and you will also have to teach the proper way only to the person that applies the paste.

Alex

 

[Genomerics]

Heat sink Paste is 'Horrible' stuff.

I had a visit from some 'Foreign Governmental Department' in their 'Black Helicopters' a while back and waved a tube of it at them. They went away pretty quickly. Probably did not want to clean their 'Black Helicopters' of 'white gunk'.

Why not give your staff tools for the job along with a mini assembly line...

First Human uses tool to deliver 'Appropriate Level Of' gunk to heat sink in the right place. Second Human plops power device in the right place and presses it down for Third Human to stick the screw in and drive it with screw driving tool, set to appropriate torque level. Everyone wears kinky gloves.

All Humans have appropriate protection. All Humans have the right tools for the job and you have a 'team'.

Perhaps, if you set up the stations and tools correctly, they will love you and you can go into mass production and take on work from elsewhere.

If not then perhaps you would like to spend a day assembling your stuff in the way you recommend and see how much 'gunk' ends up on your hands....

That might be an eye opener.

Genome.

Ooops.. As What alexan_e said whilst I was slapping my keyboard.

 

[enjunear]

thankyou,
-you have confirmed the way we have always done it...........by simply paying someone to look at the interface, and note that there is some paste squished out.

However, we have found that assembly staff are just making sure that there is a bit of paste round the outside, and they are not applying the paste properly on the inner bits........

the thing is, the paste is gooey and messy and unpleasant....staff do not like it, they suspect its carcinogenic, and keep handling of it to a minimum.......they simply press the heatsink on to a surface on which the paste is spread, ...and then make sure they get enough round the outside....but dont bother getting enough paste on the inner bits which we cannot see whan its all assembled.

...when they are running out and should get some more paste, they dont bother at first, and so loads of heatsinks are getting assembled with not enough paste......but the staff do make sure that there is enough paste round the outside so that they look properly pasted.

....some assembly staff have told us that they simply dont believe that the paste is necessary, and think just screwing metal to metal is sufficient.

I can't see how we can possibly check that all FET/heatsinks are getting properly pasted....any ideas?

It sounds like there might be a couple solutions, just "outside the box".
1. Use a different thermal compound. It may cost you a bit more, but that could easily be offset by the number of failed units that may be returned for repairs.

2. Have the person applying the thermal compound use protective gear. Maybe make one or two people the primary thermal-goop installers. They would get good, chemical resistant gloves and perhaps an industrial quality mask with super-fine filters to keep the nasty chemicals out of the air they breathe. Then, if there is a problem, you only have one or two people to blame for the screw-up (i.e. potential to fire them for failure to perform their given duties).

3. Choose a different part that attaches with a different method... maybe a larger package (increase the surface area, so you junction thermal resistance is less of an issue).

There might be more, but I think you could get pretty inventive with alternatives to get your workers away from this fear-inducing compound. Get rid of the fear, and they'll be much more willing to do the job properly.

 

[alexan_e]

1. Use a different thermal compound. It may cost you a bit more, but that could easily be offset by the number of failed units that may be returned for repairs.

You can use thermal pads that are much easier to handle and are applied very fast,
the only drawback is the price and the worse thermal characteristics
but the cost may be handled because you will be able to produce more parts for the same time
and you will also be able to maintain a heat transfer "value" in your production that may be better than the mean heat transfer ability with paste (with heat paste some will be worse and some better i think).
You will probably be able to use the worker that checks if the paste is applied to do other things too and save cost form there too.

Alex