EEVblog #744 - SMD Thermal Case Design - µSupply Part 15

[EEVblog]

Dave explains how to attach an SMD power transistor or regulator to a case to use as a heat sink in this design tutorial. And in the process talks about thermal design, the electrical/thermal analogy, and thermal vias.

This is Part 15 of the µSupply Power supply design series. Other videos are here:
https://www.youtube.com/playlist?list=PLBF35875F73B5C9B5

SilPad datasheet: http://www.bergquistcompany.com/pdfs/dataSheets/PDS_SP_A2000_12.08_E.pdf
PowerPeg thermal PCB pin/turret: http://tem-products.com/index.php/powpeg.html/
Saturn PCB Toolkit calculator: http://www.saturnpcb.com/pcb_toolkit.htm

[ColdKeyboard]

You either die a hero or you live long enough to start making crappy videos... :\

[vlad777]

Which is better : sheet mica or sil-pad?

[German_EE]

After seeing the power pegs in a mailbag I did some experiments into alternatives. A brass flat head screw driven into a countersunk hole in the PCB provides very good heat transfer, especially if the screw is used to attach a heatsink to the other side of the PCB. By clamping the semiconductor to the PCB then soldering the leads and tab pressure on the screw head will be maintained.

[T3sl4co1l]

Which is better : sheet mica or sil-pad?

Mica by a little bit IIRC, but it's a big hassle to use (has to be greased both sides).

There are also ceramic and metal products.  BeO being the classic "too good to be true" (too good to be non-toxic, more specifically), AlN and AlNO being much more popular (nearly as good performing, cheaper, non-toxic), and hard-anodize aluminum metal (very good, what with the metal core; and the advantage that it can be grounded inbetween for shielding!).  The main disadvantage being, these are all *very* rigid, very flat materials, so you need a great fit to begin with, an uneven surface may lead to fracture, the thickness wastes a lot of thermal resistance, and you still need double grease.

Kapton/polyimide is terrifically bad as a bulk material, but because it can be made in much thinner sheets than mica (which is also a bad thermal conductor, for similar reasons really), and has excellent dielectric properties, it's competitive with the others.

Rubber products are the best when you need "throw the stuff together and forget about it" simplicity.  They're generally on the low end (and the "eyed with suspicion" end) of TIMs (thermal interface materials), but they're great because of ease.

Another good way to get heat out of a board is to use a single side layout, and goop the board to the (aluminum) enclosure with one of those sticky type rubber pads (Gap-Pad seems to be the more popular trademark).  Or you can still put components on the back side, as long as the pad is tacky enough to flow between smaller components (I wouldn't suggest using anything bigger than 1206 chip capacitors if you need to do this).

If you have a thin pad (10 or 20 mil) and piles of thermal vias, you can stand to get a pretty fair power density this way (maybe up to 5 or 10W per D[2]PAK?).  Of course, you could make a sandwich pack with rubber pad and metal plate on the top side as well, which would greatly increase the heat dissipation of resistors and other low profile SMT parts.

For power applications, your best choice is to just use more parts in parallel.  Count on less than 50W per TO-220 or 100W per TO-247, and that's with a greased interface, no insulator.  Halve it if you need an insulator.  If that's not good enough, consider using a heat spreader, with a much larger insulator pad to keep the resistance low.

Tim

[rs20]

Maybe I'm missing something but where are we supposed to be sourcing these "thermal bars/spreaders" from? I realise they're pretty much just aluminium stock cut to length (although sending that contract out would get you some very crappy surface quality), but even getting this stuff simply cut to length seems like another contractor to wrangle. Not to mention getting tapped holes. Or am I missing a simpler source for these parts?

[T3sl4co1l]

You don't have a hacksaw, drill and tap?  Critical bench supplies!

No need for a huge machine shop and CNC centers!  A drill press is pretty damn handy (for straight holes to tap, and for PCB drills if you're working much FR4/G10 as well), but quite cheap as machines go, and well worth it.

Tim

[Psi]

Wouldn't more small vias be better than fewer larger ones?

The inner wall copper plated thickness would be ~the same, so it's just the surface area through the hole.
I would expect a larger hole to be worse because of the large wasted area that was drilled out (when compared with lots of smaller holes over the same area)

Imagine a large circle, now imagine one the same size but made of many smaller circles in an 360deg arc.
The total circumference of the smaller circles added up would be more than the single larger circle circumference.
And that isn't including all the extra small circles you could fit in the center.

So wouldn't you be better to have lots of 0.4mm holes, or as small/many as you can get away with before the PCB house charges you more.

[casper.bang]

Yay, finally progress on the uSupply... good on you dave!  Remember there's a non-insignificant crowd ready to throw money at you when/if this is ever finished. *Insert take-my-money smiley here*

[rs20]

You don't have a hacksaw, drill and tap?  Critical bench supplies! No need for a huge machine shop and CNC centers!  A drill press is pretty damn handy (for straight holes to tap, and for PCB drills if you're working much FR4/G10 as well), but quite cheap as machines go, and well worth it.

I hear what you're saying, and I do have a hacksaw, drill and tap (albeit only UNC 1/4"-20tpi for tripod mount threads, but hey) (and not that anyone cares, but I don't have space for a drill press but I do have a plunge router which... isn't really the same thing but I digress). I'm not sure how to get a consistent measured thickness, decent surface finish, apart from lucking out on getting the right stock to begin with, but I concede that this is just ignorance on my part. What I really don't understand is how to do this in moderate or large quantities -- I mean, he's talking about avoiding double side load for the reason of extra costs on the production line, so how do you get these bars in that sort of quantity for reasonable cost?

[EEVblog]

Another good way to get heat out of a board is to use a single side layout, and goop the board to the (aluminum) enclosure with one of those sticky type rubber pads (Gap-Pad seems to be the more popular trademark).

That stuff is ok for packages that only need to get a relatively small amount of heat out overall. But IME not so great for power packages that need real heatsinking.
I have some of this stuff lying around the lab somewhere... cost a fortune IIRC.

[senso]

You don't have a hacksaw, drill and tap?  Critical bench supplies! No need for a huge machine shop and CNC centers!  A drill press is pretty damn handy (for straight holes to tap, and for PCB drills if you're working much FR4/G10 as well), but quite cheap as machines go, and well worth it.

I hear what you're saying, and I do have a hacksaw, drill and tap (albeit only UNC 1/4"-20tpi for tripod mount threads, but hey) (and not that anyone cares, but I don't have space for a drill press but I do have a plunge router which... isn't really the same thing but I digress). I'm not sure how to get a consistent measured thickness, decent surface finish, apart from lucking out on getting the right stock to begin with, but I concede that this is just ignorance on my part. What I really don't understand is how to do this in moderate or large quantities -- I mean, he's talking about avoiding double side load for the reason of extra costs on the production line, so how do you get these bars in that sort of quantity for reasonable cost?

You can buy aluminium plates in 1mm increments in thickness,  and square/rectangular solid extrusions in increments of 2-5mm, its a matter of playing with the required thickness's required, if you choose to use a plate is cheap and fast to cut it, a bit wasteful but not that bad, and remember, thermal pads can have more than 2mm in thickness, they are squishy so you don't need to even have 0.1mm accuracy in all those parts.

And to find cheaper thermal pads, buy stuff for laptops, they use thermal pads by the metric ton, just avoid the crappy blocks of rubber from eBay that are sold as thermal pads.

You can get high thermal conductivity stuff from Fujipoly, and 3M as a huge selection, from thin to thick, more or less squishy, with glue on none, one or both sides, sold by the roll, also cheap!

[T3sl4co1l]

I hear what you're saying, and I do have a hacksaw, drill and tap (albeit only UNC 1/4"-20tpi for tripod mount threads, but hey) (and not that anyone cares, but I don't have space for a drill press but I do have a plunge router which... isn't really the same thing but I digress). I'm not sure how to get a consistent measured thickness, decent surface finish, apart from lucking out on getting the right stock to begin with, but I concede that this is just ignorance on my part. What I really don't understand is how to do this in moderate or large quantities -- I mean, he's talking about avoiding double side load for the reason of extra costs on the production line, so how do you get these bars in that sort of quantity for reasonable cost?

If it's just a few, cut slightly oversize, then use a file (or sander or grinder or..) to get the desired finish.  Using a hand file is an ancient and sorely under-appreciated task.

If you wanted to make a hundred of a part, it would be worth making up a drawing and handing it off to a machine shop.  Let them figure it out.  As long as your drawing is correct and reasonable, they have to redo it until it meets spec.  You can also make SolidWorks models (of solid machined parts, or bent/cut sheetmetal, or..) and have them rapid-fabbed at turnkey machine shops, probably for a bit more cost, but with great lead times (almost as good as quick turn PCBs!), excellent results, and reasonable prices in higher quantities.

Obviously, one cannot make prototypes without at least a little ME ability.  If you don't know drawings, it's worthwhile to take a course on it (or find as much on/offline material as you can).

That stuff is ok for packages that only need to get a relatively small amount of heat out overall. But IME not so great for power packages that need real heatsinking.
I have some of this stuff lying around the lab somewhere... cost a fortune IIRC.

I've heard of the stuff that does, or did, cost a fortune.  Unless you're buying it by the roll, it doesn't look like that's typical of even Digikey prices nowadays, though.

Like I said, the thinner stuff is good for dissipating some power; obviously you aren't going to get much through a 1/8" (3mm) pad filling in the standoff space below a PCB -- but it's a good solution if you can handle the extra buck or so, and the board is getting just too hot on its own.  In other words, it's a pretty darn good alternative to air, if not an ideal solution.

Tim

[krivx]

I'm not sure if we're dealing with a different class of machine shop but for something as simple as an aluminium bar I don't think anyone wants a SolidWorks file. A napkin with some dimensions and tolerances on it would probably be more appreciated.

[EEVblog]

I'm not sure if we're dealing with a different class of machine shop but for something as simple as an aluminium bar I don't think anyone wants a SolidWorks file.

The solidworks model was just done for the video as a visual aid.

[max_torque]

For "real" SMC thermal performance, use an IMS pcb!  (Internal Metal Substrate)

These are aluminium or copper pcbs, with a thin vapour depositied layer of insulation, and then the normal copper layer put on top.  I've done a 500Arms mosfet inverter on a circular <100mm diameter pcb usings an IMS pcb to mount the Fets! (although it was directly water cooled on the back of the pcb)

Because the layer of insulation is so thin and consistent, the thermal co-efficient between the copper layer and the alluminium of the pcb substrate is tiny 

[senso]

That stuff is ok for packages that only need to get a relatively small amount of heat out overall. But IME not so great for power packages that need real heatsinking.
I have some of this stuff lying around the lab somewhere... cost a fortune IIRC.

I've heard of the stuff that does, or did, cost a fortune.  Unless you're buying it by the roll, it doesn't look like that's typical of even Digikey prices nowadays, though.

Like I said, the thinner stuff is good for dissipating some power; obviously you aren't going to get much through a 1/8" (3mm) pad filling in the standoff space below a PCB -- but it's a good solution if you can handle the extra buck or so, and the board is getting just too hot on its own.  In other words, it's a pretty darn good alternative to air, if not an ideal solution.

Tim

You can get thermal pads with a thermal resistance of 17W/mK, you can still dissipate a lot using those pads, even with 3mm thickness.

[djQUAN]

Something I did way back.... with an aluminum case and TO-220 devices

http://quan-diy.com/projects/charger/liion.htm

[delmadord]

The 3D visualizations done by David are awesomely spectacular! Both thumbs up!  If they only had higher FPS. The video is million resolution times gazillion FPS and the visualization is photos basically.

[ion]

Which is better : sheet mica or sil-pad?

Martin Lorton did a video recently that showed silicone to be a bit better than mica.

[German_EE]

I am sorry about the delay as this page took some time to find. The following page provides a good discussion on thermal issues and includes some practical examples:

http://ludens.cl/Electron/Thermal.html

For those who are curious a .cl domain is in Chile.

[Psi]

Just made some mockups in Altium using Saturn to get via C/W

Obviously 0.2mm is crazy as the PCB house is going to charge extra for that many holes
But 0.4mm seems doable and does give some good benefit.

[Paul Moir]

Maybe I'm missing something but where are we supposed to be sourcing these "thermal bars/spreaders" from?

A local machine shop should be able to turn them out pretty cheap, especially if you talk to them beforehand about the design. 

Speaking of which, 4 tapped holes + 4 screws or is it 2 tapped holes and 4 screws?  What about 2 clearance holes and nuts & screws?  Seems like it would be a whole lot cheaper to make, and you could have pretty wide tolerances on both the case & bar.  3 screws would get much better pressure if you found it necessary:  might be worth putting the hole in your PCB.

EDIT:  or one centre hole and no outer ones? 

[overthere]

The cheapest solution in cooling is always not to have to cool. Speaking replacing 7805 with switching regulators.
If you need fast regulation, an option might be to do preregulation and use a second step LDO with fast response.
But that should be obvious, and sometimes cooling is really required. So lets face the topic.

A problem what I see for reverse mounting those TO220 is the stray inductance by the leads.
Of course it does not matter for 7805 and linear stuff, but for fast switching DC/DC Converter it counts.
Also the assembly house will be angry this that method, because mounting takes a long time.
In our company reverse mounting TO220 is forbidden, for bad serviceability.
But I think Dave already mentioned that its a  huge effort to mount these,

A solution by Infineon is to introduce so called topsidecooling in CAN-Package (Can=Tin, not controller area network)
There you can directly make contact with your PCB. However this package is to infineon, and they don't seem to offer new chips.
So it seems dead. Also TI has some package allowing top side cooling, but in my eyes are not so clean solution.

LEDs and linear stuff (7805) can also be mounted on a aluminium PCB.
Heat transfer is good, but you introduce parasitic capacistance, especially deadly if you're working on higher voltage. (>100V)
Soldering those with a good iron is also a challange, a reflow oven is highly recommended.
Heaving the Aluminium PCB, anyway a good heat contact to the thermal sink is requiered.

My current solution I have in mind for DC/DC Converter cooling.
Cutout by termal interface material
1. There are some really small high packages out for power transistors, like SuperSO8 by Infineon.
These have a high of 1mm. Top side cooling is no real option, as heat transfer is bad. (20k/W)
But you can extend the copper area of the pad a bit and make a recantagle with the IC as cutout in the interface material.
The interface material is available in 1mm high. So a good contact to the case could be made.

Let me make a rough calculation:
10mm² Area, 1mm Thickness, 1.5 W/mK:: 0.15 W/K. Perfect.
Capacitance introduced (Epsilon_R=5.5): 490pF. Not good, but okay. Maybe make the area smaller or use a ticker foil.
Or Put two over another.
The advantages of the solution is:
1. Small thermal "road". (Little bit longer then TO220 solution)
2. Cheap Mounting / of the shelf components
3. No via stiching required. / Components can be place on the top.
Disadvantages:
1. Small high package required (SON, SuperSO8, etc.). But should be no problem for new designs.
2. Introduces stray-capacitance (bare in mind, other solutions too, always calculate it.)
3. Dual side mounting (cost)

I attach a picture showing the proposed solution in detail.

Another solution, but too expensive in my eyes is to machine a custom cooling element, like Dave proposed.
The mounting can be simplified, by using pressfit, and a termal intace material. The capacitance problem of above will still exist and bigger,
and the termal road will be long.

[boffin]

I happy to see the MicroSupply back on the table.

Only thing I worry about is producing low voltages with higher currents, the DC-DC will be off, the linear reg will have 8.4v (ish) volts in, and if you ask for 2.4V out at 1A, you're dissipating 6W across the linear reg.

My wish list for the microsupply:

• single rotary encoder control with two additional buttons (load on/off, lock) - not unlike the Gopher supplies
• simultaneous display of max current and current
• 20V, 1A:  CV and CC modes
• Wireless Ethernet (what's the point of a battery power supply if you still need a piece of Cat5e)  perhaps an E.Imp on the i2c to do that?
• solid aluminium case