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

[NiHaoMike]

Actually, there are 100W audio amplifiers out there built on SMD with little additional heatsinking. The modern designs just don't get particularly hot to begin with.

[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.

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

I was referring to T3sl4co1l's post. The model in the video was a very nice visual aid 

[free_electron]

just insert a copper coin in the board and be done with it.

first picture : press fitted copper coin
second picture: embedded coin this gets laminated inside the board.

the coins can be shaped in any form you want.

[c4757p]

second picture: embedded coin this gets laminated inside the board.

That sounds expensive.

[zapta]

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*

+1.

And, hopefully it will work with a standard micro usb USB charger (up to ~10W) instead of a yet another special wall wart.

[overthere]

Actually, there are 100W audio amplifiers out there built on SMD with little additional heatsinking. The modern designs just don't get particularly hot to begin with.

As mentioned above, a good idea is always to minimize losses first, e.g. use a switching regulator. Comparing a Class D Amp is basically a switching amplifier, but can absolutely not be used for a power supply.
(You would have a hard time getting good performance.). I propose a switching pre-regulator and a following LDO for fast response and noise reduction.


A coined solution sound interesting for me. Actually pressfitting copper could work, i now some assembly houses doing that. But thats definitly not a cheap solution.
(And: You need to remove the heat from the copper coin).

[Vgkid]

That is a great video, thanks.

[T3sl4co1l]

just insert a copper coin in the board and be done with it.

first picture : press fitted copper coin
second picture: embedded coin this gets laminated inside the board.

the coins can be shaped in any form you want.

Or soldered in,

http://seventransistorlabs.com/Images/EmbedHS1.jpg
http://seventransistorlabs.com/Images/EmbedHS2.jpg

I've also heard of PCBs with not just "heavy copper" but "ludicrous copper"; surely they use machined parts instead (much as above).  I'm sure a typical quote for a board runs "if you have to ask, you don't want to know"...

Tim

[BobC]

The thermal issues bring me back to the overall packaging plan.

First, this isn't a rack-mount instrument.  The top and sides are usable, in addition to the front and back.  (Well, maybe not the sides, since that's where the case halves meet.)  Assuming simple case modifications (drilling holes) are still free/cheap in the quantities anticipated.

Even with rubber feet, the bottom will have the least air circulation, so it is the worst surface to use as a heat sink.  Which leaves the other 5 surfaces.  Since it would be convenient for the top to be removable and still have the supply operable, heat removal shouldn't be seriously impacted when the top is absent (but it would be nice if it were enhanced!).

To me, this means an internal 1" heatsink that is screwed to the top would be very convenient, with minimal layout restrictions.  Remove the screws and the top lifts off, and the exposed heatsink may work better than with the top on.  If more heat removal is needed, an exterior heatsink may be mounted on the top.

This eliminates any need for custom aluminum bars, or difficult assembly and attachment.  The cost is that the parts would not be surface-mount, but the heat path is simplified by not having the PCB be part of it.

If we're drilling holes in the top, why not use flat-mounted (upward-facing) encoders with long shafts that stick out the top?  Remove the knobs and the top can lift right off.  This not only permits encoders to be used (yay!), but keeps them far from the binding posts, and also keeps fingers from blocking the display.  And also keeps fingers from hitting the encoders when leads are being plugged/unplugged.

With no penetrations in the bottom, feet are not needed for clearance, but may be desired for non-slip purposes.

That leaves the front and rear panels available for connectors, buttons and the display.  And how about putting the binding posts on the rear?  It would improve finger access to front panel buttons, and could even leave room to have the encoders come out the front (again).

Other thoughts:

1. Don't have the PCB use both case side slots: Have it supported primarily by the front and rear panels (and optionally use one side slot).  This would permit both the front and rear panels to be PCBs soldered to the main PCB (if desired), and the united assembly dropped into the lower half of the case (rather than sliding in).  If additional support is desired, put rubber feet on the PCB, to brace it against the bottom.

2. Use 18650 battery clips soldered onto the main PCB.  These provide enough clearance for SMD parts to fit underneath.

Did I miss anything?

[free_electron]

second picture: embedded coin this gets laminated inside the board.

That sounds expensive.

not really. the press fit stuff is cheap. they simply stamp these. same principle as those press-fit connectors.

the embedded one is more pricy due to the special shape of the coin. but if you need it you need it.

[SA007]

There is one thing I am missing from the whole setup.

Why have the cooling pad of the part electrically connected to the tab?
I think in a lot of cases the added silpad is not really needed if the cooling is isolated.

Of course the board will conduct heat worse is there is no direct copper connection, but how much worse.
It think the added cost and complexity of getting a silpad nicely fitted underneath the pcb is not really needed.

[T3sl4co1l]

There are devices with isolated grounds, but they almost always perform worse.

Generally the tab is the substrate (lowest voltage in the IC), and the package generally connects that to the center pin (as with TO-220, etc. -- oddly enough, if you crack open a TO-92, you find the 'tab' is probably off-center, due to pinout variants -- but these aren't thick or extended tabs, either).

It's not convenient to attempt to make analog ICs with an isolated substrate (say by building *all* the transistors in 'wells' which isolate them over the substrate by a diode junction), nor to add an extra insulator (such as DBC (direct bonded copper), which is popular -- and necessary -- in large switching modules).  Some devices (such as MOSFETs and diodes) are available with isolated metal tabs (most likely using a DBC ceramic insulator), and many things are available in "full pack" (encapsulation all around everything, e.g. TO-220F -- also includes FWB modules, even the ones with a metal backing plate).

Some devices *do* have natural isolation (SOI and SOS, processes for CPUs and RF), but they're more expensive or special purpose, so it doesn't matter much here.

Tim

[IanB]

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.

At 17 watts per millikelvin thermal resistance, no kidding! 

[Dave]

At 17 watts per millikelvin thermal resistance, no kidding! 

No. It's watts per meter kelvin. That is actually specific thermal resistivity, not resistance.

dQ/dt = k * A * deltaT / d

dQ/dt - heat transferred per unit time (W)
k - specific thermal conductivity (W/(m*K))
A - cross sectional area (m^2)
deltaT - temperature difference between sides (K)
d - thickness of material (m)

I think they keep writing it as meter kelvins, even though they might confuse some, to avoid even greater confusion of people thinking that Km denotes kilometers. Go figure.

On a related note, why the heck doesn't the forum support greek letters other than mu, when engineers use them all the time?

[c4757p]

On a related note, why the heck doesn't the forum support greek letters other than mu, when engineers use them all the time?

Because there exist programmers too stupid for Unicode, and sadly a great many of them have written web code.

[SA007]

There are devices with isolated grounds, but they almost always perform worse.
...

Hmm, didn't even know they existed, I was aiming for an isolation on board level.
As in, place a device isolated from the cooling tab on the board.

I recently saw an image with an d-pak mounted on a board with obvious heat sinking surrounding it but not electrically connected to the heat sinking.
Of course I don't remember where I saw that board, but still.

I think there are plenty of cases where that would be enough cooling for a component.

[langwadt]

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.

whether those numbers mean anything depends on what you are doing. If for example you are trying to get heat from a dpak into a ground plane all but the outer ring of vias are
mostly redundant

[T3sl4co1l]

I recently saw an image with an d-pak mounted on a board with obvious heat sinking surrounding it but not electrically connected to the heat sinking.
Of course I don't remember where I saw that board, but still.

If you have a multilayer board, you can interpose layers, so for example, you might have:
1. Top Layer: pad, heatsink tab, copper pour, thermal vias connected
10 mil prepreg
2 . Top Inner: GND Plane
40 mil core
3. Bottom Inner: copper pour, thermal vias connecting to pad
10 mil prepreg
4. Bottom Layer: GND Plane

And then further heatsinking via the GND Plane(s), which could include SMT heatsinks for example.

Because the prepreg layers are thin, you get not-terrible conductivity between the pairs on each side (layers 1-2 and 3-4).  Not enough that it can be net pad size, so you still want copper pours, and doubling up helps that much more (even though the second pour is only connected by vias).

Again, this can be done with vias around the periphery.  The GND-GND stitching (needed to get heat from layer 2) has to go outside the pad, obviously (or use buried/blind), but the vias from the connection itself can be ViP or peripheral (in accordance with the usual advice concerning that).

This doesn't work on 2 layer boards because there's no layer pair with close spacing.  A four layer board typically has two pairs with close spacing, so you can use one, or both in parallel, to your advantage.

Obviously the added capacitance is not insignificant, so you want to direct it somewhere appropriate.  Ground is probably the best, for typical applications (switching node?).  A fully isolated heatsink node would carry essentially full noise, which wouldn't be real great on something like an offline SMPS.

Tim

[loneoceans]

I'm not sure if I'm convinced that this is a good method for heat-sinking.

In the classic approach, one needs to incur added cost of hand-soldering, but it seems like the rest of the assembly is easier - single hole to be made in case and cheaper to get this manufactured, then a screw and a sil-pad.

In Dave's approach, you not only incur greater cost of having the large device on the PCB (e.g. D2Pak is significantly larger in footprint than TO220 standing up), but you also need to have more space on -both- sides of the PCB, adding cost in drills (can add up depending on your volume), and having to design and manufacture the aluminium bar. Finally you still need to get the bar assembled (4 screws required in this case) and use sil-pads. Overall I'm not sure if I'm convinced this is any sort of cost or assembly efficient method at all.

Previously when tearing down some electrical systems, another method I came across was to have the legged device installed normally but with a cut-out on the PCB. The case is shaped to fit the device, and a sil-pad 'jacket' is slipped over the device. This is then clipped to the case with a clip that looks like a binder clip, but very strong. Aside from the one-off cost of design the custom case (probably not useful for you in this case but it's a good alternative for large volume runs), installation is quick and simple, and the result is very good.

In the picture I've attached below, you can see the TO220-shaped device (I've removed the clip), which doesn't need a jacket because it's the plastic kind which is entirely insulated. The board also had another TO247 device which was heat-sinked to the case in the same way with a silicone jacket.

[mjt]

The other day I was looking at power mosfets and I was wondering - how do companies expect you to heatsink a D2PAK / TO-263 ?

I mean, take the Infineon IPB010N06N or the  IR IRFS7530-7PPbF - surface mount chips, 300W+ power dissipation.

Obviously you can get little surface mount heatsinks but you're not going to dissipate 50 W through a 3°C/W heatsink, let alone 300W.

All the power electronics of this scale that I've seen has used big through-hole TO-220 and TO-247 packages, bolted directly onto heatsinks - but someone must be buying these surface mount power mosfets, as loads of companies manufacture them.

Does everyone use expensive insulated-metal-substrate boards? I haven't seen many of them in products, and Eurocircuits charge €148 for 10 boards at 50mm x 50mm - quite a price jump for my hobby projects, when I'm used to paying $14 for the same in FR4 and they have to be single-sided to boot.

Custom copper inlays in the PCB sound even more expensive. Or can I order them in small quantities and fit them manually? I can't find anything for thermal coin, thermal inlay or copper coin on farnell or digikey.

Would a 20mm x 20mm area of 0.4mm thermal vias drop the thermal resistance to 0.15 °C/W or would the thermal resistance of the top layer of the PCB lead to diminishing returns as you get further from the component?

More generally, how do manufacturers expect us to dissipate power from these 300W+ TO-263 MOSFETs?

[rs20]

More generally, how do manufacturers expect us to dissipate power from these 300W+ TO-263 MOSFETs?

I think they don't expect you to actually dissipate 300W+. These headline specs are theoretical capabilities, available there in reserve for those few customers that are actually using IMS board. I mean, what do you expect them to do, sell themselves short and claim that their transistor craps out at 5W just because the typical customer can't dissipate 5W cheaply enough? The transistor can dissipate 300W, so they're allowed to claim it. (Others would go further and say that the headline specs are pure marketing bull.) For the rest of us, we look at the resistance spec and get a mosfet that only dissipates 1W or less in our particular usecase.

[T3sl4co1l]

Simple: manufacturers lie about ratings.

Specifically, what they do is submerge the device in nucleated boiling liquid (frothing freon at 25.00C) and perform the test.  When they say "case temperature = 25C", they literally mean all points on the case are this temperature.  The dissipation is no different from a TO-220, as should be expected: under such fantastic cooling force, that little extra tab of copper ain't gonna do jack.

Their excuse being, they did the test.  They typically don't tell you how they did it (if so, it's buried in an obscure appnote).  Whether you can actually achieve even the teensiest fraction of that in practice, f**k you...

So you must willfully ignore the ratings they give (unless they give reasonable numbers for specified board layouts), and either calculate it yourself, or at least remember offhand what kind of ratings to expect.  You're in the single digit watts, for the most part -- true whether it's anything from SOT-89 to D2PAK.  You're primarily board-area limited.

Tim

[rs20]

Simple: manufacturers lie about ratings.

Specifically, what they do is submerge the device in nucleated boiling liquid (frothing freon at 25.00C) and perform the test.  When they say "case temperature = 25C", they literally mean all points on the case are this temperature.  The dissipation is no different from a TO-220, as should be expected: under such fantastic cooling force, that little extra tab of copper ain't gonna do jack.

I think this is a bit harsh. When you design with a MOSFET, you always have to independently check 3 things (plus a few others ): the current limit, the power dissipation limit, and the junction temperature limit. When I see 300W power dissipation limit, I say "OK, that's one less limit I need to worry about" rather than "I will angrily ignore these lies grrr". Real designs with modern MOSFETs are junction-temperature limited, so that's what you design by; the 300W figure is there to tell you "there's practically no fixed power dissipation limit, move along to the current and junction temperature limits". IRF even provides junction-to-ambient thermal resistances for certain FR4 board layouts, the actual power limits in those situations are a single division away.

Also, one day someone will might use the transistor in a bath of boiling freon on an insulated metal board. No reason to sell the part short for that user just because us mere FR4 mortals are junction-temperature-constrained.

[jb79]

Hi Dave!

Nice to see that you are working on the µSupply again. Will you update the project page in the near future, or is there a plan when the µSupply should be finished?

best regards Jürgen

[dino]

Just curious, how would one cool down a bga package (5-10w) using the enclosure? Would enclosure stress compromise the bga solder balls?