# FR4 vias vs direct to aluminium?



## RoGuE_StreaK (Apr 16, 2012)

I'm looking at getting some custom boards made up to mount XM-Ls, and am after some feedback with regards to heat transfer.
A while ago Cree released a study that investigated the effects of various via setups on an FR4 board for transfering heat, seemed that with the right setup it worked pretty well, but not as good as MCPCB. (Cree have rearranged their docs so nothing links correctly now, found it currently on digikey.jp)
Alternately, Jeff at minisystem, using advice from Kevin at Lambda Lights, used the idea of milling a slot through the PCB and soldering the XM-L directly to a slug of copper that poked through.
I'm wondering how you'd go using aluminium, whether it's better to go the milled slot vs a via-filled FR4 board (say 1mm thickness), with thermal adhesive in both instances? ie. solder the electrical contacts of the XM-L to the PCB and then thermally adhere either the via'd PCB to an aluminium heatsink, or thermally adhere the milled PCB so the XM-L's thermal pad is adhered directly to a nub on the heatsink?

I'll also investigate the cost of CNC'ing copper vs aluminium, but the heatsink in this case is also going to serve a few other purposes, which alumium would be better suited to. I could do a hybrid design of copper press-fit into the aluminium, but that would blow out the costs.

Has anyone had experience of the two methods?


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## qwertyydude (Apr 16, 2012)

Thermal adhesive onto aluminum won't work as effectively as even a regular star mount. This is because with a regular star mount, they actually have a halfway decent design, the thermal pad is soldered onto a copper foil heat spreader which, although is insulated by an enamel coat from the star, it still has a copper foil heat spreader directly soldered to the thermal pad. So even though the enamel isn't as good a conductor as the best thermal adhesives, it's still remarkably close, and the heat is distributed over a larger effective area.

With a simple thermal adhesive, even directly to aluminum you're facing a low thermal conductivity spread over a small area, which means a higher total thermal resistance. The solder joint between the copper foil heat spreader, when compared to the insulation layer essentially doesn't contribute much to total thermal resistance. So basically the big contributor to thermal resistance is the large copper foil area, with more area, even the slight disadvantage of the thermal conductivity of enamel is negated because the heat is spread over a larger area.

Basically if it were as simple as thermal compound on the pad being better, it would already be done. Certainly it would also simplify construction of stars too since it would mean less layers to silk screen on each star.


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## bshanahan14rulz (Apr 17, 2012)

I think the block of copper he was talking about using will have more mass than the heat spreader for the star, however you would have to use thermal epoxy or paste to affix the copper "slug" to the heatsink below. Also, you are making assumptions that all stars are the same. Some stars' thermal pad is the same size as the LED's thermal pad. I do think that people hate on stars too much. IMO, they are a necessary evil of trying to hand mount and hand-solder surface mount parts, and probably rob us of an unnoticable number of lumens.


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## RoGuE_StreaK (Apr 17, 2012)

I'm not hating on stars, it's primarily a mounting/connectivity issue for me, I haven't found a star that suits my needs and am having PCBs made anyway, and have some spare space on the layout. Also having parts CNC machined, so figured I may as well try to optimise the heat transfer without an intermediate layer.
Perhaps I could take a page from ahorton's Handspike, and combine it with Jeff/Kevin's slotted PCB ideas, to create something like the following (absolutely nothing to scale, simply an extremely quick mock-up)
10mm round 1mm thick FR4 PCB with shape milled in the middle and electrical contacts for XM-L. 1mm thick copper sheet then CNC routed to a suitable shape, to act as an insert in the PCB. XM-L is reflowed onto both PCB and copper slug. Assembly is thermaly adhered to aluminium heatsink?
If you are wondering, yes I am potentially looking at making these in tens/hundreds. Potentially being the operative word.


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## qwertyydude (Apr 17, 2012)

I would think the best thing to do is simply use a copper star. Your design looks alright so use that. The yellow would be bare copper and green is the enamel, gray is the traces where the electrical contacts are. The led's electrically neutral thermal pad would then be directly soldered to the star. This would be the absolute best design for a star possible, the only thing better really would be directly soldering the led to a machined block of copper heatsink. The large heat spreader area would be ideal for transferring heat. I know it works well with even non-ideal copper stars of conventional construction, direct contact of the star with the thermal pad is the best star design.

I though about this before and did an experiment, I used a sanded down copper penny, US penny's were 95% copper "red" brass prior to 1982, and in reality this is the cheapest source of copper since you're technically buying your source material below cost. I insulated trace areas and soldered an xm-l led directly to the slug. I was able to easily run 6 continuous amps through it when I used arctic silver to bond it to a good heatsink. Previously 4 amps would be the maximum with a conventional aluminum star, before long term burning of the led phosphor occurs. Even though 95% red brass has a thermal conductivity of only about 160 W/m-C and aluminum only about 138 W/m-C the soldered joint greatly helps as it's the most ideal in terms of interface between led and slug.


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## RoGuE_StreaK (Apr 18, 2012)

Actually after some more thought I'd imagine the faster (and hence cheaper) way of making the copper insert, and more effective from a heat distribution point of view, would be to start with say an 8mm rod of copper and simply run a 5mm mill bit down both sides, leaving a 2.8mm wide strip (or thereabouts, depending on tolerances of machining and PCB). Cut off in say 1cm long slugs. Drill an 8mm hole in your heatsink, press and/or adhere the slug into the hole.

Reflow would be a little trickier, could make some copper riser jigs to transfer heat to the PCB, so slug and risers/PCB sit on the reflow plate. Then again, with the PCB electrical contacts in that close proximity to a big heat-conducting copper slug, the risers may not be necessary.

Machining costs would be a lot more than material costs, so a much quicker machining process could mean it's actually cheaper to utilise a lot more copper. Will have to get in touch with my machinist and see what he reckons.

PS. I was only looking at 2-3Amps absolute max, with typical current being say 60% of max. Heat's my main worry, if I can get it working with minimal heat build-up in practice, then I might look at pushing it further.


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## SemiMan (May 2, 2012)

The green would actually be FR4 or similar with a solder mask on it ... and then the copper traces on top. That would not be level with the underlying bare copper and hence you would need a reasonably thick layer of solder to bridge that gap and may run into planarity issues. You would be running into a more complex paste mask to accomplish a reasonable soldering job.


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## RoGuE_StreaK (May 3, 2012)

Hi SemiMan, I've read your reply several times and not sure if either I'm misunderstanding your point or you're misunderstanding my diagram? To clarify, it's not a copper MCPCB, it's standard FR4 with a big hole cut in the middle, through which a shaped copper slug will protrude, bringing it level with the traces on the FR4 board. Electrical contacts on the FR4 will probably be ENIG coated, there should be very little in the way of soldermask thickness getting in the way of things. I'll be reflowing QFNL components on the corresponding driver board, so will encounter the same issues there if there are any.
That said, thanks for your input and I'll keep it in mind if I run into issues. I'm sure there'll be a lot of trial and error involved, hopefully the "error" won't result in destroyed XM-Ls and I can just try again. Just ordered a new hot air rework station the other day.

On a continued note, I've gotten back in contact with my machinist and hope to get off the designs to him today, so should get some idea of pricing. But wondering if I should modify the design to make it more usable to other people's projects; what kind of diameter/thickness would the ideal slug be for the typical CPF lumen junkie? I might be able to work the rest of my design to fit in with something more usable to the general public.


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## wquiles (May 3, 2012)

RoGuE_StreaK said:


> what kind of diameter/thickness would the ideal slug be for the typical CPF lumen junkie? I might be able to work the rest of my design to fit in with something more usable to the general public.


For a single LED, a 10mm round shape would be awesome for a bunch of small form factor lights.

The next size up would be in the 17-20mm range for the P60 drop-ins.

Anything larger would mean "C" or "D" Mag hosts.

Just my 2 cents worth of advice,
Will


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## RoGuE_StreaK (May 3, 2012)

10mm round PCB or 10mm round copper? Or both? I'm contemplating a "stepped" design, where the PCB sits on top of the copper slug, with a shaped protrusion to go to the back of the XM-L (see link in the first post for Jeff's implementation). A major design focus, as per Jeff's, is to get electrical access directly via the back, rather than having holes in the PCB that you feed the wires through and then solder onto the top.
Will check pricing comparisons, may be viable to do say a 20mm slug with a 10mm PCB on top (10mm is required to interface with other parts of my design).

Any optimal thickness for focusing purposes? Viariations in thickness won't effect implementation for me.


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## SemiMan (May 3, 2012)

RoGue, sorry I had interpreted that particular incarnation as essentially a copper star without the FR4 where the slug would be. That does give me an idea. Maybe we should PM each other on it.

Semiman


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## wquiles (May 3, 2012)

RoGuE_StreaK said:


> 10mm round PCB or 10mm round copper? Or both? I'm contemplating a "stepped" design, where the PCB sits on top of the copper slug, with a shaped protrusion to go to the back of the XM-L (see link in the first post for Jeff's implementation). A major design focus, as per Jeff's, is to get electrical access directly via the back, rather than having holes in the PCB that you feed the wires through and then solder onto the top.
> Will check pricing comparisons, may be viable to do say a 20mm slug with a 10mm PCB on top (10mm is required to interface with other parts of my design).
> 
> Any optimal thickness for focusing purposes? Viariations in thickness won't effect implementation for me.



I totally get the concept of the PCB with a hole and the metal heatsink protruding through the hole. But since you are asking about re-usability, "ideally" a 10mm round for both would be the most versatile in the most # of hosts, even hosts that use a AA size cell. If the min overall diameter of the combo board/heatsink is 20mm, then that rules out most/all of the small form factor lights - you are now in the P60 and above size range (roughly speaking, of course).

Will


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## RoGuE_StreaK (Oct 18, 2012)

Just reviving thread with a new thought; how would 4-layer PCBs with vias go? Thermal pad solders direct to a copper foil spreader, shaped something like the image above but going essentially to the edge, then the three subsequent layers getting a full plane of copper, except around the holes where the wires go through? Pepper it with vias, and...?

If I'm reading it correctly, DorkbotPDX will do 3x 1inch-square 4-layer boards for $10 + $5 international shipping; seems they'll happily do round boards etc, so theoretically I could fit 4x of these 1cm diameter boards onto a 1inch-square PCB, giving 12x boards for $15 shipped?
They use 1oz outer copper, 0.5oz inner.

I was going to get a quote from "my" machinist for a full copper heatsink/slug affair, but I'm sure even with his low costs it will be quite expensive, and surely a properly designed direct-soldered 4-layer via-filled FR4 should do quite a decent job of extracting heat? Thoughts on how it would compare to a standard non-direct-soldered MCPCB?


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## RoGuE_StreaK (Oct 18, 2012)

[EDIT] Just realised the free version of Eagle doesn't do 4-layer, then also realised that it shouldn't matter; just export the 2-layer gerber, then copy and rename the bottom layer as the two internal layers, as they should be looking the same in this case! :naughty:

[EDIT2] Actually it looks like you can squeeze in _5x_ of the 1cm diameter boards on a 1inch square, though sorting out exact meeting coordinates of connecting tabs and arcs for the outline definition could be a bit of a ***** (there's no auto-intersect)

[EDIT3] Scratch that, just lay them out in the much easier-to-determine grid manner, then group and rotate group 45degrees


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## Steve K (Oct 19, 2012)

I'm having trouble visualizing your plan... can you post images of the layers?

Thermal vias have their place, and are usually used to get heat from a package down to the power and ground layers so the heat has a chance to spread out. It gets trickier if you are using the board as an intermediate layer between the semiconductor and a heatsink, and hoping to maintain electrical isolation. 

I can see using a thin bit of FR4 with vias between the the top side where the LED solders to it, and vias connecting the top down to the bottom side where a solid layer is separated from a heatsink by a thin dielectric layer. The board would be used as a convenient mounting fixture as well as being a convenient way to make electrical connection with the LED. And now that I say this... the combination of the board and the heatsink is essentially the same as a MCPCB star (except that the star skips the bottom layer of the board and the separate dielectric layer). Oh well... 

I look forward to seeing your concept, though.


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## bshanahan14rulz (Oct 19, 2012)

I wonder if adding internal layers would really do anything to help our cause. Perhaps if those internal planes had more via connections down to the lower planes than the LED thermal pad trace can fit, there might be some benefit. I.e. perhaps you could only fit 20 vias from the thermal pad trace area to the next layer, but since the next layer is almost completely the area of the entire board, you might could fit like, 50 vias that somehow don't plate all the way through all the layers, just all the layers excluding the top layer.


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## mds82 (Oct 19, 2012)

I made some custom FR-4 boards a while back to mount a single XP-G to. the board was 5mm x 10mm in size and was 0.011 inches thick. it uses thermal vias to get the heat out and i have never once had a problem with them. Because they are so thin the heat transfer works very well.


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## RoGuE_StreaK (Oct 20, 2012)

Steve K, I did up some board designs yesterday at work but left the eagle files there; I'll try to post up some pics Monday.
Part of the whole idea is not to have / require a dialectric on the bottom; the bottom (and proposed internal) layers are only electrically and thermally attached to the thermal pad of the XM-L via soldering, the two electrical connections of the XM-L at the sides go to short traces to holes/pads, for wires to come through from underneath (holes drilled in the heatsink). I'll specify the holes in the heatsink to be slightly larger than the pad size to ensure electrical isolation.
Check out the minisystem link in my first post and you'll see what I mean.

I'm essentially wondering if internal layers will act as additional heat spreaders, so the heat can more effectively be transfered not only from top to bottom, but side to side, giving it a greater area of transfer to the final heatsink.
Then again I'm not quite deciphering the board thickness at dorkbot (too many Imperial complications...)


> The four layer board stack-up is:
> 
> 
> 
> ...


It may well be that one of the thinner 2-layer boards available at Seeed, such as the o.6mm, may work better anyway. There are some complications with Seeed however when you try to make multiple small boards on one board.

bshanahan14rulz, the connections you are describing are called blind or buried vias; they're expensive, and dorkbot don't offer them. I was pretty-much just going to add as many vias as I could fit in, without being ridiculous. Though other studies seem to indicate there's a saturation point where additional vias don't really have any added benefit. I found a good study and siimulation the other day (again at work), I'll link it through on Monday.

mds82, if I'm converting that correctly, are you saying a 0.3mm thick board? I can't find anything that thin on any of the cheap PCB sites, cost would probably be more than going for my second though of a solid CNC-machined copper slug. I think I was thinking that a 4-layer board would effectively be a stack of two much thinner boards, but looking at their figures again, I'm thinking the thickness between layers is probably thicker than thin boards available elsewhere


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## ahorton (Oct 20, 2012)

I'm a bit late to the party, but I'll throw in my experience:

I had a batch of 40mm diameter MCPCBs made a while ago with 1 XR-E, 2 XP-Gs, 7 AMC7135s and an ATTINY85 (plus a few other bits and pieces). 

140 um (4 Oz copper)
> 2 W/mK dielectric layer
good sized copper areas under the LEDs' thermal pads.
2mm copper core.

So it was really just a regular MCPCB, but it performed unbelievably well. So well that it was almost impossible to solder the power leads onto it. I basically had to heat the board up and reflow them on. So next time I'm actually considering soldering the LEDs direct to copper and leaving the driver on FR4. Not because it works better, but because it's actually easier than with the MCPCB!

Bottom line: An MCPCB can be brilliant if it's made of the right stuff.


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## RoGuE_StreaK (Oct 21, 2012)

Just throwing up the images for the hell of it

Top layer:




link to larger:
http://imageshack.us/a/img841/7474/xmltop.jpg

Red is the copper pour, as big as possible acting as a heat spreader. Vias in green. Solder mask opening is in yellow, ie. this is open copper, everything else is coated in solder mask. Big green rings are solder pads for the wires, ~1mm hole in the middle of them to allow a 
decent gauge wire to come through from the bottom (need at least 4Amp capable gauge, eg. 20AWG)
XM-L thermal pad and electrical connections are direct soldered to copper.

bottom layer, and inner layers (sans pads):




Link to larger:
http://imageshack.us/a/img502/3913/xmlbottom.jpg
Blue is copper and solder mask; all copper (except pads) is exposed, for thermal adhesion to aluminium heatsink. Idea here being as big an area of copper as possible, to spread out the heat for transfer into the Al. The queried internal layers would be identical to this, basically the whole size of the board except for an area around the pads, to ensure electrical isolation. The heatsink itself would have two holes slightly bigger that the pads, to ensure the pads don't short out to the heatsink.


Oh, and here's a link to that study/simulation I was talking about, seems to be a relatively new one, June '12.
http://www.pcb007.com/pages/zone.cgi?a=85473


Obviously the ideal solution would be a big copper slug direct-soldered to the thermal pad, but for small-scale production I think the costs are going to outweigh the gains. Likewise, custom MCPCBs aren't a solution, unless someone knows of somewhere that will do 10x or 100x for a few bucks each.


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## nickelflipper (Oct 21, 2012)

You are going to use Itead, or similar, and somehow cut the boards out?


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## RoGuE_StreaK (Oct 21, 2012)

If 4-layer, would use dorkbot, who appear to cut any old shape for free; issue there is that it seems they use a 0.1inch router, which leaves a decent chunk of connection between the five boards. Dropping it to 4 boards would mean a bigger gap between the boards, so the router could reach in further. Could be worth the slightly higher per-board cost to save a little hassle on my end.

Previous design with the internal milling was done through seeed (same fab house as itead), they use a 0.8mm router bit so much closer finish. But any of these 1cm diameter boards are seen as sub-boards, which aren't allowed, or they charge extra. Not sure what would happen if you sent a 5cm x 5cm file with say 20x of these boards on them, whether they'd flat-out reject them, charge you as 20x orders, or just charge an extra $1 for panelisation? My last (and first) batch had a main board approx 5cm x 3cm, with one of the milled-out LED boards attached to the side; they charged an extra $1 each, so the usual $10 for 10x boards, plus $10 for the sub-boards.

Here's the bottom of one of the internal-milled boards, still attached to the main board; sorry it's not a good shot, it's all I could find in my files at the moment.


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## nickelflipper (Oct 21, 2012)

Yeah, the Dorkbot router is pretty hefty, as seen on my Desklamp Driver. You shouldn't get charged extra for the router width though. I would think you would send them a single design, and let their script place however many you want in their panel. Unless the 4-5 pieces together is for some reason? You kind of have to watch rotated rectangular designs, as it can give a different price then an orthogonal image. 

My impression from the Itead site, was you could put four designs on a say a 5cm board, but they do not include routing, v grooves, or drill lines in their base price. I've never used them, so can't say from experience.


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## mds82 (Oct 22, 2012)

RoGuE_StreaK said:


> mds82, if I'm converting that correctly, are you saying a 0.3mm thick board? I can't find anything that thin on any of the cheap PCB sites, cost would probably be more than going for my second though of a solid CNC-machined copper slug. I think I was thinking that a 4-layer board would effectively be a stack of two much thinner boards, but looking at their figures again, I'm thinking the thickness between layers is probably thicker than thin boards available elsewhere



These were custom made boards for a large project where i ordered about 32,000 boards. In that quantity the boards cost only about $0.20 each, but i had to pay for all the tooling for them however


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