Eliminate the MCPCB?

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ZPositive
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Eliminate the MCPCB?

Long time lurker here. I have CNC gear and a soldering microscope. The thought occurred to me that machining a pill from scratch with the proper wire holes in it would let me reflow the LED right to the pill.  Soldering the wires directly to the LED die through the holes underneath would be tricky, but possible.  I did a quick and dirty model of it:

 

Copper pill with LED reflowed to the top.

 

Underside view:

Pill underside, access to cathode and anode pads for soldering wires.

 

What do you guys think about the thermal conductivity here?  Would it be better than the traditional LED->Solder->MCPCB->Thermal grease->Pill stack?

BlueSwordM
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Yes, it would be better in terms of heatsinking.

However:

1. Trying to solder the thermal pad to that huge heatsink would not work well at all.

2. Interchangeability would be an absolute nightmare to manufacture.

3. RIP standards for MCPCBs.

4. RIP easy repairability.

Finally, while this would work very well as a heatsink, heat dissipation would actually be quite bad.
The heatsink will get heat saturated very well quickly, and will not be able to dissipate heat effectively. Therefore, the benefit of soldering directly the LED would be null.

TLDR: You forgot about the the fact you need to dissipate the heat effectively.

Your design: LED —- Solder —- Heatsink —- ? —- Fins
That ? step is the reason why your design wouldn’t work all that well. With a DTP MCPCB, that is essentially a classical pill arrangement.
Better, but still not as good as the next one.

Modern designs: LED —- Solder —- MCPCB —- Thermal paste —- Integrated “heatspreader” to the head — Horizontal/vertical finning
Now, while that design will not be able to hold as much energy before heating up, it’ll be able to dissipate the heat much better

My very own high current Beryllium Copper springs Gen 3:
http://budgetlightforum.com/node/67401
Liitokala Aliexpress Stores Battery Fraud: http://budgetlightforum.com/node/60547

ZPositive
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Awesome, thanks for the quick reply!

I should’ve been more clear and not even called this thing a “pill”. This was just a quick and dirty CAD job to convey the idea I’m most concerned about; the heat flow into the copper piece even though those two ovals are cut away. If I build this into a flashlight, the copper piece will become a monolithic, complete center section of the flashlight with proper heat sinking fins. The only soldered connection is between the LED and the copper piece. No thermal paste, just one piece. The battery tube will screw onto one end of it, and the optics will screw to the other end. Make sense?

1. Although I haven’t actually done it yet, it doesn’t seem that difficult to reflow the die onto a pill like this, no matter how massive the pill is. I have a small kiln I can set to a precise temperature and heat the whole piece of copper at once.

2. This is my flashlight, not a production idea, and replacing the LED and driver seems pretty straightforward with my tools. Heat the whole thing, pluck the LED off, cool, clean, reheat, reflow.

3. It’s my design for my flashlight, so I don’t mind it being non-standard. Is it possible I asked this question in the wrong forum, and that’s why you assume I’m thinking of replacing an entire aftermarket worth of parts from scratch?

4. Again, this is for me, not for Joe Flashlight with a 35 Watt Weller.

Lightbringer
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If you make the shelf thick, and the holes for the wires just barely bigger’n the pads on the LED, you could maximise heat-flow through the critter. As drawn, the LED’s thermal path is just resting on that toothpick of metal which’d constrict heat-flow.

Once it’s mounted, quick tack-soldering of wires to the LED could be done before the center-pad’s solder melts.

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ZPositive
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Quote:
If you make the shelf thick, and the holes for the wires just barely bigger’n the pads on the LED, you could maximise heat-flow through the critter. As drawn, the LED’s thermal path is just resting on that toothpick of metal which’d constrict heat-flow.

Yep, that was my main concern, that the restriction of the two ovals would negate too much of the benefit of getting rid of the MCPCB. The smaller the holes and thicker the shelf, the more difficult the soldering job becomes.

Quote:
Once it’s mounted, quick tack-soldering of wires to the LED could be done before the center-pad’s solder melts.

I don’t think I’m steady enough to pull that off, but it’s a great thought.

Lightbringer
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Treat the pill like a hotplate. Heat it with, say, a hot-air gun, and once the LED’s soldered to it, cool it with a fan.

09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

contactcr
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This seems pointless just reflow a good mcpcb onto your piece. No paste needed that way either.. people have been doing this forever

ZPositive
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Okay, more ideas struck me.  How about something like this:

 

No big oval holes in the copper right next to the LED.  There's a small tombstone boss in the center of the copper for the LED to solder to.  The green part is a layer of Kapton tape.  The two orange pads are the cathode and anode solder pads.  Here's a section view:

 

Sorry that Fusion screws up all the colors in the section view.  I hope you can still make sense of it.  See how the copper piece is still monolithic, and the kapton is just insulating the two pads?  I'd need some sort of high temp adhesive to attach the pads to the Kapton.  With some careful temperature control, the whole thing could be reflowed without coming apart.  

 

Again, the copper piece is going to be way bigger than what's shown.  It'll be a big monolithic heat sink that the rest of the flashlight screws onto.  We're just looking at the interface of the LED and the copper because that's where my questions lie.

 

Edit:  I just discovered that there's a double sticky Kapton tape!  It seems like that would work to hold the two pads down during the reflow. 

ZPositive
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contactcr wrote:
This seems pointless just reflow a good mcpcb onto your piece. No paste needed that way either.. people have been doing this forever

From what I understand, solder isn’t the best conductor. I read on some old BLF post that soldering the MCPCB to the pill is worse than lapping the surfaces and screwing the MCPCB down to the pill. Surely eliminating the MCPCB and going with a monolithic center section would be even better, right?

BlueSwordM
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That’s true, but the only way for a TIM to be better is to use liquid metal.

A soldered down MCPCB would still be a better idea.

However, the idea has already been executed, but only with watercooling.

My very own high current Beryllium Copper springs Gen 3:
http://budgetlightforum.com/node/67401
Liitokala Aliexpress Stores Battery Fraud: http://budgetlightforum.com/node/60547

contactcr
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Idk I’ve never felt like that was a bottleneck in any of my builds. Especially what I can only assume is a single 3V LED you are doing this for. I don’t want to discourage you but unless you have unlimited budget for weight you should focus on outside host and driver/LED efficiency to make bigger gains than whatever thermal paste transfer bottleneck you are trying to solve.

Xoden
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ZPositive wrote:
How about something like this:

Seen it done a few times. Back before DTP MCPCB became available.
A basic insulated MCPCB with a drilled out central pad and a copper or brass pill with a nub machined to fit it.
ZPositive
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contactcr wrote:
Idk I’ve never felt like that was a bottleneck in any of my builds. Especially what I can only assume is a single 3V LED you are doing this for. I don’t want to discourage you but unless you have unlimited budget for weight you should focus on outside host and driver/LED efficiency to make bigger gains than whatever thermal paste transfer bottleneck you are trying to solve.

I’m going for increased turbo times from a FET driver on a small EDC light. And unless I misunderstand things, removing heat is exactly the main bottleneck when trying to increase turbo times. Also, if I decide to build something really extreme someday, this type of attention to detail is exactly what that’ll need.

contactcr
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The LED shelfs on even our budget lights are good now but once it gets saturated you need surface area (fins) to make any more headway without the light just being a brick of copper.

That part is way harder to do right. We already get the heat off the LED good enough. Work on a good design to get it out of the light

moderator007
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I have made a copper heatsink similar to your design many years ago. It wasn’t my idea, I just followed what someone else had already done. I saw it done a couple of times over at the other flashlight forum long ago.
It does work well. The only fault was that the wires being directly soldered on to the led was a little hard to do and was fragile while trying to assemble everything.
This was before any of us used flexible silicone wire which would take some strain off the connection. I tried to find somewhere that someone had posted up how it was done. I know I have seen it a few times years ago but I can’t seem to find the exact method posted anywhere now. Once the copper direct thermal path Sinkpad was invented there was no need to do it this way any more. It might be better than a Sinkpad if soldered directly to a copper heatsink but the copper mcpcb’s are just so much easier to work with and easier to change out if you want to change something. More modder friendly I guess.
I used a propane torch to heat the heatsink to reflow the led directly to it while held in a vise. It was a crude uncontrolled reflowing method that would cook a emitter every now and then. Back then, gaining 100 lumens from better heatsinking was well worth the effort.
.

contactcr wrote:

That part is way harder to do right. We already get the heat off the LED good enough. Work on a good design to get it out of the light


I would have to agree with contactcr, thats where the bottleneck is.
ZPositive
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Quote:
I would have to agree with contactcr, thats where the bottleneck is.

Seems to me that building this into a big monolithic center section covered in fins is as good as it can get. Or maybe a press-fit of something like this into a larger aluminum heat sink. But other than that, what else can be done?

contactcr
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How about this. Make a heavily finned light then create a ball bearing sleeve around the handle so when you want turbo you give it a spin and go to town. You can add a latch to keep it in place for lower modes.

Practical and fun and still mostly water resistant!

ZPositive
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contactcr wrote:
How about this. Make a heavily finned light then create a ball bearing sleeve around the handle so when you want turbo you give it a spin and go to town. You can add a latch to keep it in place for lower modes.

Practical and fun and still mostly water resistant!

Why?

Phlogiston
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OK, that’s different. Instead of having a fan, spin the heat dissipation fins for more air circulation Smile Beer Beer Beer

FlashPilot
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I think you’re on the right track for high wattage applications. Interestingly, Lambda Lights used to mod and resell high amp LED maglites with the same mod. He had to heat the entire chunk of copper (around +2 pounds as I recall) in order to flow the emitter onto the gargantuan copper pill. There were technical challenges he had to overcome before they would adhere. Here are some of his creations*including-video*-of-Lambda-VPT3-SBT90-MB-flux-bin. I owned several before reselling.

kiriba-ru
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You can try another way.
Regular pcb (fr4 or alu based) with central pad hole, and copper plate with extruding pin of same shape. This way have no user benefits in comparation with dtp pcb, but making fr4 or alu core pcb of any shape and design is widely available (while dtp pcb is not).

ZPositive
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kiriba-ru wrote:
You can try another way. Regular pcb (fr4 or alu based) with central pad hole, and copper plate with extruding pin of same shape. This way have no user benefits in comparation with dtp pcb, but making fr4 or alu core pcb of any shape and design is widely available (while dtp pcb is not).

Brilliant! I’ll make my pads out of standard double sided PCB. I can machine pockets into the copper base material for the pads, and even reflow the pads onto the copper. It’s basically making my own MCPCB, but the MCPCB is a huge piece of copper, with heat sinks machined into it. I’ll make sure to cut the pad reliefs a few thousandths deeper than needed, so the center section (the tombstone that the LED mounts to) is proud just a tiny bit. This will ensure the thinnest film of solder at this critical point.

I’ll model it up and post it in the morning.

ZPositive
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FlashPilot wrote:
I think you’re on the right track for high wattage applications. Interestingly, Lambda Lights used to mod and resell high amp LED maglites with the same mod. He had to heat the entire chunk of copper (around +2 pounds as I recall) in order to flow the emitter onto the gargantuan copper pill. There were technical challenges he had to overcome before they would adhere. Here are some of his creations*including-video*-of-Lambda-VPT3-SBT90-MB-flux-bin. I owned several before reselling.

Holy F! I remember those lights from back in the day! Like I think I even bookmarked that exact thread in my early days of lurking. LED technology sure has progressed in a decade.

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This made me think of the mods that have attached a star MCPCB to the base of a heatpipe based heatsink.
Too bad there is nothing akin to that or even a vapor chamber for the pill to move heat away, far faster than any solid metal you can think of.

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ZPositive wrote:

Okay, more ideas struck me.  How about something like this:


 



No big oval holes in the copper right next to the LED.  There’s a small tombstone boss in the center of the copper for the LED to solder to.  The green part is a layer of Kapton tape.  The two orange pads are the cathode and anode solder pads.  Here’s a section view:


 



Sorry that Fusion screws up all the colors in the section view.  I hope you can still make sense of it.  See how the copper piece is still monolithic, and the kapton is just insulating the two pads?  I’d need some sort of high temp adhesive to attach the pads to the Kapton.  With some careful temperature control, the whole thing could be reflowed without coming apart.  


 


Again, the copper piece is going to be way bigger than what’s shown.  It’ll be a big monolithic heat sink that the rest of the flashlight screws onto.  We’re just looking at the interface of the LED and the copper because that’s where my questions lie.


 


Edit:  I just discovered that there’s a double sticky Kapton tape!  It seems like that would work to hold the two pads down during the reflow. 


This is exactly what a DTP MCPCB is…
So your idea is basically just a bigger mcpcb?
If you use liquid metal between the mcpcb and a heatsink it’s basically like having no mcpcb, the extremely high thermal transfer and large area on the bottom of the PCB make it so that heat flow restriciton is as low as possible for the LED.
ZPositive
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Quote:
This is exactly what a DTP MCPCB is… So your idea is basically just a bigger mcpcb? If you use liquid metal between the mcpcb and a heatsink it’s basically like having no mcpcb, the extremely high thermal transfer and large area on the bottom of the PCB make it so that heat flow restriciton is as low as possible for the LED.

Yep, the idea is to eliminate the thermal resistance of the MCPCB->pill connection and pill->heat sink connection by making everything from a monolithic piece of copper. No need for liquid metal connection to the heat sink when the LED is already directly attached to the heat sink.

djozz
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ZPositive wrote:
Quote:
This is exactly what a DTP MCPCB is… So your idea is basically just a bigger mcpcb? If you use liquid metal between the mcpcb and a heatsink it’s basically like having no mcpcb, the extremely high thermal transfer and large area on the bottom of the PCB make it so that heat flow restriciton is as low as possible for the LED.

Yep, the idea is to eliminate the thermal resistance of the MCPCB->pill connection and pill->heat sink connection by making everything from a monolithic piece of copper. No need for liquid metal connection to the heat sink when the LED is already directly attached to the heat sink.


If you do make it, you may test the performance compared to a copper DTP-board with ordinairy Arctic silver 5 paste connection to the pill. Back in 2012 our BLF member match tested the performance of the first Sinkpads against his machined copper slugs and found to his disappointment that it was the same, all his hard work did not have unique performance anymore, any layman could by a cheap Sinkpad and get the same performance in a much easier package. It so appears that the thermal performance at the tiny solderpads of the led is way dominant over what happens further away from the led spread out over larger surface areas.
ZPositive
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Okay, here's the latest. Two pieces of FR-4 circuit board material are set into the copper monolith.  They're copper on both sides, so I can reflow them down to the monolith underneath.  There's a thin tombstone right under the LED's thermal pad, just a couple thousandths above the FR-4 pads and the rest of the monolith.  This ensures a thin solder connection where it counts (from the LED thermal pad to the monolith), and lets the solder be thicker for the electrical connections.from the die to the FR-4.

 

 

Section view.  The only FR-4 I have is thicker than I'd like (0.056"), but I don't think it'll make a massive difference here.  The chamfers on the FR-4 should keep any solder bridging from happening during reflow.  

 

 

Again, this is just the LED->monolith interface.  The rest of the monolith and heat sinks aren't shown.

Marc E
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Always good to see creativity Smile
I’m not knowledgeable enough to comment on the existing plan but is there scope/neccessity to raise the emitter up a little, away from the shelf, for better focus in a reflector?

Enderman
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ZPositive wrote:
Quote:
This is exactly what a DTP MCPCB is… So your idea is basically just a bigger mcpcb? If you use liquid metal between the mcpcb and a heatsink it’s basically like having no mcpcb, the extremely high thermal transfer and large area on the bottom of the PCB make it so that heat flow restriciton is as low as possible for the LED.

Yep, the idea is to eliminate the thermal resistance of the MCPCB->pill connection and pill->heat sink connection by making everything from a monolithic piece of copper. No need for liquid metal connection to the heat sink when the LED is already directly attached to the heat sink.


The thing is there is no significant thermal resistance between the MCPCB and pill.
The area of contact is extremely large, hundreds of times larger than the area between the LED thermal pad and MCPCB.
The bottleneck is there, not between the PCB and pill.
If you want every bit of extra performance then use a larger 32mm pcb and liquid metal, but you can do tests and you will probably not be able to measure any difference than if you used a 20mm one, or if you used a solid copper block.

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