Thermal vias - filling with solder

[harryn]

Hi, I am working on an LED related project. (Lumileds Rebel in case you are interested). The basic design is just single sided, but there is a lot of heat to move.

The application note calls for using
- about 30 thermal vias to move the heat from the top copper layer to the bottom.
- each thermal via is a plated through hole
- Finished hole size approx 0.5mm (but this is somewhat adjustable)

I actually want to put multiple LEDs on the board, so inserting a piece of copper wire into each thermal via is not really practical.

With a machined board, plated through holes are more challenging, so I am toying with the idea of trying to fill the drilled holes with solder paste instead to make a connection from the top copper to the bottom. I wonder if anyone has experience with something like this?

Thanks

Harry

[langwadt]

Originally Posted by harryn Hi, I am working on an LED related project. (Lumileds Rebel in case you are interested). The basic design is just single sided, but there is a lot of heat to move. The application note calls for using - about 30 thermal vias to move the heat from the top copper layer to the bottom. - each thermal via is a plated through hole - Finished hole size approx 0.5mm (but this is somewhat adjustable) I actually want to put multiple LEDs on the board, so inserting a piece of copper wire into each thermal via is not really practical. With a machined board, plated through holes are more challenging, so I am toying with the idea of trying to fill the drilled holes with solder paste instead to make a connection from the top copper to the bottom. I wonder if anyone has experience with something like this? Thanks Harry

I'm sure that if you put a piece of copper wire in the hole it will have much
less thermal resistance than just a plated through via, so I don't think you'll need nearly as many. maybe you could mill a slot and solder a short piece
of wire same thickness as pcb horisontal in the slot

there's really only one way to find out, build one and measure the temperature

-Lasse

[Jay C]

Harry, I read something on the web last night where the guy used the windshield antenna repair kit from auto parts store. He uses a vacuum table to suck the solution he squeegeed on into and through the vias.

There there is this: http://www.lpkf.com/products/rapid-p...ting/index.htm

Update: that "guy" is actually guruflorida on this board so maybe he'll chime in

[wildwestpat]

Hi Harry

You need to take a hard look at the total heat that is being generated and then work from the allowable jucnction temperature of the LED. Each junction from the LED die inside the plastic package to the ambient free flow air has a thermal resistance. The resistance is quoted in terms of watts per degree temperature rise. Your thin copper laminate of the PCB will have a high value of thermal resistance from the LED mounting flange (if I can call it that) out to the free air. Mounting several LED modules on the same piece of PCB will mean using more area of PCB per LED than would be require for a single LED multiplied by the number of LEDs. Clamping the LED packages to a solid metal surface will help but you MUST ( yes absolutely must!) calculate the heat dispersal capability of your heat sink to see what the LED junctions are going to be ok. You care to look at the mounting methods used by some of the 3 watt plus LED's from other sources. You can get a rough guide as to the size of metal heat sink required from the total wattage assume a 50 degrees ambint for passive cooling and a further 20 degrees from the heat sink mounting flange to the LED inside its package. Thus for 30 watts of LED you are looking for a heat sing with a thermal resistance of 0.43 watts per degree centigrade. To give you an idea of what would be required for the hyperthetical example look at page 19 heat sink part number SK507 and don't neglect the use of thermal paste between the LED mounting flange and the heat sink.

http://docs-europe.electrocomponents...6b80d520d8.pdf

Not exactly compact so you may need to go the forced air route.

The tabs on the LED packages are designed for surface mount but with a little cunning can be used to directly mount to a suitably profiled heat sink whilst preserving the insulation and isolation required to satisfactorily meet the operating conditions for each device.

Hope this helps before you start to test as you could blow the LEDs if you over heat them.

Also the drive conditions may require you to provide separate power supply sources for each LED package particularly if you need equality in light output from each source.

Regards

Pat

[harryn]

Thank you for the links and comments.

I actually have made a number of power LED projects, but in those cases I had some special tricks to move heat separate from the board. For this project, those methods are not available - and for once I am trying to follow industry practice - except in one area. The path I am attempting to follow is the Lumileds thermal recommendations board design data sheet. The challenge I have, is that small runs often don't included plated through holes - at least when cnc drilling is used.

I have used silver filled epoxy before - similar to what the other guy had done. Mcmaster sells it as do others. The problem that the guy in the link has is that he didn't cure it sufficiently - at least so it appears. It also has mediocre thermal conducticity, in spite of what we wish. If this project were for me, I could use it, but for this project, the economics do not work.

Back to the original question I guess:

- Is it possible to use solder paste alone to make a conductive bridge between top and bottom copper boards or is this just crazy? By this, I mean to squeegy on the solder paste - perhaps with vacuum if needed, then melt it to form a conductive bridge.

Alternative 2 - drill the hole down from the top - stop on the bottom copper - and then fill the hole with paste - then melt it to make the connection.

Thanks

Harry

[Megahertz]

I would say "yes" it is possible to make a conductive bridge between top and bottom copper layers of a PCB with solder paste. Your first post said you were using a single sided milled PCB.

However, I think you should be more concerned about the thermal transfer of heat. Production made PCBs have copper plated through-holes. Copper has excellent thermal conductivity when compared to other materials like solder paste. Refer to the Wikipedia lists.

http://www.engineeringtoolbox.com/th...ity-d_429.html

Copper = ~400 k, lead solder= ~50 k (Big Difference 8x)

You cannot replace copper (plated through-holes) with tin/lead (solder paste) and expect the same thermal transfer

[Mariss Freimanis]

Jeez,.. How hard can this be? Choices:

1) Have thermal via holes with nothing in them except for the 1mil thru-hole copper.

2) Fill the thermal via holes with heat-conductive metallic solder.

I'll take door #2.:-)

Mariss

[wildwestpat]

Hi Harry

Now that you have indicated what you are trying to replicate. Stuffing the through holes with heat conductive paste will not give you anything like the conduction via a commercial through plated hole. On the refference design has the board not only been through plated but also flow soldered? I ask as flow soldering will fill the through plated holes with solid solder unless special precautions are taken. Flow soldering will also increase the weight (thickness) of the metal and also aid heat dissipation. Eyeletts and dummy component bridges are used in industry to bond oposing sides of the copper clad board. If you have the link for the refference design it might be possible to comment further.

Have you looked into the spec of the copper clad board there are a wide variety of types available with specs and prices to match. Any refference design should specify this.

Regards

Pat

[Jay C]

Originally Posted by wildwestpat If you have the link for the reference design it might be possible to comment further.

Here

Also, the question is not whether to fill the vias or not, it's how to DIY filled vias. And the design is single-sided, but the PCB is double sided to use the bottom layer as a heatsink too.

Jay

[wildwestpat]

Hi JaY

Thanks for the data on the reference design I will read this after tea! The question on DIY through holes is that the holes are drilled then the PCB is placed in an electrolyte bath and copper deposited in the holes. This is a bit of a black art as the coating has to be homogenous and coat only the sides of the hole as the holes may also be required to take the leads of components. I don't know of any DIY methoid for producing through plated holes.

Regards

Pat

[wildwestpat]

Hi Jay

First point is that the data sheet covers the use of double sided PCB with conventional through plated holes. The thermal data relates to boards that have either through plated holes that are left vacant prior reflow and those that have been filled with resin and then plated over. The penultimate sentence on page 10 indicates that filling the vias with thermal paste achieves little when added to the copper pipe left by the through plating.

This leads to the conclusion that the data sheet is aimed at mass production PCB manufacture using electroplating to build the heat path through the vias and supplementing this by copper plating over the vias as a method of decreasing the thermal route away from LED to the exposed PCB copper. This process is not compatible with the DIY milling of the isolating lands on a PCB as the copper plating is integral to the drill plate drill cycle. The use of PCB bonded to a metal backing plate is your way forward. However the insulating material between the copper and the alloy backing will be very thin and hence the milling accuracy will have to be high if the insulation is not to be compromised. Suggest you use this material but photo etch - not too difficult as a DIY operation provided you mask the alloy well!

Your next problem will be to position and reflow the chips in place with out frying them.

There are other chips and mounting solutions out there and these might give the optical / physical features. You might strike lucky and find a source of ultra thick single sided copper clad board - where thick means copper thickness. This could give improved thermal conduction to the exposed copper to the cooling air. You will need to experiment but you will be doing this anyway as the data sheet is not a completed design as the case and cooling air will play a great part in the final application.

Hope this helps you reach a satisfactory conclusion.

Regds Pat

[Jay C]

Pat a very thorough post. This is my first thermal board, and I volunteered to mill a few for Harry so we were looking for wisdom on the topic. LPKF do have a manual plating system using a conductive paste and a vacuum fixture so it is possible. They also sell a system with small copper rivets.

I suspect you are correct and that using a board house to manufacture the design after the initial prototypes is the best course. I will start by using copper plugs for the vias. Somewhat painful . And the PCB is 3oz dual-sided which makes it a bit more difficult to find a "cheap" board house to make the design.



Jay