High power LEDs on heatsinks

Hang them from the ceiling...

Solder to aluminium clad pcb and heatsink the metal side.

This type of pcb has a very thin insulator layer between the copper and aluminium so has good thermal conductivity.

Robert Baer wrote in news:aIqdnd2H_dF-

they generally get soldered to aluminum substrate PCBs,which then are bolted to larger heatsinks.

for examples;

you can buy the aluminum substrates from Deal Extreme.(Hong Kong)

Bat glue, of course! ;-)

Are they soldering iron solderable or is it an oven job?

The kind that come attached to a small board - with screws.

The kind that come without board (just the LED case and two electrical tabs) - with epoxy or double sided tape. I think there are some epoxies/tapes optimized for heat transfer. The LED base of some units is not electrically isolated, so this will be a consideration as well.

Is there something wrong with (standard) copper?

Does anyone sell un-etched aluminum-clad PCB to individuals?

I've had to resort to double-sided 0.01 inch PCB glued to a heatsink. It works well thermally but bonding anything to copper is a PITA. I have to use copper pipe adhesive sealed with anti-oxidation lacquer. It's hobby-grade at best.

On standard board most of the thickness is fibreglass. On aluminium board the insulator layer is thin and most of the thickness is the aluminium. Vastly better heat transfer from front to back of the board.

For prototypes it can be soldered by hand.

Until we got our new oven we had to have production boards made out of house to make sure we kept within the soldering profile of the LEDs.

How about sticky backed copper foil on a Al heatsink?

It should be OK for a prototype.

How many LEDs in series? You might want to check the voltage that can be applied to the insulator on the foil. My last design the LED string had 70V across it.

I use thermal epoxy. Gets tricky IF the LED is not isolated. Easy if it is. For insulation I used diamond dust.

greg

Robert Baer wrote in news:TdadnfbuS6qzzaPQnZ2dnUVZ snipped-for-privacy@posted.localnet:

if you mean standard copper PCB,yes. epoxy-glass substrate doesn't conduct heat as well as aluminum. I don't know if copper substrate PCBs exist,but they would be better than aluminum,I suppose,barring Cu oxidation.

the Deal Extreme substrates are PCBs with an aluminum substrate,and low priced. I bought a pair of 3W CREE XR LEDs that have the AL-PCB substrates. the LEDS only cost USD $5.something apiece,postpaid. (and you can get reflectors to suit,also.) I used them for parts for my homemade bicycle LED headlamp.It came out very nicely. 8-)

my only complaint about Deal Extreme is their lack of data sheets on their current regulator boards.

My favorite prototyping epoxy for gluing heatsinkable LEDs to heatsinks is the "4-minute" epoxy for metal that some auto parts stores have. IIRC, twin-tube syringe-style dispenser, one part black, the other gray. In my experience so far, dries quickly, gets pretty-much as hard as a rock, sticks well to metal and almost anything else, and conducts heat better than plain epoxy. After that, look for "plastic steel" epoxy.

My favorite electrical insulator between a semiconductor's heatsinkable surface and a heatsink in prototypes: Plastic grocery bags, especially ones that "crinkle" more loudly in the process of being squeezed into a little ball. I find that epoxy sticks usably well to those, even though most of these are made of HDPE and HDPE is a form of polyethylene, noted often to be a "hard to glue" plastic. If you can get somke nice thin plastic grocery bags with the /2\ recycling symbol (HDPE), that has better heat conductivity than others. - And a ~.02 mm thickness of plastic is less of a barrier to heat than .25 mm (estimate) of mica. Mica's heat conductivity is only twice that of HDPE and around 2.5-4 times that of most other plastics, maybe 4.5 times that of PETE (recycling symbol /1\) and 4.5-5 times that of a few others. -

J-B Weld.

Bonding via flow soldering?

Ah. Thanks.

I haven't figured out how to do that easily. Maybe the board could be soldered to a plate with high temperature solder then the LEDs applied to the board with low temperature solder. I don't know if high temp solder remains strong enough for that to work without the PCB popping off and scattering $150 of LEDs all over the place. Another option may be creating a silicone slab that could press the hot PCB down without touching the LEDs. It would require some testing to see how big those holes need to be to allow for thermal distortion. A grid of screws won't work because they will interfere with surface mount lenses.

I still think etching aluminum-clad PCB would be easier for personal projects.